Pregnancy

Article curated by Rowena Fletcher-Wood

There are many unknowns when it comes to pregnancy, and many accepted phenomena are still unexplained, or simply attributed to "hormones" or "the placenta" (a complex and poorly understood organ!).

Morning Sickness

  

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2. By-product – sickness is a nasty side effect that serves no direct purpose, caused by mother and baby competing over essential resources or the hormones associated with viable pregnancies.^[3].

3. Social indicator – letting others know a woman is pregnant to ward of sexual partners and increase social support (there seems to be nothing in the literature at all about this!).

  

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During the critical first trimester, most women get sick and go off bitter things, including coffee. This has completely messed up medical research into the effects of caffeine, because it’s difficult to untangle high consumption from lack of sickness. Some research suggests excessive caffeine consumption magnifies other negative effects^[4].
Learn more about Alcohol and Caffeine (Things We Don’t Know about Pregnancy Series #9).

  

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Advice on relieving morning sickness is also controversial. Many tricks don’t have an explanation behind them, such as ginger, used for centuries as a sickness remedy, and still unexplained – or sea bands, which apply acupressure, an eastern medical technique that currently remains unsupported by scientific evidence. Another controversial remedy is vitamin B6, which comes with both studies showing no effect^[5], and studies showing effectiveness in treating morning sickness^[6].

  

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The Thalidomide Scandal

In 1953, thalidomide was prescribed for morning sickness, but then over 10,000 babies were born with deformities. The molecule, it turns out, is handed, which means it comes in right-handed and left-handed versions, and the left-handed version appears to be responsible for birth defects.

But is the right-handed version safe? We don’t actually know. Initially, thalidomide was identified as harmful when birth defects were statistically observed in 20% of thalidomide babies (above the normal average of 1.5%). However, years later, higher than usual rates of dyslexia, autism, or epilepsy were also observed, and we’re not sure how thalidomide causes any of these.

  

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There are more than 30 proposed mechanisms for how thalidomide caused these birth defects, and in fact, it looks like more than one could be true. Mechanisms include causing mutations in DNA and cartilage, interfering with neural networks, generating reactive oxygen species (which accelerate cell ageing in the body), and inhibiting some proteins^[9]. It is also antiangiogenetic, which means it stops the development of new blood cells – exactly what happens when the placenta is forming between 6 and 12 weeks of pregnancy.
Learn more about The Placenta (Things We Don’t Know about Pregnancy Series #4).

  

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Cravings and aversions

Along with sickness, many pregnant women experience cravings. The reason for this is often vaguely explained away by hormonal changes, but the specific reason is still unknown. Medical advice is to follow cravings, which may signal the need for certain nutrients.

 

At the extreme end, some women suffer from pica, a craving for non-food things like dirt. Many crave ice cubes – which is directly linked to iron deficiency^[10]; however, research has shown that pica sufferers with low iron do not necessarily crave iron rich foods – scientists can’t explain why.

  

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Embryo development

There are many things we still don't know about the very first hours of an embryo's development. In this time, does an embryo separate its back from front? Left from right? Or is this decided later?

We know some of the orders of development, but usually in more detail later in the cycle when the embryo is looking more like a baby. We still don't know what drives cells to specialise into organs and functions and how the initial shape and structure takes hold. This is of especial interest to those making moral decisions about when an embryo changes from a bunch of cells to an independent living thing.
Learn more about Embryo development.

 

Menstruating when pregnant

  

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Morning sickness is a common symptom of pregnancy, but traditionally, a woman twigs she is pregnant when she misses a period, but this doesn’t happen to everyone. Some continue having “periods” for one, two, or more months. Whilst they are not truly menstruating, doctors can’t explain every bleed. Common explanations are implantation bleeding (when the embryo embeds into the uterus lining), subchorionic haematoma (where blood collects between the placenta and uterus lining), or miscarriage. Minor causes might be tears, inflammation, or infections. Around One in five women experience some kind of bleeding during pregnancy^[11].

  

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Unusual pregnancies

  

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We don’t know how long cryptic pregnancies last because the women who get them didn’t know about it at the time. However, some think they last longer or shorter than recognised pregnancies.
Learn more about Cryptic Pregnancies (Things We Don’t Know about Pregnancy Series #20).

  

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Phantom pregnancies
Some weird pregnancies are not really pregnancies at all. For example, phantom pregnancies. Sometimes, like in many cryptic pregnancies, there is no medical evidence of pregnancy, but the woman still labours under the impression that she is pregnant. This is known as pseudocyesis, or delusion of pregnancy, or phantom pregnancy. Phantom pregnancies can also occur in males, and can last significantly longer than recognised pregnancies, some for many years. We don’t know why phantom pregnancies come about, but it correlates strongly with the post-menopause period and weakly with psychiatric disorders including schizophrenia, other psychotic disorder, mood disorders and organic brain disorder ^{[12][13][14][15]}.

  

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Parthenogenesis
Parthenogenesis, or asexual reproduction, has never been observed in humans, but does occur in animals, including sharks, snakes, lizards and insects.

Considered a last-resort tactic for species survival, parthenogenesis occurs when two of the mother’s eggs combine to create the embyro. In 2001 at the Henry Doorly Zoo in Omaha, a female bonnethead shark that had been kept in all-female captivity for three years gave birth to a female pup that was later DNA tested and found not to have any male shark contribution. It is not known to what extent parthenogenesis has impacted animal lineages.

  

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Sleep

  

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We can’t measure the brain activity of a human foetus – not whilst they’re inside their mother. Researchers into brain activity instead perform EEG (electroencephogram) exams on premature babies and monitor eye movement to tell them about sleep cycles, although errors are common.

  

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Using these methods, REM (rapid eye movement) sleep sleep is detected from around 7 months, when the brain cycles in and out of restful and REM sleep every 20 to 40 minutes and the foetus sleeps 90% of the time. Very little is known about foetus sleep before this.

  

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Opinion is divided when it comes to whether foetuses sleep more at night or day. Until 3 months old, they don’t produce their own melatonin – so rely on mum. But does it work to make babies sleep? Many babies are soothed by their mother walking, and this persists after birth, with motion such as rocking sending them to sleep, whilst stillness makes them wake up.

  

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Another area of research is the relatively un-navigated territory the relationship between foetal health and maternal sleep. Some studies think that the mother can influence her child’s health by sleeping well when she’s expecting, but the cause-effect relationship between impaired sleep and infant health is a difficult web to untangle!
Learn more about Foetal health and maternal sleep.

  

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Skin

  

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Weight

In the US, health professionals obsessively monitor women’s weight whilst pregnant. However, the correct amount of recommended weight gain during pregnancy is debated scientifically^[16], and the weighting process is often counter-productive because it makes women worry.

Weight gain goes towards growing the placenta (1.5 pounds), amniotic fluid (2 pounds), increased tissue masses (4 pounds), extra fluids (4 pounds), increased blood volume (4 pounds), and, of course, the baby (7.5 pounds). A further 7 pounds is stored nutrients, including fat, believed to be a vital resource during pregnancy and breastfeeding. Scientists don’t know exactly how this happens – only that it’s controlled by hormones.

Because baby growth is affected by what you take in, weight affects baby size. Very large or very small babies are at risk of more complications, especially small for gestational age (SGA) babies. SGA babies carry a risk of cardiovascular, respiratory and digestive complications, whereas SGA (large for gestational age) babies cause birth problems – usually leading to caesarean^[17]. Or, in other words, small is bad for baby, big is bad for mother.

Statistical analyses of competitive athletes (not climbers) has shown that women who are extremely fit are less likely to have overweight babies and equally likely to have underweight babies, with a slightly lower average weight. Since women who are very fit are less likely to be overweight, and overweight women tend to have heavier babies, this could just be a biased sample effect.

But weight gain during the pregnancy is not the only thing that affects baby weight. Even more important is when the baby is born – early, late, or close to their due date, any conditions the mother may get like gestational diabetes, and her starting weight.

Over- and underweight women can be at increased risks of pregnancy complications. However, overweight and obese women are still recommended to gain weight during pregnancy – just less. Being obese carries a chance of reduced fertility, higher incidents of miscarriage, stillbirth, birth defects, gestational diabetes, preeclampsia, sleep apnea, blot clots, and a tricky delivery/recovery. It can also complicate spotting birth defects. Being underweight carries the risk of premature birth and SGA at birth – and so risk of cardiovascular, respiratory and digestive complications in the newborn.
Learn more about Weight (Things We Don’t Know about Pregnancy Series #2).

  

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Height

Why do shorter women have shorter pregnancies? Researchers looked at the records of nearly 3500 births and discovered a connection between height and pregnancy length. The team don't know the exact cause yet, but believe it must be in a set of unknown genes. It was added that a "woman's lifetime of nutrition and her environment" are likely factors too.

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Microbes and genes

  

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Caesareans

Are caesareans risky? Maternity mortality rate for caesarean section is now only 3 times that of vaginal births^[23], but this may be because a number of caesareans are only carried out in emergencies. Caesareans may also reduce the risk of complications in some cases: for women who have previously had a caesarean section, choosing an elective one for a subsequent baby over a vaginal birth reduces the risk of complications or consequential health problems (such as womb damage) from 1.8% to 0.8%^[24]. Overall risk remains minute.

  

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Regardless of whether elective or emergency, a review of 20 million births across 61 studies has shown that babies born by caesarean are a third more likely to develop autism and a sixth more likely to develop ADHD^[25]. Possible explanations include:

• Confounding factors that contribute to the likelihood of a caesarean, like an older mother, defective placenta, or premature baby.
• Use of antibiotics during the operation.

  

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Studies have shown that parents recognise their own baby’s cries above those of others and are more responsive to their needs due to an amplifying effect of the hormone oxytocin^{[26] [27]}. The effect, however, is not as strong if the baby is delivered by caesarean section^[28]. We’re not sure why.

  

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Interestingly, fathers are equally good at recognising their own baby’s cry as mothers, but, for some reason, they are less sensitive to oxytocin and giving them more doesn’t seem to increase their sensitivity to baby cries^[29][30].

  

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We can use oxytocin to initiate labour, but how does it happen naturally? Does the foetus initiate labour? If so, how? We don't actually know, but the foetus may be responsible – or one of many factors.

Researchers have found two proteins in mice that eventually lead to the promotion of labour. These proteins, SRC-1 and 2 (steroid receptor coactivators) produce pulmonary surfactant components, which in turn begins the birth. Two of these components specifically, SP-A and PAF, are very important in the start of labour because a lack of them caused an up to two day delay in the mice. If that was in a woman, it could be up to a month late! Further, when injected into mice lacking these components, labour was initiated^[31].

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Exercising whilst pregnant

Exercise is good for you – but how much exercise is good for you when you’re pregnant? Recommendations are ever-changing, and for some sports, such as climbing, there are no controlled studies for performing whilst pregnant.

However, one literature review found that women who exercised at high intensity had a range of reduced pregnancy symptoms, including:

• 1/3 the rate of gestational diabetes
• 2/3 the rate of preeclampsia
• 1/4 the rate of low back pain
• Lower chances of deep vein thrombosis

• 1/3 the rate of epidurals
• 1/4 the rate of induced labour
• 1/4 as many caesarean sections
• Lower chances of umbilical cord tangling

  

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• Dizziness
• Headache
• Extreme thirst
• Swelling

• Chest pain
• Vaginal bleeding
• Contractions

  

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Scientists are investigating how, but it seems getting too hot and staying too hot for prolonged periods has been linked to malformations and birth defects in unborn babies, especially during the first trimester. This has led some people to avoid exercising hard. However, you’d have to run very fast for at least a couple of hours to get your body temperature near the danger zone. Studies on competitive athletes who work out at above 90% their recommended maximum heart rate have so far shown no increase in birth defects. “Hot yoga”, “hot pilates” and saunas, however, are deemed hot enough to pose some risk.
Learn more about Exercise (Things We Don’t Know about Pregnancy Series #8).

  

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Miscarriage

Miscarriage is a common medical complication that leads to the loss of a pregnancy before 23 weeks, and affects 1 in 4 women during their reproductive lifetime. Most miscarriages make themselves evident with symptoms including abdominal pain, discharge, and vaginal bleeding. These symptoms can abate, or progress to the expulsion of the foetal tissue after days or even weeks. However, once it’s started, we don’t know any way to stop it: nature simply runs its course. Sometimes the uterus doesn’t empty out properly, and an operation has to be performed called dilatation of the cervix and curettage of the uterus (or D & C).

There are five main reasons for a miscarriage:

  

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(i) genetic or chromosomal abnormalities, where a foetus inherits a faulty gene, or copying errors occur when cells are dividing early on^[32].

(ii) lifestyle factors such as smoking, drinking, obesity, or malnutrition

(iii) health conditions like uncontrolled diabetes, thyroid disease, high blood pressure, food poisoning, infections (especially in the uterus), trauma, or hormonal disorders – or the use of non-compatible medications.

  

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(iv) mis-implantation – when an egg implants outside the uterus (usually on the walls of the fallopian tube). This is a medical emergency because the placenta develops a complex network of blood vessels between the mother’s tissues and the baby’s – meaning that when it peels away at birth, massive internal bleeds can happen if it’s not attached to the uterus, a muscle that contracts to stem the bleeding. Around 1 in 80-90 pregnancies are ectopic in the UK.

(v) placental problems: which can be affected by blood disorders, trauma, or multiple pregnancies, amongst many things. Amongst things that can go wrong, insufficiency (not passing the baby enough nutrients) and abruption (coming off the uterus wall too early) are linked to miscarriage.

  

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When the amniotic sac that usually contains a foetus develops without the foetus in it, this is known as a blighted ovum or anembryonic pregnancy. The empty sac means that the fertilised egg never implanted, or didn’t develop properly, and instead got reabsorbed back into the uterus. Whilst this happens early on, if the placenta continues growing, it makes all the hormones associated with pregnancy anyway, masking the loss. Normal pregnancy symptoms progress, and the sac often has to be removed surgically. Scientists don’t know what causes a blighted ovum, but it could be linked to complications chromosome 9 and is more common if the parents are biologically related.

Read more about miscarriage and ongoing research on the topic here.

  

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The placenta

The placenta is a complex and poorly understood organ. A two-sided disc, one side develops from the mother’s tissues 7-12 days after conception and sticks to the womb, and the other forms 17-22 days after conception from the blastocyst that starts off the foetus, once it has connected up with the mother’s blood supply. Scientists are still studying how the placenta forms.

  

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Mostly when something foreign enters a woman’s body, her immune system attacks it. Scientists are still trying to uncover why she doesn’t also kill off a foetus. Understanding the process could reduce miscarriage frequency (~10% of pregnancies).

  

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At the end of its life cycle, the placenta starts to break up. For placenta-sharing twins, this can sufficiently limit their oxygen and nutrient supply that doctors will induce the mother early. We don’t know why it breaks up early, but it might help detach it from the womb wall so that it can be expelled in the “third stage of labour”.

  

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A new study has reported the vertical transmission of the coronavirus COVID-19 from mother to unborn baby during the second trimester of pregnancy^[37]. We don't know how it overcome the placenta. Abnormalities were found in the foetus, and placental insufficiency diagnosed. It’s not clear whether the virus took advantage of the placental insufficiency, or caused it, but no other cause was identified.

For more about the placenta, see our blog post or article on the subject.

  

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Toxoplasmosis

One such disease is toxoplasmosis, contracted from toxoplasma gondii, a protozoan parasite that will infect a third of people over their lifetimes.

Women who’ve contracted it toxoplasmosis before becoming pregnant are immune, and will pass on that immunity. The danger is contracting it for the first time whilst pregnant and passing on congenital toxoplasmosis, which can cause miscarriage or stillbirth, or birth defects that may not develop until adulthood, including seizures, jaundice, liver enlargement, hearing or vision loss, low IQ, neurological disorders and mental disability.

Doctors still don’t understand how toxoplasmosis works, and can’t treat it, though new anti-malaria, anti-schizophrenia and nanoparticle-borne antibody drugs are being explored.
Learn more about /toxoplasmosis.

  

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Healthy adults humans are usually asymptomatic (though some get flu-like symptoms) and are considered dead-end accidental hosts, because toxoplasma gondii can only reproduce in cats – and wants to get back in cats. Some people think that toxoplasmosis can even change your behaviour, driving you to like cats, hang out with cats and, if you’re a rat, end up eaten by cats so the parasite can get back in cats.

  

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However, you can’t toxoplasmosis directly from cats: you have to handle cat poo (where the eggs are excreted) or soil. This is why some foods, such as “unwashed greens” are best avoided during pregnancy – they might be soil contaminated and the soil might contain toxoplasma gondii – washed greens are safe to eat. Meat is the primary source of toxoplasmosis (30-63% of infections in Europe^[38]), and is dangerous when undercooked (especially pork, lamb, and venison), cured (like salami), raw (such as oysters, clams, or mussels), or unpasteurised (dairy products).
Learn more about Toxoplasmosis (Things We Don’t Know about Pregnancy Series #6).

  

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Things to avoid and environmental exposures

There are lots of things pregnant women are advised to avoid, including mobile phones, fire retardants, non-stick coatings, and endocrine disrupting chemicals such as bisphenol A and phthalates, found in many plastics and personal care products.

Whilst eating fresh rather than processed foods, minimising painting and use of cosmetics (or fancy pregnancy oils, creams and other unnecessary personal care products) are sensible moves, the exposure levels to these things are so low that they’re pretty much negligible anyway. However, these things do create confounding factors that make assessing the impact of particular foods, drinks or chemicals difficult if not impossible to quantify.

 

Many women turn to herbal teas as an alternative to caffeinated beverages, but be careful to do your research if you choose this path: not all herbal teas are safe for pregnant women. Herbal remedies tend to be unregulated, and natural concentrations of components can vary wildly, making amounts on packets rough guidelines at best. One recommended, safe tea is peppermint tea (which can even ease nausea), whilst chamomile is not a good idea (it can trigger uterine contractions).

 

Nettle tea, meanwhile, is sometimes recommended, and sometimes recommended against.

  

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Research in mice has shown that the reproductive potential of mice born to paracetamol-ingesting mothers is reduced. The males were less aggressive to other males and couldn't mate with the females, whilst the females were born with fewer eggs, becoming infertile earlier. We're not sure how the study translates to humans, however. At the moment, women are advised not to take ibuprofen, aspirin, and other painkillers, but paracetamol is considered okay. Is this wrong?

  

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Vitamins and supplements

Supplementing is generally advised for all pregnant women – but should it be? Do we actually absorb many of the nutrients in tablets, and do they make any difference? Scientists have shown that taking high, regular doses of multivitamins is actually proven to increase your risk of heart disease or cancer^[39]. We don’t know why (it could be because of the fibre).

However, supplementing is advised to ensure you get enough of some nutrients, including vitamin B12 (cobalamin). This vital nutrient supports the functioning of the nervous system and blood formation and is only found in trace amounts outside meat and dairy products, except in fortified foods like cereals. Researchers are unsure whether vegan sources of B12 are adequate.

It's also unclear whether or not pregnant women should eat much oily fish. Fish contains omega 3 fatty acids – vital nutrients essential for brain development, blood and heart regulation – but also mercury. Is the risk worth it, or should it be avoided? Mothers are being pulled in two different directions.

Vitamin D may need to be supplemented for in winter: helping the body absorb calcium for building baby bones, it's found in fish, eggs, dairy, fortified breakfast cereals, beans, and green leafy vegetables, and is made by the body when exposed to sunlight.

However, more care should be taken over vitamin A. Whilst essential for growth and development, and healthy eyesight and immune system, the liver stores it, and can accumulate dangerous levels, whereupon it starts to act as a toxin and cause birth defects in babies. Healthcare professionals can advise on the correct amount.

Similarly, folic acid is needed in high quantities (10-20 times the normal requirements) in early pregnancy, but research suggests that getting too much folic acid later in pregnancy can mask vitamin B12 deficiency^[40] and may increase the risk of autism^[41]. Research continues.

  

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Other diseases

Autoimmune disorders may be triggered by harmful bacteria, viruses, or certain drugs, which mess up the body’s ability to differentiate between normal, healthy tissue and harmful substances. It could also have a genetic factor.

  

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If your red blood cells are coated in an antigen known as rhesus D, you are rhesus positive. If they’re not, you’re rhesus negative. Rhesus state is inherited from the mother or a father, so a rhesus negative mother could carry a rhesus positive foetus if the father is also rhesus positive. In the UK, 85% of people are rhesus positive.

A pregnant rhesus negativewoman can come into contact with rhesus positive blood either when the placenta peels away during childbirth, or earlier if maternal and foetal bloods mix, such as after a fall or during a prenatal test. This sensitises the mother to rhesus positive blood, amplifying her immune response to it next time – usually during a second pregnancy. If the mother’s immune system attacks a foetus, this can lead to jaundice, heart failure, or enlarged organs. The mother herself won’t feel symptoms though – only the foetus suffers.

Rhesus disease is normally treated blood transfusion or early delivery.

  

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Anti-phospholipid syndrome (APS) is an autoimmune disease that pregnant women can get, where anti-phospholipid antibodies are produced by the immune system. These attack the pre-embryo or trophoblast (outer cells from the blastocyst that give nutrients to the embryo and eventually become placenta), preventing it from staying implanted, and alter blood flow to the uterus. No one is sure why this comes about.

  

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Mental health

Pregnancy is a time of great vulnerability, so it may come as no surprise that mental health conditions are higher for women on average around pregnancy (including during and just after) than any other time. Women's mental health, however, remains misunderstood and chronically underdiagnosed, as do the instances of pregnancy- or birth-centred mental health conditions in men. Many women’s conditions such as complicated births are treated physically, but the mental aspect is under-researched and rarely addressed, including birth trauma, postnatal depression, and grief following a miscarriage.
Learn more about /perinatal mental health.

  

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Postnatal depression is thought to occur in ~1 in 10 mothers, making it a common form of mental illness. The onset and peak of the illness may be weeks or even months after the birth of a baby, and the condition lasts for weeks, months, or longer.

  

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It also occurs in men, where it is chronically underdiagnosed.

  

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The Avon Longitudinal Study of Parents and Children looked at 3,176 father and child pairs, where they found not only did 1 in 20 fathers developed postnatal depression, but this correlated with a small but significant increased risk of daughters (but not sons!) developing depression at age 18^[43]. Scientists are unsure why the effect is only seen in girls.

  

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There is no conclusive medication used to treat postnatal depression, but many options including drug alternatives such as hormone therapies, acupuncture, electroconvulsive therapy (ECT), or transcranial magnetic stimulation (rTMS). Evidence is inconclusive^[44].

  

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Postpartum bipolar disorder, the least well-known postpartum mental health disorder, is characterised by mood episodes of mania, hypomania or depression that interfere with everyday life and performing ordinary tasks. We don’t know the cause.

  

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Postnatal (puerperal) psychosis is an uncommon but severe condition, which can include low mood – or manic mood – delusions, hallucinations, and out-of-character behaviour. No one’s sure what causes it, but trauma or a history of other mental illnesses are risk indicators.

  

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Nesting
Is nesting, cleaning, organising, decorating and stockpiling in preparation for a baby biologically or socially driven?

Animals nest from birds to fish, rodents, cats, dogs, pigs, and 80% of pregnant women. Pregnant dogs will steal blankets, cats will climb into haylofts, rabbits pluck out their own fur to line the burrow, sows leave the herd to travel up to 6.5km^[50], and broody birds will insist on constant nest sitting. Hormones have been linked to it, but up to 90% of men in a relationship with a pregnant woman show some kind of symptom (weight gain, morning sickness, mood swings, fatigue, disturbed sleep, labour pains(!)…) – leading to the alternative suggestion that it may be social.
Learn more about Nesting (Things We Don’t Know about Pregnancy Series #5).

  

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The endometrium

Scientists know very little about the endometrium – the uterus lining. The wall of the uterus changes during the menstrual cycle and accepts the implantation of a fertilised egg during pregnancy^[51]. As such, it is known that the endometrium responds to female sex hormones, but not which or how. It also produces its own chemicals.

  

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IVF and infertility

Now, scientists think that IVF failure may be just due to timing. IVF works by suppressing the natural menstrual cycle, stimulating and gathering eggs, and then reintroducing fertilised eggs. If they are introduced too early or too late, however, the womb may not be ready to receive them, and they will be lost. For most women, this timing is guessed, but a pilot study that individualises treatment by studying women’s uterine cycles has shown a 33% success rate.

Immunological treatment, however, remains controversial. It’s hard to entangle immunological factors in success stories from other factors such as individualised support, changes in diet, and medical attention.

  

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Some clinicians think high levels of natural killer cells from the uterus, might be associated with miscarriage. However, they may have a different role there to in the blood, remodelling blood vessels through the placenta; in fact, some research suggests low numbers of natural killer cells in the uterus may obstruct pregnancy and that they perform a benign role, preventing other immune cells from attacking the foetus^[52][53]. There’s also no agreed way to get natural killer cell counts. Blood levels and uterine levels are not necessarily linked, and levels in the uterus vary with the menstrual cycle.

Statistical analyses of 21 antibodies have found no differences in implantation rates, pregnancy rates, or pregnancy outcomes for higher or lower concentrations. Some treatments have been outright refuted, such as leukocyte immunisation therapy, which may be dangerous^[54][55].

  

List of Institutes Researching Infertility

Things We Don’t Know (1 researchers)

Alloimmune disease might also affect fertility. Alloimmune disease happens when the female body launches an immunological attack on male tissues – basically killing off his sperm.

  

List of Institutes Researching Alloimmune disease

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Semen and pre-eclampsia

However, in contrast to alloimmune responses against a partner’s sperm, exposure to sperm may sometimes prevent pre-eclampsia, a life-threatening pregnancy complication that sometimes arises in the second half of pregnancy^[56]. This may be due to immune modulating factors in the seminal fluid.
Learn more about Reproductive Immunology (Things We Don’t Know about Pregnancy Series #7).

  

List of Institutes Researching Semen to Prevent Pre-eclampsia

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Biological sex

Identifying the biological sex of an unborn baby can be difficult, even with ultrasound: sometimes the baby just doesn’t cooperate, and accuracy only rises to 100% after 14 weeks gestation, with a success rate starting at 75% during the 12-week scan ^[57]. For some, knowing the sex of their baby early is especially important if they carry a serious genetic disease that affects one sex. For these women, invasive techniques may be used to collect foetal DNA, which can increase risk of miscarriage, so increasing ultrasound accuracy could save lives. Possible develops include cross-section imaging (allowing us to see “through” legs that are in the way).

To find out more about determining the sex of a baby, read our dedicated article on the subject.
Learn more about Determining the Sex of a Baby (Things We Don’t Know about Pregnancy Series #12).

  

List of Institutes Researching sexing a baby

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Types of twins

Scientists are very interested in twins because it helps us identify the differences between genetic and environmental factors that influence health and behaviour. However, how twins form and the various types out there remains mysterious. For example, no one has ever seen cell fission – the explanation we rely upon to explain monozygotic (identical) twins.

Other types of twins exist too. For instance, semi-identical or sesquizygotic twins: “in-between twins” that share 50% to 100% of their DNA in common: halfway between monozygotic (identical) and dizygotic (fraternal) twins. They are rare, we don't know how they come about, and there are several theories.
Learn more about /twins.

  

List of Institutes Researching Sesquizygotic twins

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²

Other strange types of twins include conjoined twins. These may arise from fission or fusion^[58], but we don't know which. Do they come from incomplete egg splitting, or the twins growing together? If the latter, are they linked to additional limbs (which could be vestiges of absorbed twins), or even fetus in fetu, or one twin entirely inside another?

  

List of Institutes Researching Conjoined twins

Things We Don’t Know (1 researchers)

²

Although as many as eleven internal foetuses have been identified, some scientists say these are not actually fetuses in fetu, but in fact a teratoma: a type of tumour that forms into whole organs or tissues such as hair^[59].

  

List of Institutes Researching Is fetus in fetu actually a teratoma?

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²

Partcularly common in twins is chimerism, a strange phenomenon where cells with the DNA of one person are found in the body of another. In particular, remnants of DNA of the mother have been found integrated in the brain of the child and vice versa^[60][61][62]. Exchange of cells occurs across the placenta, twins exchange cells in the womb, and older siblings can even pass cells to younger ones via the mother. Male cells found in the brains of some women are notably fewer amongst women with Alzheimer's, suggesting they contribute to brain health.

  

List of Institutes Researching Chimerism in twins

Things We Don’t Know (1 researchers)

²

What causes SIDS, and why mention it in pregnancy?

Sudden Unexplained Infant Deaths (SUIDs) of infants under a year old occur unpredictably and don’t have an obvious cause. Of around 200 such deaths in the UK every year, around 80% are classified as SIDS – Sudden Infant Death Syndrome (also know as cot death). These are the deaths that can’t later be explained by suffocation, infections, or genetic disorders, even after autopsy.

But what does this have to do with pregnancy?

Maternal health during pregnancy is a key indicator for SIDS risk. Some risk factors are a bit connected to pregnancy and a bit not, for example, minor illnesses (such as anaemia^[63]); these can be out of your control, but chances may be reduced by staying healthy during pregnancy (in the case of anaemia, eating tons of iron-rich food) and getting all the requisite vaccinations.

Scientists think SIDS may be due to a combination of developmental challenges and environmental stressors, but there's still a lot we don't know.

Check out our article on SIDS for more information.
Learn more about /sids.

  

List of Institutes Researching What causes SIDS?

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This article was written by the Things We Don’t Know editorial team, with contributions from McDowellG, Ginny Smith, Cait Percy, Johanna Blee, Kat Day, Rowena Fletcher-Wood, and Joshua Fleming.

This article was first published on 2019-10-19 and was last updated on 2021-03-29.

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why don’t all references have links?

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Blog posts about pregnancy

Baby Tastes (Things We Don’t Know about Pregnancy Series #22)

Friday 26th of June 2020

Superfetation (Things We Don’t Know about Pregnancy Series #21)

Sunday 14th of June 2020

Cryptic Pregnancies (Things We Don’t Know about Pregnancy Series #20)

Saturday 30th of May 2020

The Causes of SIDS (Things We Don’t Know about Pregnancy Series #19)

Monday 11th of May 2020

Depression During and After Pregnancy (Things We Don’t Know about Pregnancy Series #18)

Tuesday 28th of April 2020

Birth trauma (Things We Don’t Know about Pregnancy Series #17)

Wednesday 15th of April 2020

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