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!).
Reports suggest two thirds or more women will experience some sort of morning sickness – which can include vomiting, nausea, food aversions, and feeling “under the weather”. Most get it between weeks 6 and 14, some get it earlier, and some suffer for 9 long months. Some get it at specific times of day, or have it triggered by brushing their teeth, particular foods, or particular smells. Unfortunately, we don’t know what causes morning sickness, nor why it varies so much woman to woman, but it may be something to do with the placenta, because it’s whilst the placenta’s forming that most women experience morning sickness.
Animals also get it. In a study of 202 female rhesus monkeys, it was found they systematically rejected foods or certain foods both whilst they were ovulating and in early pregnancy, peaking around week 5.
Women who feel sick also have a higher probability of a healthy pregnancy, and lower probability of spontaneous abortion. However, this does not explain Hyperemesis Gravidarum, which makes the woman so sick she is at risk of dehydration, malnutrition, and even death.
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. 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!).
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, and studies showing effectiveness in treating morning sickness.
The most mysterious thing we’ve found about morning sickness is its apparent predictive power over your offspring’s salt preferences. Studies in infants and adults have correlated how sick expectant mothers got with how much their offspring like salt, and found it statistically significant. There appears to be no explanation for this!
For more about morning sickness, see our blog post on the subject.
Pica and cravings
Along with sickness, many pregnant women experience cravings. 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; however, research has shown that pica sufferers with low iron do not necessarily crave iron rich foods – scientists can’t explain why.
Some women experience a persistent metallic flavour in their mouth (dysgeusia). Dysgeusia can stick around, fade away, or come and go throughout pregnancy, including after birth: the reasons why are unknown. Dysgeusia might happen to push women towards avoiding risky foods – but the taste persists even with “safe” foods. Or it might serve to drive a woman to eat things with trace salt elements in them, like calcium, sodium and iron. Alternatively, it could just be a side effect, and serve no biological purpose.
Anything associated with food during pregnancy is considered a food craving or aversion. This includes a change in sense of smell. Women have reported being able to suddenly smell a non-burning candle across a room. Some think this develops to sharpen sensitivity to bitter flavours that might be poisons, but it doesn’t happen to all women, and isn’t selective towards bitter flavours. Scientists think it is also caused by rising oestrogen levels, or increased snot and mucus production, which can interfere with the senses! Mucus can even change the length or quality of vocal chords, and some pregnant women report their voice deepening!
Menstruating when pregnant
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 1 in 5 women experience some kind of bleeding during pregnancy.
Pregnancy is associated with more and poorer sleep and some women have bizarre and vivid dreams.
Most studies conclude that concerns about labour or becoming a mother drive nightmares during pregnancy, but these don’t explain all dreams, including the commonly reported sex dreams!
Other scholars think rising progesterone may be responsible for sleepiness and dream intensity. Increased tiredness increases the volume of sleep and so the volume of dreams. Pregnancy discomfits also break up the rhythm sleep, meaning you wake more and recall more. We can’t ignore these physiological factors (hormones and discomfits), otherwise we’d expect to see the same kinds of and vividness of dreams in (involved) expectant fathers. Some expectant fathers do report increased dreaming and more vivid and anxious dreams, but not all or as much.
Although they’re not sure why, some researchers have linked poor sleep to longer and more difficult labour and delivery, with women sleeping < 6 hours a night 4.5 times more likely to undergo caesarean operation. Others found that shorter and easier deliveries were linked to labour nightmares, and hypothesised that this was because women were “practising” in their dreams!
For more about sleep, take a look at our dedicated article on the subject.
Pregnant women can experience several skin conditions, including mild itching, intrahepatic cholestasis, a serious liver condition caused by the build-up of bile, which doctors think may be triggered by high levels of oestrogen, or melanin overproduction, leading to dark skin patches around the face, nipples or a line on a pregnant stomach, especially for darker skinned women, known as the linea nigra. However, some pregnant women get PUPPP (pruritic urticarial papules and plaques of pregnancy) – a nasty, itchy rash that often starts on the stomach and spreads. The origin is unknown, but it mostly happens on first pregnancies (we don’t know why, but it rarely recurs) and when weight gain is rapid (scientists think rapid skin stretching might provoke an inflammatory reaction).
For more about bizarre pregnancy symptoms, see our blog post on the subject.
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, 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. Or, in other words, small is bad for baby, big is bad for mother.
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.
For more about weight during pregnancy, see our blog post on the subject.
Microbes and genes
Some have suggested that heavier mother or fast weight-gaining mothers (often the same group) may cause their children to grow up to have higher BMIs. If so, the mechanism behind this could be insulin resistance. However, it’s impossible to entangle weight factors from genetic and environmental factors such as family eating habits. Other research has suggested that in some cases increased BMI can skip a generation, or is in fact linked to the microbiome. For more about the microbiome, see our dedicated article on the subject.
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.
Humans, bats and shrews menstruate, shedding the blood lining of the unimpregnated womb, but most other animals don’t. Some scientists reckon that spontaneous decidualisation (the body preparing for pregnancy monthly “just in case” rather than in response to an embryo implanting) and shedding to evacuate the womb may have evolved to protect the mother from an aggressive burrowing foetus. Once formed, the placenta acts as a barrier between mother and baby: protecting her from its demands and it from her immune system.
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). The placenta is believed to be instrumental in conveying “immune privilege” to the foetus. Amongst its many activities, the placenta secretes neurokinin B – the chemical used by parasitic nematodes to avoid detection by the immune system of their host!
However, it’s not the only time when genetically different cells have been found living inside our bodies. Because some cells cross the placenta, mothers keep some of their babies’ cells, and their offspring carry some of their mothers. Cells may also be exchanged between unborn twins, and younger siblings may carry some of their older brothers and sisters. What these microchimeric cells do and how benign they are remains unsolved, but they have been implicated in brain health: fewer are found in the brains of women who get Alzheimer's.
One Nature study suggested that schizophrenia could develops during pregnancy when “schizophrenic genes” are turned on in the placenta. These findings are the first to link early life complications, genetic risk and mental health.
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”. During this stage, women are often given an oxytocin injection their thigh to stimulate contractions and help expel the placenta. For some reason, this reduces postpartum bleeding, but scientists think there may be conflating adverse effects, and more research is needed.
After birth, the cord to the placenta is cut. However, some research suggests it would be better to wait, allowing the baby absorb nutrients from placental blood that could help it adapt and decrease chances of anaemia. Scientists are interested in what special qualities placental blood may have. Others have suggested delaying cord cutting may be linked to increased risk of jaundice.
The placenta has long been thought sterile, but recently, some researchers have claimed to find healthy and diverse microbes – this remains controversial.
We still don’t know how the placenta controls what can and cannot pass. It’s possible viruses can get through where bacteria can’t because they’re smaller. Yet the placenta collects and stores some chemicals, including medications from the mother’s bloodstream. If doctors could understand what triggers the storing mechanism, they could design medications that could treat the mother and not affect the baby.
Modelling the placenta is hard because placental cells do not spontaneously grow into a placenta, and trophoblast starter cells do not divide. Scientists have managed to get some growth using a microgravity bioreactor system to model shear stress and rotational forces.
Towards the end of pregnancy, the placenta passes antibodies from the mother to the baby that weren’t allowed to transfer earlier, conveying passive immunity to the baby for about 3 months. But only some kinds of antibodies can pass – ones acquired a while ago. Diseases caught and fought off whilst pregnant don’t transfer!
For more about the placenta, see our blog post on the subject.
ToxoplasmosisOne 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 Estersantibody drugs are being explored.
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.
However, you can’t toxoplasmosis directly from cats: you have to handle cat poo (where the eggs are excreted) from cats that have contracted the parasite for the first time (or lambs!): after that, like humans, they become immune. Another risk factor is handling soil, possibly because cats toilet outdoors. This is a bigger risk but can be mitigated by wearing gloves and hand-washing.
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), and is dangerous when undercooked (especially pork, lamb, and venison), cured (like salami), raw (such as oysters, clams, or mussels), or unpasteurised (dairy products).
However, risks vary with cultural eating habits, livestock health and local soil. Social trends also play a role, such as the fashions to eat raw food diets, including uncooked vegetables and meats. Free-range and organic meats are more likely to carry the parasite, simply because the animals spend more time outdoors on untreated land.
One study showed that 14 to 49% of infections still had an unidentified cause, primarily because of the difficulties involved in monitoring, detecting toxoplasma gondii in food, and in confirming sources . Scientists think there may be other unidentified sources in food and the environment.
For more about toxoplasmosis, see our blog post on the subject or our specialist article.
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, and broody birds will insist on constant nest sitting. Marsupials don’t nest. They carry their young with them in a pouch, and scientists think this might be why.
When a pregnant female animal starts to nest, her oestradiol, prolactin and progesterone levels soar. When she stops, her oxytocin is high – the hormone responsible for contractions in labour. Shortly after the birth, progesterone levels drop, oestradiol stays steady, and prolactin keeps going up. The exact timings vary from species to species.
But not all nesters are pregnant females. Male and non-pregnant female animals sometimes nest. Scientists think this behaviour is performed to regulate temperature or endear themselves to a potential mate.
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(!)…) – called sympathetic pregnancy, or sometimes couvade syndrome. Some doctors think this is psychosomatic, whilst others think there may be stress hormones behind it.
For more about nesting, see our blog post on the subject.
This article was written by the Things We Don’t Know editorial team, with contributions from Ginny Smith, Cait Percy, Johanna Blee, and Rowena Fletcher-Wood.
This article was first published on 2019-10-19 and was last updated on 2019-11-04.
why don’t all references have links?
 Czaja, John A. Food rejection by female rhesus monkeys during the menstrual cycle and early pregnancy.Physiology & behavior 14.5 (1975): 579-587.
 Flaxman, Samuel M., and Paul W. Sherman. Morning sickness: a mechanism for protecting mother and embryo. The Quarterly review of biology 75.2 (2000): 113-148.
 Flaxman, Samuel M., and Paul W. Sherman. Morning sickness: adaptive cause or nonadaptive consequence of embryo viability? The American Naturalist 172.1 (2008): 54-62.
 Schuster, K., et al. Morning sickness and vitamin B6 status of pregnant women. Human nutrition. Clinical nutrition 39.1 (1985): 75-79.
 Chittumma, Porndee, Kasem Kaewkiattikun, and Bussaba Wiriyasiriwach. Comparison of the effectiveness of ginger and vitamin B6 for treatment of nausea and vomiting in early pregnancy: a randomized double-blind controlled trial. Journal-medical association of thailand 90.1 (2007): 15.
 Crystal, Susan R., and Ilene L. Bernstein. Infant salt preference and mother's morning sickness. Appetite 30.3 (1998): 297-307.
 Crystal, Susan R., and Ilene L. Bernstein. Morning sickness: impact on offspring salt preference. Appetite 25.3 (1995): 231-240.
 Borgna-Pignatti, Caterina, and Sara Zanella. Pica as a manifestation of iron deficiency. Expert review of hematology 9.11 (2016): 1075-1080.  Şükür, Yavuz Emre, et al. The effects of subchorionic hematoma on pregnancy outcome in patients with threatened abortion.Journal of the Turkish German Gynecological Association 15.4 (2014): 239.  Abrams, Barbara, Sarah L. Altman, and Kate E. Pickett. Pregnancy weight gain: still controversial. The American journal of clinical nutrition 71.5 (2000): 1233S-1241S.  Emily Ostler, Expecting Better: Why the Conventional Pregnancy Wisdom is Wrong - and What You Really Need to Know (2013) Penguin Press.  Fraser, Abigail, et al. Association of maternal weight gain in pregnancy with offspring obesity and metabolic and vascular traits in childhood. Circulation 121.23 (2010): 2557.  Reynolds, R. M., et al. Maternal BMI, parity, and pregnancy weight gain: influences on offspring adiposity in young adulthood. The Journal of Clinical Endocrinology & Metabolism 95.12 (2010): 5365-5369.  King V, Dakin RS, Liu L, et al. Maternal obesity has little effect on the immediate offspring but impacts on the next generation. Endocrinology. 2013.  Chan, William FN, et al. Male microchimerism in the human female brain. PLoS One 7.9 (2012): e45592.  Gammill, Hilary S., et al. Effect of parity on fetal and maternal microchimerism: interaction of grafts within a host?.
 Robert-Gangneux F, Dardé ML. Clin Microbiol Rev. Epidemiology of and diagnostic strategies for toxoplasmosis. Clinical microbiology reviews 25.2 (2012): 264-296.
 Singh A.K., Verma A.K., Jaiswal A.K., Sudan V., Dhama K. Emerging food-borne parasitic zoonoses: A Bird’s eye view. Adv. Anim. Vet. Sci. 2014;2:24–32. doi: 10.14737/journal.aavs/2014/2.4s.24.32.
 Hussain, Malik, et al. Toxoplasma gondii in the Food Supply. Pathogens 6.2 (2017): 21.  Wischner, D. Kemper, N. Krieter, J. (2009). Nest-building behvaiour in sows and consequences for pig husbandry. Livestock Science. 124 (1–3): 1–8. doi: 10.1016/j.livsci.2009.01.015.  Woodside, B., and Leon, M. (1980). Thermoendocrine influences on maternal nesting behavior in rats. Journal of Comparative and Physiological Psychology, 94(1), 41-60. doi: 10.1037/h0077652.  Klein, Hilary (1991). Couvade syndrome: Male counterpart to pregnancy. International Journal of Psychiatry in Medicine. 21 (1): 57–69. doi:10.2190/FLE0-92JM-C4CN-J83T. PMID 2066258.