In many ways, I hit the jackpot with my first pregnancy: my energy stayed pretty high the whole time, my knees and hips only ached a little, but most spectacularly, I didn’t get any of the so-often-depicted nausea of early pregnancy. With my second, I wasn’t so lucky; around 4pm every day, I was pretty reliably searching for a bucket or a toilet.
It has become an overused trope in a lot of way in the media, with a newly pregnant woman running off to the bathroom first thing in the morning or at the mention of any particularly potent food. For many people, it is the first sign that something might be up, and perhaps they should pee on a stick and see.
While it gets the name ‘morning sickness,’ pregnancy-related nausea can hit any time of day - in fact, only 1.8% of pregnant people report morning-only symptoms, and 95% report symptoms before and after mid-day (Lacroix et al 2000, Hadsby et al 1993) . It impacts up to 85% of pregnant people (O’Donnell et al 2016), about 35% of whom say it negatively impacts their day-to-day lives (Attard et al 2002). Some studies have suggested difference across cultures in the prevalence of “morning” sickness, but many more indicate that the awful experience is universal for pregnant people cross-culturally (Fez et al 2019). These early-pregnancy “morning” sickness symptoms of nausea and vomiting typically begin around 2-4 weeks after fertilization (so, when you are 4-6 weeks pregnant) and generally abate by 20 weeks, unless you are in the unlucky minority with the extreme hyperemesis gravidarum, in which vomiting and nausea continues all the way through the pregnancy (Flaxman and Sherman 2002, Verbery et al 2005).
Frustratingly, and like so much related to pregnancy, childbirth, and postpartum care, the root cause of morning sickness and early-pregnancy nausea remains unclear. While the atrocious hyperemesis gravidarum is considered a disorder worthy of treatment, run-of-the-mill early pregnancy nausea and “morning” sickness are often shrugged away as just a typical part of pregnancy.
So what is happening here? Why would early pregnancy, during which extra calories are needed and macronutrient requirements are critical for proper fetal development, have vomiting up everything you eat as normal standard operating procedure? And, why don’t any other mammals seem to have nausea in early pregnancy?
Let’s first explore what are called proximate causes; in other words, what is happening in the bodies of pregnant people that trigger the nausea and vomiting? Then, we’ll try and tackle the ultimate causes, those that explain why this trait appeared in our species - and only our species - at all.
Potential proximate causes for morning sickness include:
Hormones. The catch-all scapegoat for all things that go wrong in pregnancy (and menopause, and puberty…). The American Pregnancy Association points to both HCG (human chorionic gonadotropin) and estrogen as possible hormonal culprits, but this is based purely on correlation: HCG, estrogen, and nausea all increase rapidly in early pregnancy, so it is assumed that hormonal rises must be causing the nausea. Some studies show that pregnant people with particularly severe nausea have higher than average levels of HCG, not everyone with high HCG levels has extreme nausea (O’Donnell et al 2016). The actual physiological explanation that links increased HCG to nausea is also unclear.
Genetics. Another scapegoat for all things that go wrong! Some research has linked a sensitivity to bitter tastes to a higher likelihood of hyperemesis gravidarum (Sipiora et al 2000). Perhaps pregnant people with more sensitivity to bitter tastes develop stronger food aversions because of that hyper-tasting ability during pregnancy, but the link between bitter taste receptor genes and increased morning sickness remains unclear. A study from Norway found that pregnant people who had hyperemesis gravidarum in one pregnancy were more likely to have it in subsequent pregnancies, and anyone whose mother had hyperemesis gravidarum when she was pregnant is also more likely to develop the extreme disorder (Trogstad et al 2005, Vijanes et al 2010). Both of these point to a genetic component for the extreme hyperemesis gravidarum, but don’t tell us much about the more common run-of-the-mill morning sickness. A twin study from 2016 does suggest that all “morning” sickness might be in part genetic, using identical twins to estimate the heretability of early-pregnancy nausea at 73% (Colodro-Conde et al 2016).
Psychological factors, such as a history of depression or depression during the first trimester, also correlate with higher likelihoods of morning sickness and nausea (Dekkers 2019). This finding stresses the importance of being attentive to both the physical and mental health of pregnant people when assessing risks for pregnancy-related complications, but doesn’t provide any direct link. Studies show that pregnant people who experience morning sickness often feel more isolated, lonely, or depressed than those who don’t (Tan et al 2014), and who is surprised? I certainly feel less social when I am constantly nauseous. Drawing the line of causality - does depression cause morning sickness, or does having morning sickness bring on depression? - is something that is so far unclear.
A better understanding of the underlying cause of this enormously unpleasant pregnancy symptom (that impacts up to 85% of pregnant people!) could go a long way to better treatment options. For the moment, treatments are often difficult because of the fragility of the embryo in early pregnancy. Many medications that would otherwise be used to treat nausea and vomiting could harm the embryo and thus are off the table for pregnant people. For me, 15 weeks pregnant and staring at the upcoming dinner hour with horror and no relief in sight, an explanation of why this was happening to my body would have been more than welcomed. And while knowing exactly which hormone or genetic variant in my DNA was the culprit might help the medical field design a better treatment, it wasn’t going to help me push through my discomfort. Is there a reason for this misery? Something in our species’ history that at some point in the distant past made this useful?
Maybe.
It is completely possible that early pregnancy nausea and vomiting are a completely non-adaptive side effect of hormones, or a bad draw of the genetic lottery, but that is unlikely because:
How enormously prevalent it is;
Its presence in only humans (as far as we know);
A pile of correlation evidence suggesting that pregnant people who experience nausea and vomiting the first trimester have fewer birth defects, better birth weight, and lower risks of preterm delivery, miscarriage, stillbirth, and death during delivery (summarized very helpfully in Patil et al 2012).
If early-pregnancy nausea isn’t just a nasty side effect, then there must be something deeper in our evolutionary past that would have selected for the suffering, something that explained why it was adaptive - in other words, what made pregnant people with nausea and vomiting in the first trimester of pregnancy more likely to have offspring that survived and have offspring themselves (the evolutionary holy grail: pass on more copies of those genes through more babies!).
And never fear, there is no shortage of hypotheses as to what this deeper-rooted cause might be.
The evolutionary explanations offered to explain why early pregnancy nausea is adaptive range from the plausible to the insultingly absurd. For example, in 1994 a paper proposed that nausea was adaptive during early pregnancy because it prevented sexual intercourse, which was harmful to the embryo - notwithstanding there is zero evidence that intercourse during the first trimester causes any harm at all, or that early pregnancy nausea is associated with any change in sexual behavior (Deutsch 1994).
On the more plausible side of things, there are two major hypotheses for why this early pregnancy nausea might have been helpful in the past:
The “maternal and embryo protection hypothesis,” which suggests that the nausea and associated food aversions stop pregnant people from consuming foods that could be harmful or toxic to either the fetus or the pregnant person (who is immunocompromised during pregnancy, a topic for a later post).
The “placental growth and development” hypothesis, which argues that restricting the nutrient intake of the pregnant person actually helps the embryo grow a larger, more robust placenta, therefore increasing nutrient transfer to the eventual fetus.
Let’s start with the “maternal and embryo protection hypothesis.” All plants have secondary compounds developing throughout its unique evolutionary past that deter herbivores (and humans) from eating them. These compounds often have a bitter or otherwise distinctive taste to them. Clever humans, we often seek these out for our own purposes, like caffeine in coffee beans or capsaicin in chili peppers, but these secondary compounds have the potential to be toxic, carcinogenic, or otherwise harmful in large doses. This hypothesis suggests that the early pregnancy nausea response stops pregnant people from eating bitter-tasting or strong-tasting foods in early pregnancy when the embryo - and the newly-immunocompromised pregnant person - are the most vulnerable (Sherman and Flaxman 2002).
There is mixed support for this in the literature. Pregnant people all across the world tend to have aversions to similar foods, many of which contain toxins and pathogens (summarized in Patil et al 2012, Table 2). Flaxman and Sherman (2000) show that many of the food aversions during this first trimester are related to animal foods (I couldn’t even look at salmon, but I craved hamburgers - go figure). Meat products are more likely to carry harmful pathogens than plants (broadly speaking). The researchers also find that nausea and vomiting are less common in societies without meat as a “staple” in their traditional diets (although the connection between early-pregnancy nausea and societal dietary norms deserves much more careful study). Fesler (2002) found that meat aversions were statistically more common across societies than other food aversions, which makes sense again given that meat is the most likely food item to be loaded with dangerous pathogens or parasites. However, there are also plenty of counterexamples to these observations; some studies find that sometimes pregnant people crave meat more than other foods (ex: Orloff and Hormes 2014). There is also no clear data that aversions to meat or bitter vegetables would change the risk profiles of pregnant people in an evolutionary significant or meaningful way. Nonetheless, as meat products have undoubtably played a pivotal role in human evolution, it is not impossible that limiting their consumption during this vulnerable few months would have been adaptive in the time before USDA quality standards.
The second theory with some evidence behind it is called the “placental growth and development” hypothesis. This one is a little counterintuitive at first: it posits that mothers who restrict nutrient intake during early pregnancy end up with a larger placenta, which in turn means more nutrient transfer to the fetus through the rest of development. The reason for this is because unstable or nutrient-poor environments during early development cue increased growth of the placenta and nutrient transfer mechanisms; basically, the growing placenta “thinks” that there isn’t much food to go around, so it better grow as big as possible to gather up as much as possible. When the nausea goes away in the second trimester, the growing fetus then has a robust placenta (and, hopefully, plenty of food to go around after all) (Huxley et al 2000, Coad et al 2002, Patil et al 2012). Evidence for this hypothesis is more mixed, and much of it is based on studies in animals (like sheep and cows) that may or may not be true for humans. A more foundational issue, there is no definitive evidence that nausea in early pregnancy actually does decrease nutrient intake overall; many women make up for the aversion to meats or other foods by increasing calories from carbohydrates and dairy products, for example (Sherman and Flaxman 2002).
So, what does this mean for the pregnant and nauseous among us?
Take heart: although it might not be clear exactly why nausea in early pregnancy was adaptive in our evolutionary past, it is pretty likely that it was. While you’re cursing your partner for grilling a steak inside the house, think to your far-away ancestors who survived and reproduced just that much more effectively because of it.
Should I avoid meats and plants with lots of secondary compounds, even if I’m not feeling nauseous? Not necessarily. There are some secondary compounds - like alcohol and caffeine - that have demonstrated negative impacts on your developing embryo and fetus, and you should absolutely follow medical guidance on that front. But if you make sure your meat is well-cooked and rinse off your spinach, there is very little chance of pathogens making their way to you via your foods today.
What about treatments? Some studies have shown that homeopathic treatments, like ginger and even acupuncture, do help with mild nausea better than a placebo (O’Donnell et al 2016). If those are appealing to you, do it! If your nausea is causing you to get dehydrated, feel lightheaded, or otherwise negatively impact your wellbeing, call your doctor or midwife right away.
Remember that the psychological impacts of this are real, and anyone who tries to dismiss feelings of loneliness, depression, or anxiety as being “just in your head” doesn’t know what they are talking about. Your brain is “just” in your head! Your head is filled with mission-critical things, including your emotional wellbeing. There is help out there - speak up and reach out. We were never meant to grow tiny humans alone.
References
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