Pineapples Don't Belong on Pizza: The Evolutionary Function of Morning Sickness

by Amy Fox, Natalie Cox, Brandy Shell, Exree Hipp, and Steven Jackson

Though the debate of pineapples on pizza can be quite controversial to avid pizza lovers, most pregnant women would agree with the statement that pineapples don’t belong on pizza. Little do most people know that there are potential evolutionary purposes for the food aversions and nausea that characterize morning sickness. Pineapple, for example, is potentially dangerous to fetuses, and often causes vomiting when consumed by pregnant women.

What is morning sickness?

Pregnancy sickness or "morning sickness" is a set of symptoms that commonly occur during the first trimester of pregnancy. Many women know that some of these symptoms include food aversions to things that they previously enjoyed, nausea and vomiting. This side effect can be one of the first signs of pregnancy, as it usually is most severe 2-8 weeks after conception, peaking in severity in weeks 4 or 5. 

As many pregnant women will tell you, "morning" sickness is actually an inaccurate label because it is not limited to the morning.

Furthermore, research suggests that even the "sickness" part of this term may be inaccurate, as pregnancy sickness might actually have a helpful function. Because morning sickness is generally thought of to be an awful side effect of pregnancy hormones, the idea that it might have benefits was not explored until relatively recently. We'll discuss this possibility in more detail later. 

What triggers morning sickness?

Hormonal factors

One of the most commonly believed triggers of morning sickness is Human Chorionic Gonadotropin (hCG). This hormone causes ovulation and is important to fetal development. Exactly how this hormone leads to sickness is not known, but it is suspect due to its synchronized timing and abundance with morning sickness. Two hormones, progesterone in conjunction with estrogen, may also cause or contribute to morning sickness. 

Food triggers

Specific smells, especially those of foods, may trigger a sick feeling or nausea in pregnant women. For example, Samuel Flaxman and Paul Sherman conducted research (described here; see here for original article) indicating that meat, fish, poultry and eggs are common triggers of nausea and vomiting during pregnancy. Their research indicated that many bitter foods (e.g., certain raw vegetables) and beverages (e.g., coffee, alcohol) may also trigger pregnancy sickness. Also, as alluded to in our title, tropical fruits such as pineapple are also commonly aversive to pregnant women. 

Heredity

Heredity may also play a role, as the severity of morning sickness symptoms seems to run in families, as demonstrated by twin studies and studies examining correlations between morning sickness severity in mothers and daughters. Many of the other factors discussed here could actually be hereditary, so at any time there may be multiple factors at play, leading to differing degrees of morning sickness.

Other factors

Lifestyle factors, such as diet and activity level, as well as medical history have been seen to correlate with pregnancy sickness, though these factors are more likely to contribute instead of cause pregnancy sickness. 

The sickness caused by these potential factors has obvious physical effects such as nausea, vomiting, and headaches, but they also come along with psychological effects. These unpleasant physical sensations can cause food avoidance or the avoidance of undesirable smells. In very extreme cases, morning sickness can cause substantial physical harm the mother and/or fetus, although this is very rare. In fact, as discussed in the next section, morning sickness may actually be predictive of positive health outcomes for the fetuses. 

Could morning sickness be beneficial?

In the 1980s, Margie Profet of the University of California in Berkeley offered a functional theory to explain morning sickness. She said that morning sickness might be an evolved adaptation that protects a fetus from naturally-occurring toxins found in certain foods. This is known as the “Embryo Protection Hypothesis.” By avoiding these foods, women may be able to improve their chances of having a healthy child. Thus, women who experienced nausea and revulsion to particularly harmful foods would have more successful pregnancies, and those same aversions would be passed onto future generations. 


There are a number of findings that support Profet’s hypothesis. For example, as mentioned above, morning sickness symptoms are most severe from 2-8 weeks after conception, the period in which organ and brain development are occurring. During this time, the developing fetus is most vulnerable to the negative effects of food-borne toxins. Pregnancy sickness also tends to diminish significantly or disappear completely by the fourteenth week after conception. This correlates with the the most vulnerable period of fetal development.

Furthermore, the foods that commonly trigger morning sickness in women are more likely to high levels of toxic substances. As mentioned above, bitter foods and foods with strong "bacteria-y" smells (e.g., fishy odors) are particularly likely to cause nausea in pregnant women, and are also very likely to contain things that would be likely to harm the developing baby. Meats are also commonly avoided by pregnant women, and are also likely to contain parasites and bacteria.

More interesting still, evidence indicates that the severity of morning sickness correlates with positive health outcomes for the baby. Women who do not experience morning sickness at all are three times more likely to have a miscarriage (see this study also). Women with morning sickness also have a reduced risk of low birth weight and preterm births. Furthermore, women with moderate to severe morning sickness have better fetal outcomes compared to women with mild morning sickness. However, it should be noted that about 20-30% of pregnant women who do not experience pregnancy sickness give birth to healthy children.

All of these pieces of evidence are consistent with Profet's hypothesis that pregnancy sickness serves the purpose of protecting the fetus.

Are there alternative theories about morning sickness?

Some competing theories to Profet's are that pregnancy sickness is simply a byproduct, or non-adaptive side effect, of pregnancy. For instance, the mother-offspring conflict hypothesis suggests that the fetus "wants" an abundance of resources to survive but this is often in conflict of what is best for the mother and potentially her future pregnancies, and that this conflict may result in certain negative side-effects for the mother. Pregnancy sickness could simply be a result of over-active thyroid function in pregnant women. The fetus causes the mother's thyroid to overproduce certain hormones for the fetus because they are important for early neurodevelopment, and pregnancy sickness is just a side effect of that overproduction. This would suggest that pregnancy sickness is not an adaptive trait to protect the fetus, but rather a byproduct of the mother-offspring conflict in which the fetus attempts to obtain more resources from the mother's body than she is willing or physically able to give.

The main distinction between the mother-offspring conflict hypothesis and the embryo protection hypothesis is that the former suggests that nausea is just a side-effect of pregnancy, whereas the latter suggests that nausea per se has a purpose, that is, to promote the avoidance of harmful foods.

However, notice that both hypotheses predict that the severity of pregnancy sickness symptoms would correlate with the overall health of the baby. Particularly healthy babies would be able to secure more resources from the mother during development, and this would cause greater nausea as a side-effect.

Take-aways

Certain foods and substances are avoided by pregnant women because they cause nausea and vomiting. These same foods also are likely to cause harm to developing fetuses. This is consistent with the hypothesis that women's bodies (and minds) have developed adaptations that protect their fetuses. The current evidence doesn't completely rule out the idea that pregnancy sickness is a byproduct of fetal development, but either way, it is suggestive of an adaptive function. So, pregnant women will continue to avoid pineapple on their pizza, rare steaks, and perhaps sushi as scientists search for answers to pregnancy sickness.