Morning Sickness is an Adaptation, not a ... Sickness

There is new information from an older idea (from about 2000) by Paul Sherman and colleagues. The idea underlying this research is simple: Symptoms of illnesses may be adaptive. Indeed, this may be true to the extent that we should not call certain things illnesses. Like "morning sickness."

ResearchBlogging.orgBroadly speaking, there are two different kinds of reasons that a woman may experience nausea in association with pregnancy. 1) This pregnancy thing is a complicated mess with all kinds of hormonal (and other) things going on, so you puke; or 2) a woman who is pregnant feels nauseous for good evolutionary reasons.

From an article in American Naturalist by Flaxman and Sherman:

"Morning sickness" is the common term for nausea and vomiting in early human pregnancy (NVP). Recent interest in why NVP occurs-that is, in the evolutionary costs and benefits of NVP-has spurred the development of two alternative hypotheses. The "prophylaxis," or "maternal and embryonic protection," hypothesis suggests that NVP serves a beneficial function by expelling foods that may contain harmful toxins and microorganisms and triggering aversions to such foods throughout pregnancy. The alternative "by-product" hypothesis suggests that NVP is a nonfunctional by-product of conflict-over resource allocation-between the pregnant woman and the embryo. The critical predictions of the prophylaxis hypothesis have been developed and tested, whereas the by-product hypothesis has not been subjected to similar scrutiny. To address this gap, we developed a graphical model and used it to derive predictions from the by-product hypothesis under two different assumptions, namely, that NVP is either (i) a by-product of current conflict between a pregnant woman and an embryo or (ii) a by-product of honest signals of viability produced by the embryo. Neither version of the by-product hypothesis is fully consistent with available data. By contrast, the timing of NVP, its variation among societies, and associated patterns of food cravings and aversions are consistent with the prophylaxis hypothesis.

In the prophylaxis, or more easily pronounced, "adaptive" hypothesis, a pregnant woman becomes hypersensitive to certain inputs (smell and taste of certain foods) that she should be avoiding for the health of her fetus. There are lots of things that typical adult humans can eat without causing harm that can do bad things to a fetus, especially during the first several weeks of development when organs are forming. Many plant products, for instance, are suspect.

After testing the two dominant theories (one adaptive and the other non-adaptive) for why two-thirds of women around the world -- but seemingly no other mammals -- experience nausea and vomiting in pregnancy, only one holds water, says Paul Sherman, Cornell professor of neurobiology and behavior and a Weiss Presidential Fellow.

"Our study, which tested theories and predictions about the nature of parent-offspring conflict in human pregnancy, shows that nausea and vomiting in pregnancy is beneficial by expelling such foods as meat and strong-tasting vegetables that historically and still may contain harmful toxins and microorganisms that could potentially sicken the woman and damage her fetus just when its organs are developing and are most vulnerable to chemicals," said Sherman, who is an expert in Darwinian medicine -- viewing diseases from an evolutionary perspective.

"Morning sickness" is rare after the first 18 weeks ... the organ forming weeks ... of pregnancy. Severe morning sickness is associated with lower rates of spontaneous abortion. The most pathogen-containing foods (historically, as in, pre-refrigerator) are those that tend to induce nausea in these women. Morning sickness is found more often in societies with risky foods as part of their day to day diet. And, this phenomenon ... morning sickness ... is only known in humans. Humans have a broad and, in terms of toxins, relatively dangerous diet.

This last part could be investigated further, I would think. There must be enough variation out there among omnivores to find some other species with morning sickness. Keep looking guys!

(I would add that slow development is another feature of humans that could be important.)

If the alternative theory that morning sickness is a non-adaptive outcome of an evolutionary tug-of-war between the mother and fetus for resources were correct, then the nausea should peak in the final trimester, when the fetus requires more nutrients and blood than at any other time. But it doesn't. Neither does it occur with every pregnancy. If morning sickness were the result of the fetus signaling its viability to the mother, then all humans and other mammals should experience it.

"All this leads us to suggest that morning sickness is a misnomer," Sherman said. "It doesn't occur just in the morning, and it's not an illness. It can occur any time of day and it appears to be beneficial -- we could call it a form of evolutionary wellness insurance."


Flaxman, S., Sherman, P. (2008). Morning Sickness: Adaptive Cause or Nonadaptive Consequence of Embryo Viability?. The American Naturalist, 172(1), 54-62. DOI: 10.1086/588081

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Dogs get morning sickness. It sometimes starts around implantation, about eighteen days post breeding. Weeks four and five, during organogenesis, are usually the worst; many dogs will refuse to eat during that time, or become incredibly picky. My own dogs definitely prefer cooked meat to raw at that point.

I'll wait for more data on this one but am certainly interested.
In my case, I was already violently ill when I got out of bed in the morning. There didn't seem to be anything that made it either better or worse. Interestingly enough, I only experienced "NVP" in my first and third pregnancies, none in the second nor fourth. (Girl, boy, boy, boy... all healthy.)

Some people get morning sickness much worse than others. If we examined the extremes of these two groups you would expect the "non-sick" group to have more damaged babies than the "high-sick" group if the adaptive hypothesis is correct. I don't think anyone has shown that.

Sailor: The hypothesis you suggest has been proposed and tested in various ways:

""Morning sickness" is rare after the first 18 weeks ... the organ forming weeks ... of pregnancy. Severe morning sickness is associated with lower rates of spontaneous abortion. The most pathogen-containing foods (historically, as in, pre-refrigerator) are those that tend to induce nausea in these women. Morning sickness is found more often in societies with risky foods as part of their day to day diet. And, this phenomenon ... morning sickness ... is only known in humans. Humans have a broad and, in terms of toxins, relatively dangerous diet."

But more needs to be done.

Please keep in mind that this research has been going on for way over 10 years, and there are a number of papers. this is starting to look like a well tested and supported idea.

Jess: I have a very hard time believing that Sherman would not know about dogs. Also, dogs seem to puke for totally different reasons than humans do (they LIKE to puke!).... Nevertheless, it would be interesting to know what Sherman thinks of dogs.


"Severe morning sickness is associated with lower rates of spontaneous abortion."
That does it for me, thanks.

I buy the fact that morning sickness causes aversion to foods because they're likely to harbor pathogens. What I'd like to know, however, is why the morning sickness tends to abate when you (well, the female "you") manage to eat protein but continues or gets worse when pretty much anything else is eaten.

GAAK. Over five months of barfing my guts out at least once a day. And that was with Bendectin. About once a week I'd try to go a day without it. Then I'd be barfing up the previous three days' meals, or at least that's what it felt like.

I lived on potatoes, rice, pasta (no sauce, just butter), bread, eggs, cheese, chicken, and milk. All the bland stuff. Veggies, especially corn, were almost guaranteed to "bounce," as was red meat. Cooking smells were hell. The mere mention of fish (which I hate anyway) was enough to make me run like hell for the bathroom. I couldn't eat normally until I was at least 6 months along.

And on top of that, I was supposed to be taking those horse-pill vitamins? Double GAAK! I can barely choke down an Advil!

They took Bendectin off the market shortly after my son was born. AFAIK it was never shown to cause birth defects in all the years it had been on the market, but the company got spooked by a bunch of nuisance lawsuits. I'm just glad it was there when I needed it.

At least they've stopped saying that morning sickness is a subconscious reaction to not wanting the baby............

By themadlolscientist (not verified) on 28 Jun 2008 #permalink

I'm not surprised by symptoms in dogs. Many dogs tend to be alarmingly omnivorous. In my experience, for example, a Labrador retriever will eat almost anything, no matter how disgusting, and rely on the gag reflex to decide if it's bad for them.

Greg Laden: I wouldn't be the least bit surprised if Sherman didn't know about dogs (always assuming that they do show morning sickness). Several years ago, a neuroscientist specializing in music perception told me, "Humans are the only animal that shows rhythmic entrainment." But any horse trainer will disagree, pointing to the ease with which horses synchronize their performances to music. (And recent research has show rhythmic entrainment in birds, too.)

In a case like this, the safe thing to do is to call a bunch of dog breeders and ask. :-)

I once had a knock down drag out fight (well, a heated discussion) regarding music and animals with The Man himself (Matt Cartmill).

My evidence was anecdotal but large. Two elephants who habitually would tap their feet and sway their trunks to music that no humans watching could hear unless they listen very carefully. Polka music coming from the beer garden at the Milwaukee County Zoo.

But I'm still surprised with Sherman. Until I hear otherwise, I'll assume he's considered dogs and ruled them out. I've sent off an email. We'll see.

Another health problem of many expecting mothers (apart from 'mechanical' problems like varicose veins in the legs) is pre ecclampsia (high blood pressure with the risk of developing more serious symptoms if untreated).

Would anyone know if this occurs in other mammals, and speculate about its cause/ purpose?

There absolutely is an evolutionary hypothesis regarding pre-clapsia, proposed by David Haig of OEB at Harvard. David casts preclapsia in terms of parent offspring conflict.

A query in response to this comment:

"If we examined the extremes of these two groups you would expect the 'non-sick' group to have more damaged babies than the 'high-sick' group if the adaptive hypothesis is correct."

If we're assuming that the reason for becoming sick in the first place is because the mother ate something potentially harmful to the fetus, then couldn't we also assume that the non-sick mothers have a better diet and therefore don't need to rid themselves of harmful substances?

If you eat nothing, the fetus only gets your mobilized fat stores along with all the fat-soluble poisons stored there as you waste away.

If a little food aversion is a good thing in early pregnancy, full-blown NVP (vomiting even when no food has been consumed) is a dangerous over-reaction. (And no one can convince me it's not a sickness!)

On another note, differences in degree of NVP from one pregnancy to another may depend on the asymmetry between the right (which drains to the posterior vena cava) and left (which drains to the left renal vein) ovarian veins. You get a different dose of hormones from the corpus luteum depending on which ovary produced the egg.

McGroovy: The ides is that the adaptation provides females with increased sensitivity for nausea under a range of conditions. This keeps her from the bad food. She didn't eat a certain food and got sick. She is just sick a lot and avoids certain foods and is thus saved from her own cuisine.

Isn't it possible that most foods popular among humans are pretty much harmless to a developing fetus, while NVP is simply a broad-spectrum reaction to a narrow-spectrum problem? Maybe being sickened by the smells or tastes of many different foods is just a side effect of the overall warning response being ramped up -- better to reject a few safe foods than to eat something that's really full of toxins or pathogens.

Also, if the response varies not only among mothers but among pregnancies in the same mother, there are probably both genotype X genotype and genotype X environment interactions going on.

By Julie Stahlhut (not verified) on 30 Jun 2008 #permalink


nasty stuff - nearly killed me.

By PennyBright (not verified) on 01 Jul 2008 #permalink

Julie: Yes and no. Yes, the whole idea of Sherman's is that there is a ramped up warning response. That is the proposed mechanism. However, that is not seen as a side effect, but rather, a very sensible adaptation.

I don't think there is any evidence that this occurs differentially across pregnancies. That some individuals have had different experiences with different pregnancies (noted in the comments) may be confirmation bias and is not admissible as factoid. But as you say, Julie, there would presumably be different outcomes under different conditions. A ramping up of sensitivity does note mean nausea. A ramping up of sensitivity followed by encountering certain situations might mean nausea.

As a general note: As is the case with all behavioral research on humans, it is difficult to make the case. Individuals with personal experience will often attribute to their own experience a higher level of confidence or relevance than it may deserve in the context of a broader study. Folklore is powerful, and some societies have the quaint belief that "there is truth behind every story." And so on.

People also have different experiences with nausea in general. I've thrown up about half a dozen times as an adult, but I know people who throw up every time they get a cold.

They seem to assume that all NVP is food-related here, but it's not just smells and tastes that do it. Pregnancy is the only time I can remember actually vomiting because of things I saw on TV (I actually became certain I was pregnant and took a test because one night my ex and I rented *Monty Python's The Meaning of Life* and *American Pie*, both of which had scenes that literally made me vomit), and my gag reflex became crazy--it was hard to brush my teeth without setting it off.

What is the evolutionary explanation for an increased sensitivity manifest as nausea to Monty Python?

PennyBright: Me month ago yesterday, in fact. If my seizure had occurred a mere five minutes later, no one would have been around to witness it and call an ambulance.

As to nausea, if it were purely food related, then why during my first pregnancy would I be eating a meal, get nauseous, throw up, then go back and continue eating on the same meal and not get sick again? If my body was trying to purge unsafe foods why would it let me keep eating what was left on my plate, the same type of food I had just expelled not ten minutes earlier?

And I'm with Web Design: NVP is not just food-related. The main trigger for me during this second pregnancy was the shower. Didn't matter what I tried, I wound up vomiting immediately after showering. Thankfully I could still watch all the Monty Python I wanted :)

Jyotsana: I'd be very interested to know what that meal was. Do you remember?

Greg: I do indeed remember. It was a tofu and vegetable stir fry with noodles. Nothing fancy, just something I whipped up for dinner that night using a bag of frozen mixed veggies and a bottle of store-bought sauce (and the tofu, of course). I ate all of five bites and started to get severely nauseous. After vomiting I felt much better and was able to keep on eating and everything stayed down. I even did something similar during my second pregnancy. Ate some toast with butter and honey for breakfast, hurked it up, felt hungry again so I made some more toast using bread from the same loaf, same butter and honey, and just like with the stir fry, it stayed down on the second try.

I (oddly) feel better knowing that there might be a reasonable evolutionary explanation for my morning sickness. I hate the idea that it was just a "medical mystery".

My first pregnancy I had cravings for protein rich, slightly salty foods, even before I knew I was pregnant. Upon discovery I suffered severe morning sickness and motion sickness and often throughout the day. I had heard from older women that it was signal of what the baby 'preferred to eat' and I should 'listen'. My favorite foods would make me sick. No doubt, when I ate what the 'baby' wanted I could avoid sickness.

My second pregnancy I had no cravings and no sickness. But my sense of smell of super-hero like. I later miscarried.

By Anonymous (not verified) on 26 Dec 2008 #permalink

On TV once I heard about a captive Gorilla that started vomiting a lot in her first couple of months or so after conception.

Currently in my 11th week of pregnancy. This is our second child, this being the worse MS of the two and the AC, or change of wind sends me into forceful vomiting, as well as Social media and pictures of things i once loved to buy. I dont know that I buy into the theory that this is an evolutionary advantage. Not all the trigger makes any sence. I have heard that people who experienced motion sickness are more likely to have worsening morning sickness and coincidentally mottion sickness medication helps. I wonder why it affects the equallibrium so much and i bet there are some answeres in researching the changes in the mothets brain.

Alisha, the idea is that one's sensitivity increases overall. The specific thing that triggers nausia, an item, a condition, motion, a smell, etc., is not what the adaptation is to (nor could it be, that seems impossible to arise through evolution). Just an increase in sensitivity.

I hope you feel better soon!

Very interesting. Intensely curious about morning sickness being adaptive. My best friend and I are both pregnant and she has MS bad. At times vomiting up water. I've had mild nausea this time around and none with my first pregnancy. I do consider my diet healthier than hers. I've also read about correlations between MS and women who felt nauseous when taking birth control or are prone to motion sickness. Apparently MS is also more likely in Caucasians than in any other ethnic background. Its also more likely in expectant mothers who's own mothers experienced MS. Again, in my case, any nausea I've had mirror my mom's experiences.