It’s Q&A time! You can ask your own question here for a chance to have it answered in an upcoming edition.
This is a little Q&A grab bag where I delve into the science to answer several questions I’ve gotten (and asked myself) about pregnancy. The first answer is available to everyone (about whether the sex of the baby predicts nausea levels), and the others are for paid subscribers (about whether methylated folate is better than folic acid, whether dates (the fruit) have really been shown to induce labor, and whether there’s an increased risk of stillbirth after 39 weeks for IVF babies).
Is morning sickness really worse when you’re pregnant with a girl? I saw a post saying “science proves it,” but of course they didn’t give any details about the study.
FYI: my answers here are for educational and informational purposes only, aren’t a substitute for medical or mental-health advice, and don’t constitute a provider-patient relationship.
When I set out to answer this question, I was pretty sure there wouldn’t be any real science to speak of—maybe one study in rats or something like that. So I was surprised to find there is actually quite a bit of (human) scientific literature on this question. It’s all observational and has some limitations, so I don’t think we can definitively say science “proves” that nausea and vomiting of pregnancy (NVP, aka morning sickness) is worse with girls. But it does seem that there’s at least an association.
Population studies going back several decades have found a link between a very severe form of NVP called hyperemesis gravidarum and being pregnant with a girl. For example, a very large 1999 study of Swedish birth and hospital records from 1987–95 found that among mothers hospitalized for hyperemesis gravidarum during the first trimester, the sex ratio of the fetuses was significantly altered (55.7 percent girls in the hyperemesis patients, compared to 48.6 percent girls in the general population). However, there seemed to be no difference in sex ratio for women admitted with hyperemesis gravidarum during the second or third trimester.
Even the common, milder form of NVP seems to be correlated with fetal sex. A 2020 survey of moms’ self-reports from 4,320 pregnancies found that women pregnant with girls reported higher frequencies of NVP than those pregnant with boys. This held true even when controlling for a number of confounding variables, including maternal age, number of previous pregnancies, general susceptibility to nausea, location, and preconceived beliefs about the relationship between fetal sex and NVP. The sex difference was statistically significant, but not necessarily clinically significant: Girl moms rated their symptoms an average of 6.35 on a 1–9 scale, whereas boy moms rated theirs an average of 6.04. So it’s not true that the mere presence of nausea and vomiting—or even of hyperemesis—can predict the sex of the baby. But it does appear that people carrying girls are more likely to report slightly worse nausea and vomiting.
The question is why, and we don’t really know for sure. It may have something to do with the presence of certain hormones in utero with female fetuses, including higher levels of human chorionic gonadotropin (hCG, or the pregnancy hormone), estrogen, and/or a variety of other hormones. But there’s scientific disagreement about whether these explanations are really true; for example, a 2014 systematic review and meta-analysis found inconsistent associations between hyperemesis and HCG, thyroid hormones, leptin, estradiol, progesterone, and white blood cell count. There’s also a theory that NVP is related to normal immune-system changes in pregnancy that result in higher levels of inflammation, but so far there’s no real evidence to bear that out (even the recent, small study testing that theory didn’t find a statistically significant link between inflammatory markers and NVP).
The bottom line: there does seem to be a correlation between worse NVP symptoms and carrying a girl, though it’s unclear why, and nausea and vomiting can certainly affect people pregnant with a fetus of any sex.
Thank you so much for your recent coverage of frequently recommended supplements. You are one of the few I know I can trust to do a legitimate deep dive into these types of things. In this same vein, I am curious what the research says about the value of methylated folate vs. folic acid during pregnancy and at other times. There is a lot of hype about how methylated folate is supposedly better than folic acid, but I am skeptical. From what I understand, even for those with the MTHFR gene mutation, they are still able to absorb an effective (if lower) amount of folic acid from unmethylated versions for things like reducing the risk of neural tube defects. I would LOVE to get your take!
Thanks for your kind words and great question. I think this is one of those cases when wellness culture takes a small grain of truth and blows it up into a huge bowl of misinformation popcorn.
