Somewhere around week six, many pregnant people get their first real introduction to morning sickness — and if you’re carrying a girl, new research suggests you may be in for more of it than most. For just as long as pregnant people have been miserable, we’ve been told the same thing. All that nausea means you’re having a girl.
Turns out, there’s more to that claim than folklore. A new analysis of over 1.8 million symptom logs from the What to Expect pregnancy app found that mothers carrying girls were statistically more likely to experience nausea or vomiting than those carrying boys. Out of 67 commonly tracked pregnancy symptoms, nausea and vomiting showed the strongest statistical difference between the sexes.
The likely mechanism is hCG (human chorionic gonadotropin, the hormone detected by pregnancy tests), which tends to run higher in pregnancies with female fetuses. Higher hCG, more nausea. The biology was always there; the data to back it up at scale just hadn’t been.
What makes this particular analysis more credible than earlier attempts is the data source itself. Previous studies had a built-in problem in that researchers often knew the baby’s sex going in, which introduced bias. The pregnancy app data sidesteps that entirely. Participants logged symptoms in real time, early in pregnancy, before gender was known. The difference is 3.2 percentage points, statistically meaningful but not exactly a slam dunk for predicting what’s in your future nursery.
Explore all articles: Family health and wellness
“Mothers carrying girls are 3.2% more likely to experience nausea or vomiting than those carrying boys. It was the strongest statistical difference out of 67 tracked pregnancy symptoms.”
The belief itself has been around for a very long time. Hippocrates noted the pattern in his writings on medicine in Aphorisms. Which is either reassuring or slightly alarming, depending on how you feel about ancient Greek medical theory.
For what it’s worth, I have both a son and a daughter, and my pregnancies were not especially useful data points here. I was barely sick with either of them, no dramatic first trimester familiarity with the bathroom floor, no meaningful difference from one pregnancy to the next. So I would have been a poor candidate for the old wives’ tale. My kids, apparently, were not interested in confirming anyone’s theories.
That said, the study doesn’t claim that all girl pregnancies bring severe nausea, or that easy first trimesters mean boys. It’s population-level data, a trend across millions of logs, not a promise about what your specific pregnancy will look like. Morning sickness varies enormously, from barely-there queasiness to hyperemesis gravidarum, which is severe and requires medical care. What you’re feeling in week seven is not a reliable oracle.
Still, there’s something satisfying about one of those handed-down pregnancy beliefs getting actual support in the data, even if the answer comes with an asterisk and a shrug, as most things in pregnancy do.
Read the full What to Expect report at whattoexpect.com
