Morning sickness does tend to be slightly worse when carrying a girl, but the difference is small enough that it won’t reliably tell you your baby’s sex. In a study of over 4,300 pregnancies, women carrying a female fetus rated their nausea frequency at 6.35 on a 9-point scale, compared to 6.04 for women carrying a male fetus. That’s a real, statistically significant gap, but not the kind of dramatic difference that turns nausea into a gender reveal.
What the Research Actually Shows
Multiple lines of evidence point in the same direction: girl pregnancies are associated with somewhat more nausea. A large retrospective study of 2,543 mothers found that women who carried a female fetus consistently reported more frequent nausea and vomiting during the first trimester, even after accounting for other factors like age and how many previous pregnancies they’d had. The connection also shows up at the extreme end of the spectrum. Hyperemesis gravidarum, the severe form of pregnancy sickness that can require medical treatment, lists a female fetus as one of its recognized risk factors alongside things like young maternal age, first pregnancies, and carrying multiples.
So the old wives’ tale has a kernel of truth. But it’s important to keep the size of that truth in perspective. The difference between the average nausea scores for boy and girl pregnancies was about 0.3 points on a 9-point scale. Plenty of women carrying boys feel terrible, and plenty carrying girls breeze through the first trimester with minimal symptoms. Your individual experience of nausea is shaped by so many other variables that fetal sex plays only a minor role.
Why Girls May Cause More Nausea
The most promising explanation involves a protein called GDF15, which the body produces in large quantities during pregnancy and is now considered a primary driver of pregnancy nausea. Women carrying a female fetus have significantly higher levels of this protein in both their blood and their spinal fluid compared to women carrying a male fetus. After delivery, those differences disappear completely, suggesting the fetus itself is influencing production. Researchers have speculated that the higher nausea reported in girl pregnancies may be directly linked to these elevated GDF15 levels.
Hormones likely play a role too. In normal pregnancies, women carrying a girl had nearly double the levels of hCG (the hormone pregnancy tests detect) compared to women carrying a boy: roughly 28,800 versus 16,300 units per liter. Since hCG has long been considered one of the triggers for pregnancy nausea, this substantial difference could contribute to the pattern. Interestingly, other hormones like estrogen and progesterone don’t appear to differ by fetal sex during pregnancy, so the hormonal explanation seems to center on hCG specifically.
Other Factors That Matter More
If you’re trying to understand why your morning sickness is particularly mild or severe, your baby’s sex is one of the least useful explanations. Several other factors have a stronger influence on how nausea plays out.
Your general tendency toward nausea outside of pregnancy is a significant predictor. Women who are prone to motion sickness, migraines, or nausea from strong smells tend to have worse pregnancy sickness regardless of fetal sex. Age matters too: younger women often experience more intense symptoms, while women 35 and older are less likely to have nausea that drags into the second trimester.
How many times you’ve been pregnant also shapes the pattern, though not in the direction you might expect. With each additional pregnancy, nausea symptoms are more likely to persist beyond the first trimester. Race and ethnicity influence the experience as well. Non-Hispanic Black women were over four times more likely to have a delayed onset of symptoms compared to white women, and Hispanic women had about twice the risk. At the same time, Black women were less likely to have symptoms lasting beyond the first trimester.
Even the age you started your period may play a role. Women who began menstruating at 12 or 13 were nearly three times as likely to have a delayed onset of nausea symptoms compared to those who started at 14 or later.
Can You Use Nausea to Guess the Sex?
As a party trick, it’s fun. As a prediction tool, it’s not much better than flipping a coin. The statistical difference between girl and boy pregnancies, while real, is far too small to apply to any individual pregnancy. A woman with severe morning sickness is only marginally more likely to be carrying a girl than a boy. The overlap between the two groups is enormous: most women in both categories cluster around similar nausea levels, and the averages are separated by a tiny margin.
The study that confirmed the nausea-sex link also identified general nausea proneness, maternal age, and number of previous pregnancies as independent predictors of how sick you’ll feel. All of these factors are doing work at the same time, making it impossible to isolate the fetal sex signal from everything else happening in your body. An ultrasound at 18 to 20 weeks or a blood test as early as 10 weeks will give you a definitive answer. Your nausea, no matter how miserable, simply can’t.