Old wives’ tales about predicting a baby’s sex — relying on clues like the way the woman carries and the fetal heart rate — are usually more fantasy than fact.

But the notion that morning sickness can sometimes indicate that a girl is on the way may be an exception. A number of large studies in various countries have examined the claim, and almost all have found it to be true, with caveats. Specifically, studies have found that it applies to women with morning sickness in the first trimester, and with symptoms so severe that it leads to hospitalization, a condition known as hyperemesis gravidarum.

Other studies in The Lancet and the journal Epidemiology, among others, have repeated the findings. It is thought that certain hormones produced by female fetuses may be the culprit.

Severe morning sickness may indicate a higher likelihood that the baby will be a girl.