Morning Sickness and Gender: Are They Linked?

The old wives' tale says: if you have severe morning sickness (nausea and vomiting), you are having a girl; if you have mild or no sickness, you are having a boy.

This is one of the oldest and most widely believed pregnancy myths. It appears in cultures around the world and has been passed down for generations. Unlike most old wives' tales, however, this one has a kernel of scientific truth — which is why it persists even though it is not a reliable predictor.

For the full list of gender prediction myths, see our complete guide to old wives' tales.

Several studies have found a weak statistical association between severe morning sickness and carrying a girl. The most commonly cited is a Swedish study of over 1 million births that found women admitted to hospital for hyperemesis gravidarum (extreme, dehydration-causing sickness) were very slightly more likely to be carrying a girl (Askling et al., Lancet 2006).

The proposed mechanism is biological: female pregnancies may produce slightly higher levels of human chorionic gonadotropin (hCG) on average, and hCG is one of the hormones that drives nausea in pregnancy. So if female pregnancies have marginally higher hCG, they might also have marginally more nausea.

However — and this is the crucial point — the effect size is very small. In the Swedish study, the difference was a matter of a few percentage points. At the individual level, the overlap between boys and girls is enormous. The data shows a statistical signal across a million births, not a reliable prediction for one person.

Even though there is a statistical signal, morning sickness fails as a predictor for several reasons:

• The overlap is huge. The distributions of sickness severity for boys and girls overlap almost entirely. Many women with severe sickness have boys; many with no sickness have girls.
• Sickness has many causes. hCG is one driver, but oestrogen, thyroid hormones, nutritional factors, stress, and individual sensitivity all play a role. The baby's sex is just one small variable among many.
• Sickness varies between pregnancies. The same woman can have severe sickness in one pregnancy and mild sickness in the next — regardless of the sex of the babies.
• Severity exists on a spectrum. "Severe" is subjective. Where do you draw the line? The statistical association is strongest at the extreme end (hospital admission for hyperemesis), not for ordinary morning sickness.

The bottom line: the association is real but weak. It is interesting science, not a useful prediction.

Morning sickness (nausea and vomiting of pregnancy, or NVP) is one of the most common pregnancy symptoms, affecting up to 80% of pregnancies. Its exact cause is not fully understood, but several factors are known to contribute:

• hCG (human chorionic gonadotropin): levels rise rapidly in early pregnancy and peak around 10–12 weeks, which coincides with the worst period of nausea for most women. The higher the hCG (as in twin pregnancies or molar pregnancies), the worse the sickness tends to be.
• Oestrogen: oestrogen levels increase dramatically in pregnancy and can contribute to nausea, particularly in women who are sensitive to hormonal changes.
• Thyroid hormones: hCG can stimulate the thyroid gland, and elevated thyroid hormones can cause nausea.
• Evolutionary theories: some researchers suggest that nausea may have evolved as a protective mechanism to avoid foods that could harm the developing fetus during the vulnerable first trimester.

None of these factors are determined by the baby's sex — which is why sickness severity cannot reliably tell you whether you are having a boy or a girl.

Hyperemesis gravidarum (HG) is the extreme form of pregnancy nausea: persistent vomiting, dehydration, weight loss, and often the need for hospital treatment. It affects about 1–3% of pregnancies and is far more severe than ordinary morning sickness.

The statistical link with female sex is strongest for HG, not for ordinary NVP. In the Swedish study of over 1 million births, women hospitalised for HG were slightly more likely to carry a girl — but again, the difference was small (roughly 55–56% girls vs the expected 48.7% in the general population). This means that even with HG, you still have a roughly 44% chance of having a boy (Askling et al., 2006).

If you are experiencing HG, please seek medical help — dehydration and weight loss can be dangerous for both you and your baby. Your GP or midwife can prescribe anti-sickness medication and, if needed, arrange IV fluids. The sex of your baby is the least important question when you are seriously unwell.

If you are spending your first trimester with your head over the toilet and wondering whether it means you are having a girl, the honest answer is: it is a clue, but a very weak one. Here is how the real methods compare:

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