Boy or girl?

Does the mother determine the baby’s sex?

Checked against the sources listed on this page. Not reviewed by a clinician. Last updated 5 November 2026. How we check.

Myth

No. The egg always carries an X chromosome. The sperm carries either an X (girl) or a Y (boy), and that decides the chromosomal sex.

This matters because, historically, mothers have been blamed for the sex of their children. Biologically, it is decided at fertilisation by the sperm.

References

We found no study that tests this directly, so this answer rests on general biology and the lack of evidence for the claim.

Related weeks

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More boy or girl? myths

Myth

Does carrying high mean a girl and carrying low mean a boy?

No. The shape and height of a bump depend on your muscles, body shape, how many pregnancies you’ve had and the baby’s position, not the baby’s sex.

 Myth

Does a heart rate over 140 mean it’s a girl?

No. Studies comparing early heart rates in boys and girls found no meaningful difference. Heart rate depends mostly on gestational age.

 Partly true

Does bad morning sickness mean you’re having a girl?

There’s a small statistical link between severe sickness and girls, but it can’t predict the sex of any one baby.