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Medical Myths

Can you predict a baby’s sex from the size of the bump?

About the author

Claudia is a writer, broadcaster and lecturer in psychology. She presents Health Check on BBC World Service every Wednesday and her new book is titled Time Warped: Unlocking the Mysteries of Time Perception.

If there are myths you’d like Claudia to bust in future columns, she’s on Twitter @claudiahammond.

(copyright: Thinkstock)

(copyright: Thinkstock)

The shape of the bump, a difficult labour, or dangling a wedding ring over the belly. All are supposed to predict the sex of a baby. But only one works.

If a pregnant woman has a neat bump that sticks out in front like a netball, then it is a boy. If the weight is more spread out around her middle then it is a girl. Or so they say.

As any mother will tell you, there is no shortage of family members and friends offering folk stories about how to tell the sex of your baby during pregnancy. Even strangers stop pregnant women in the street to pronounce on the sex of their baby based on such “conclusive” signs as the shape of their bump. 

Sadly, it is not as simple as that.

Two variables determine the nature of a pregnant woman’s bump. The first is the size of the baby. It is true that on average baby boys weigh more at birth than baby girls, and so this could make the bump for a boy slightly bigger. But this small difference in weight does not change the shape of the bump.

The second is the position of the foetus in the womb. If it has its back alongside the mother’s front this makes her belly stick right out. If the baby’s back is parallel with the mother’s back the result is that the abdomen looks flatter. And as the position the developing baby adopts is not dependent on its sex, it is a myth that the shape indicates whether it is a boy or a girl.

Tall tales

So, if you cannot predict the baby’s sex from the shape of the bump, then how about the other folklore tales? Dangling a wedding ring on a string over the bump and looking to see which way it spins will not reveal the answer, because the foetus can have no impact on how something external moves. Nor is there any evidence that the types of food craved by the mother-to-be are related to the sex of the foetus.

But how about the idea that morning sickness is worse if the woman is expecting a girl? The theory is that if you are carrying a girl you get a double dose of female hormones, and this makes you feel sick. Again this is a myth. Most morning sickness occurs during the first 12 weeks when the developing embryo is very small and the levels of sex-related hormones are low. 

The only reliable way of knowing the sex of a baby is medical screening – through ultrasound scanning, amniocentesis or through chorionic villus sampling where a sample of cells is taken from the placenta. The latter two tests are only used to determine the sex if there is a risk of a gender-related problem with the foetus. Ultrasound scans are far more common, but some hospitals have a policy of not telling parents the sex of the baby.  

But there is one indication, admittedly rather late on pregnancy, which could give you a clue. For years, midwives have joked during long labours that “If it is difficult it must be a boy”. But it seems there could be some truth in this. An Irish study published in the British Medical Journal examined 8,000 births at a Dublin hospital between 1997 and 2000. The authors found that on average labour lasted longer when delivering a baby boy, and that there were more complications requiring interventions such as caesarian sections. 

So if you find yourself in the midst of a difficult labour this could hint that you are having a boy. But remember that these are only average figures and there are plenty of difficult labours with female babies too. And that once labour is finally over, you will find out the sex soon enough.

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