Seasons affect sex of babies, study reveals

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The Independent Online

Some try crystals under the bed, others a Harley Street doctor but scientists have found the best way of improving the chances of conceiving either a boy or a girl.

Some try crystals under the bed, others a Harley Street doctor but scientists have found the best way of improving the chances of conceiving either a boy or a girl.

Couples who want a boy should try to conceive in autumn and those who want a girl have a better chance if they can conceive in spring, a study has found.

Nature is designed to favour the conception of boys from September to November and girls from March to May because of an evolutionary mechanism aimed at keeping the overall sex ratio as near to 50:50 as possible, the scientists said.

A team led by Angelo Cagnacci, a gynaecologist at the Policlinico of Modena in Italy, investigated the conception dates of 14,310 births at his clinic between 1995 and 2001 to see if the sex ratio varied substantially.

The normal balance of the sexes is tilted in favour of girls because of a higher mortality of males in the womb and at birth. That was why nature had tried to level the playing field by favouring boys in the best months for conception, Professor Cagnacci said. Couples are known to conceive more easily in autumn than in spring – a leftover from the days when ancient humans were seasonal breeders – but what was not known was that this also coincided with a significant shift in the sex ratio.

Research in the journal Human Reproduction shows that 535 males were conceived in the most fertile month of autumn as opposed to 464 females, while only 487 males were conceived, compared with 513 females, in the worst month of spring.

"What is fascinating is the degree of disparity in the sex ratio, that the numbers of boys conceived compared with girls was so much higher in the favourable months when overall conception rates were high and so much lower in the unfavourable months when overall conception rates were low," Dr Cagnacci said.

The scientists could not explain the mechanism, but Dr Cagnacci suggested it occurred early in pregnancy.

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