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Every year, slightly more boy babies than girl babies are born worldwide. But back when sperm meets egg, the two sexes are conceived in equal numbers, a newly published study suggests.
That contradicts the idea found in many textbooks and scientific articles that males are in the majority at conception, researchers said. And it implies that more females than males die before birth, resulting in the excess of male births, says Steven Orzack, a study author. "We don't have good information on the cause of this difference," he said.
The work, released Monday by the Proceedings of the National Academy of Sciences, also estimates the ratio of males to females at various points in pregnancy. It finds a see-sawing pattern over that time in which sex is more prone to die in the womb, as various genetic influences take their toll. "We're gaining fundamental new insights in the biology of humans in the first nine months of life," said Orzack, president of the nonprofit Fresh Pond Research Institute.
In general, around 105 boys are born for every 100 girls worldwide. To expand upon that statistic, the study authors drew on a variety of sources for information on the male-female ratio throughout pregnancy, including abortions, genetic sampling of fetuses in the womb, and fetal deaths. To estimate the sex ratio at conception, they examined data on nearly 140,000 embryos that had been routinely screened at fertility clinics in the United States and elsewhere for genetic problems. The embryos were 3 to 6 days old. Analysis concluded that by that point, the sex breakdown was virtually 50-50.
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