Japanese researchers surveyed 10,000 mothers about the age when they started menstruating and the sex of their babies, and found that women who start their periods before age 10 are more likely to have girls. New Scientist reports:
Forty six per cent of the children born to women who began their periods at age 10 were boys. This figure rose to 50 per cent when the woman began her period at 12, and 53 per cent when the women entered menarche at age 14
Previous research has shown women who start menstruating earlier have higher levels of the hormone oestradiol, which may lead to the spontaneous miscarriage of fertilized male eggs.
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