The probability of families having more children of the same sex increases, with a 61% chance for families with three boys and a 58% chance for those with three girls. This overturns previous assumptions that each pregnancy has an equal chance of resulting in a boy or girl. Researchers found that older mothers, particularly those 29 or older, had a higher probability of having only boys or girls. Changes in vaginal pH as women age may play a significant role in determining offspring sex.
In families with three boys, there is a 61% chance that the next sibling is male, while families with three girls have a 58% chance of another female.
The findings challenge what people have been told about their baby's sex, indicating that the chances of having a different-sex child are less than 50:50.
Women aged 29 or older when having their first child have a 13% higher chance of having only male or only female children compared to younger mothers.
Changes in vaginal pH as women age may influence the child's sex by affecting the survival odds of sperm carrying X or Y chromosomes.
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