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The orthodox view that, in every singleton pregnancy, there is a more or less equal chance of the baby being a boy or a girl may not be right. An analysis of the sex of children born to 60 000 nurses in the United States between 1956 and 2015, finds that families where all the offspring are of one sex are commoner than one would expect from a simple binomial distribution ( Sci Adv doi:10.1126/sciadv.adu7402). Among families with three boys or three girls, the chances of the next sibling being of the same sex were around 60%.