Data for 6910 families each containing one set of twins have been obtained from genealogical records in the Archives of the Mormon Church in Salt Lake City and analysed according to intervals between births. The findings indicate that intervals following singleton females are shorter than those following singleton males, particularly in sibships in which the sex ratio (excluding twins) is high. This relationship holds, with few exceptions, when sibship size is held constant and whatever the sex of the following child. There is no evidence in these data that the interval between births is shorter when the children are of the same sex. Our most striking finding is that twins tend to be preceded by an unusually long interval.