Fertility declines at 30 and is almost gone by 40

Women perform best at childbearing when they are young, just as they do in gymnastics and marathon running. Aristotle recommended that “marriage be set for girls at 18, for men at 37 or somewhat less” because these were the ages thought to produce the finest children. In the days when life expectancy was low, it was clearly prudent to begin a family while stillyoung. But the price of early marriage was usually a train of pregnancies that did not halt until infertility, menopause, or death intervened. With the advent of efficient contraception, we became biologically a new breed. Women could choose whether to have children--and when.

For the first time in recorded history, the number of births for every thousand British women in their early 30s has recently exceeded that in women in their early 20s. The rate in older women is climbing too.1 The trend towards later maternity …