Dizygotic twinning as a measure of human fertility.

There is widespread concern about a possible decline in human fertility in recent decades. The spontaneous dizygotic twinning rate provides a way of measuring a combination of male plus female fertility as it reflects the frequency of double ovulation, the probability of fertilization, and the survival of the zygote. There was a decline in dizygotic twinning rates in developed countries which began around 1960 and continued until the late 1970s. The exact cause of the fall remains unknown. We suggest that it could have been due to a depression in the twin ovulation rate in women who stopped taking the oral contraceptive pill. The rise in the dizygotic twinning rates which occurred from the 1980s onwards in developed countries is almost certainly due to increasing use of ovulation-inducing agents, but this rise may have masked a continuing decline in dizygotic twinning. Monozygotic twinning rates have remained remarkably constant or increased only very slightly in recent decades. This makes it possible to use the dizygotic:monozygotic twinning ratio to monitor dizygotic twinning in populations where true incidence rates cannot be calculated, e.g. in hospitals where there may be selective referral of twins.