DIZYGOTIC TWINNING AS A MEASURE OF HUMAN-FERTILITY

There is widespread concern about a possible decline in human fertilit y in recent decades. The spontaneous dizygotic twinning rate provides a way of measuring a combination of male plus female fertility as it r eflects the frequency of double ovulation, the probability of fertiliz ation, and the survival of the zygote, There was a decline in dizygoti c twinning rates in developed countries which began around 1960 and co ntinued until the late 1970s. The exact cause of the fall remains unkn own. We suggest that it could have been due to a depression in the twi n ovulation rate in women who stopped taking the oral contraceptive pi ll. The rise in the dizygotic twinning rates which occurred from the 1 980s onwards in developed countries is almost certainly due to increas ing use of ovulation-inducing agents, but this rise may have masked a continuing decline in dizygotic twinning. Monozygotic twinning rates h ave remained remarkably constant or increased only very slightly in re cent 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 m ay be selective referral of twins.