The decreased rates of triplet births: temporal trends and biologic speculations.

Recent data from the US and from England and Wales demonstrate decreasing rates of higher-order multiple births and represent, for the first time, a striking change in trend when compared with the previous steep 4-fold increase since the early 1980s. However, the incidence of other multiples--twins--continued to escalate. The most probable reasons for this change are new embryo transfer guidelines and availability of multi-fetal pregnancy reduction procedures. Because actual numbers of higher-order multiples are by far lower than the number of twins, and because twins are predictably associated with significant perinatal morbidity and mortality, the implications of the ever-increasing multiple birth rates are no less alarming. As long as the incidence of twins is not reduced, the decreasing incidence of higher-order multiples, per se, does not herald the end of the epidemic of multiple births.