Increasing trend in the monozygotic twinning rate.

Recent changes in the estimated incidence of monozygotic twinning in 15 European populations are described. The overall trend was an increase in the monozygotic twinning rate (MZTR) since the 1960s, particularly in those countries in which the use of oral contraceptives (OC) was widespread. A slower increase or even a decrease in the MZTR was observed in countries with low use of OC. Some countries, eg, Sweden, demonstrated an unexpectedly sharp increase since the 1960s. In Poland and the Federal Republic of Germany the MZTR was already strongly increasing as early as in the 1950s, clearly before the introduction of the pill. The influence of several other factors on the MZTR is discussed, such as toxic and teratogenic agents, pelvic infection diseases caused by the use of intrauterine devices, the increased use of ovulation inducers and neuroleptics as well as changes in the registration of perinatal deaths.