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Twinning rates were studied in Swedes, Åland Islanders, Finns, Germans, and Dutch during years of starvation when death rates were two to three times higher than average. In contrast to the situation among some animals, this study suggests that nutrition above a certain threshold is unimportant for human reproduction, including twinning. The twinning rates for these different populations display marked temporal differences, but low values in the twinning rate are not consistently associated with periods of epidemics, famine, or similar nutritional stress. After years of privation and/or separation of spouses, a rapid “catch-up effect” can often be seen in the twinning rates, as well as marriage and birth rates. Psychoendocrine factors and interparental immunological conditions that may be involved in this phenomenon are discussed.
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.
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