Seasonality in twin and triplet births

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Abstract
Twin births in England and Wales between 1952-1959 and 1963-1975 were studied. There is good evidence that the incidence of opposite-sexed (and by inference, dizygotic) twin births is seasonal about 5% higher in Dec. than June. Triplet births seem to show seasonal variation at about the same location and with double the amplitude. Seasonal variation of polyzygotic maternities is probably due to seasonal variation in the rates of multiple ovulation rather than to seasonal variation in any of the other reproductive parameters which could, in principle, be responsible (coital rate, spontaneous abortion, probability of fertilization). Such an hypothesized variation may be due to seasonal variation in food consumption. Evidence in regard to seasonal variation in monozygotic [MZ] twinning rates is equivocal, but a number of considerations suggest that they are subject to seasonal variation. If this is so, peak incidence of MZ twin births occurs around Nov.

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