Twinning in Twin Pedigrees

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Abstract
From ten years'' official statistics for England and Wales it is demonstrated that incidence of dizygous twinning increases significantly with maternal age, while the rate for monozygous twinning is constant at about 3 maternities per thousand, and both rates increase with birth rank. Hellin''s law regarding triplet births is discussed. Three suggestions are given for amending the form of birth registration of twins. From a sample survey of 1415 volunteer twin pairs the author concludes that the twinning rate is higher than avg. among relatives of twins; and that almost certainly a tendency to dizygous twinning is inherited through the female line; distr. of like and unlike-sexed pairs is not nandom; birth-order of unlike-sexed twins is influenced by maternal age; and sex-differences of twins increases the time interval between the births of a pair.

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