The mothers of twins and their fertility.
D000328:Adult; D001722:Birth Order; D001723:Birth Rate; D005260:Female; D005298:Fertility; D005387:Finland; D006801:Humans; D008423:Maternal Age; D008875:Middle Aged; D011247:Pregnancy; D013548:Sweden; D014310:Triplets; D014427:Twins; D014798:Vital Statistics