Second survey of secular trends in twinning rates.
D000328:Adult; D001208:Asia; D005060:Europe; D005202:Asia, Eastern; D005260:Female; D005306:Fertilization; D006801:Humans; D007843:Latin America; D008423:Maternal Age; D009656:North America; D011247:Pregnancy; D013019:South Africa; D014427:Twins