Retrospective studies on the twinning rate in Scandinavia.
D005260:Female; D005298:Fertility; D005387:Finland; D049671:History, 18th Century; D049672:History, 19th Century; D049673:History, 20th Century; D006801:Humans; D008393:Marriage; D008423:Maternal Age; D011247:Pregnancy; D011783:Quadruplets; D012189:Retrospective Studies; D012424:Rural Population; D013548:Sweden; D014310:Triplets; D014427:Twins; D014505:Urban Population