title: Changes in the twinning rate in Canada 1926-70.
mesh_terms: D000328:Adult; D002170:Canada; D005260:Female; D006801:Humans; D008423:Maternal Age; D010298:Parity; D011247:Pregnancy; D013223:Statistics as Topic; D013997:Time Factors; D014427:Twins; D014505:Urban Population