Twinning rates in Virginia: secular trends and the effects of maternal age and parity.
D000293:Adolescent; D000328:Adult; D001722:Birth Order; D001723:Birth Rate; D044383:Black People; D005260:Female; D006801:Humans; D008423:Maternal Age; D008875:Middle Aged; D010298:Parity; D011247:Pregnancy; D018566:Pregnancy, High-Risk; D011272:Pregnancy, Multiple; D014427:Twins; D014429:Twins, Dizygotic; D014768:Virginia; D044465:White People; D001741:Black or African American