Multiple births: an upward trend in the United States.
D000293:Adolescent; D000328:Adult; D001723:Birth Rate; D005260:Female; D005299:Fertility Agents; D006801:Humans; D008423:Maternal Age; D011247:Pregnancy; D044469:Racial Groups; D014427:Twins; D014481:United States
D005299:Fertility Agents