Factors related to the frequency of twinning in the State of Nuevo León, Mexico during 1977 and 1978.
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D001722:Birth Order; D005260:Female; D006801:Humans; D008297:Male; D008423:Maternal Age; D008800:Mexico; D008875:Middle Aged; D010331:Paternal Age; D011247:Pregnancy; D012621:Seasons; D014427:Twins; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic