title: Parental age, sibship's size, and sibship's sex ratio in twins: a progress report.
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D005260:Female; D006801:Humans; D008297:Male; D008423:Maternal Age; D008875:Middle Aged; D010290:Parents; D011247:Pregnancy; D011272:Pregnancy, Multiple; D012744:Sex Ratio; D012801:Sibling Relations; D014427:Twins; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic