Rarity of multple births.
D005260:Female; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D011247:Pregnancy; D011272:Pregnancy, Multiple; D011783:Quadruplets; D011811:Quintuplets; D044469:Racial Groups; D013223:Statistics as Topic; D014310:Triplets; D014427:Twins; D014481:United States