Fertilization of a second ovum after a pregnancy has begun

• Results in two fetuses of different ages in the uterus at the same time

Wiktionary Word definitions in Wiktionary
n. 1 The formation of a fetus while another fetus is already present in the uterus. 2 An excessive accumulation; a superfluous addition.

WordNet Word definitions in WordNet
n. fertilization of a second ovum after a pregnancy has begun; results in two fetuses of different ages in the uterus at the same time; "superfetation is normal in some animal species"

Wikipedia Word definitions in Wikipedia
Superfetation (also spelled superfoetation – see fetus ) is the simultaneous occurrence of more than one stage of developing offspring in the same animal. In mammals , it manifests as the formation of an embryo from a different estrous cycle while another ...

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Superfetation \Su`per*fe*ta"tion\, n. [Cf. F. superf['e]tation.] (Physiol.) The formation of a fetus at the result of an impregnation occurring after another impregnation but before the birth of the offspring produced by it. This is possible only when ...