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

• Fertilization of a second ovum after a pregnancy has begun

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.

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 ...

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"