SUPERFETATION
superfetation
(noun) 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”
Source: WordNet® 3.1
Etymology
Noun
superfetation (countable and uncountable, plural superfetations)
The formation of a fetus while another fetus is already present in the uterus.
An excessive accumulation; a superfluous addition.
Source: Wiktionary
Su`per*fe*ta"tion, n. Etym: [Cf. F. superfétation.] (Physiol.)
Definition: 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 there is a
double uterus, or where menstruation persists up to the time of the
second impregnation.
In then became a superfetation upon, and not an ingredient in, the
national character. Coleridge.
Source: Webster’s Unabridged Dictionary 1913 Edition