What is "superfetation"

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 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.

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

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"

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 embryo or fetus is already present in the uterus. When two separate instances of fertilisation occur during the same menstrual cycle, it is known as superfecundation.

Superfetation is claimed to be common in some species of animals, but is extremely rare in humans. In mammals, it can occur only where there are two uteri, or where the estrous cycle continues through pregnancy. The risk with superfetation in humans is that the second baby is often born prematurely, which can increase its odds of experiencing lung development problems.