- 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.
Meaning of 'superfetation' (Princeton's WordNet)
1 . superfetation
[ n]
Meaning (1): - fertilization of a second ovum after a pregnancy has begun; results in two fetuses of different ages in the uterus at the same time