superfetate
To conceive after a prior conception, but before the birth of the offspring.
The hare, for instance, which is hunted alike by beasts, birds, and men, breeds so abundantly as even to superfetate, a thing which is true of no other animal. You find in a hare's belly, at one and the same time, some of the young all covered with fur, others quite naked, others again just fully formed in the womb, while the hare perhaps has lately conceived afresh.
Where it comes from
From Latin superfetare, from super (“above, over”) + fetare (“to bring forth”).
Adapted from the Wiktionary entry superfetate (CC BY-SA 4.0).