SUPERFETATE
superfetate
(verb) conceive when a fetus is already present in the uterus
Source: WordNet® 3.1
Etymology
Verb
superfetate (third-person singular simple present superfetates, present participle superfetating, simple past and past participle superfetated)
To conceive after a prior conception, but before the birth of the offspring.
Source: Wiktionary
Su`per*fe"tate, v. i. Etym: [L. superfetare; super above, over +
fetare to bring forth.]
Definition: To conceive after a prior conception, but before the birth of
the offspring.
The female . . . is said to superfetate. Grew.
Source: Webster’s Unabridged Dictionary 1913 Edition