superfetate

American Heritage® Dictionary of the English Language, Fourth Edition

1. v. To conceive when a fetus is already present in the uterus.

Century Dictionary and Cyclopedia

1. To conceive after a prior conception.
2. n. See superfetate, superfetation.

Wiktionary

1. v. To conceive after a prior conception, but before the birth of the offspring.

GNU Webster's 1913

1. v. To conceive after a prior conception, but before the birth of the offspring.

WordNet 3.0

1. v. conceive when a fetus is already present in the uterus