superfetation

https://doi.org/10.1351/goldbook.11268

Presence of two fetuses in the uterus at different stages of development, resulting from fertilization of ova released in two successive periods of ovulation.

Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 809 [Terms] [Paper]