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 (https://doi.org/10.1515/pac-2015-1202)