Ovulation is normally suspended during pregnancy to prevent further ova becoming fertilized and to help increase the chances of a full-term pregnancy. However, if an ovum is atypically released after the female was already impregnated when previously ovulating, a chance of a second pregnancy occurs, albeit at a different stage of development. This is known as superfetation.

• Leda lies with both her husband Tyndareus and with the god Zeus, the latter in the guise of a swan. Nine months later, she bears two daughters: Clytemnestra by Tyndareus and Helen by Zeus. This happens again; this time Leda bears two sons: Castor by Tyndareus and Pollux by Zeus.
• Alcmene lies with Zeus, who is disguised as her husband Amphitryon; Alcmene later lies with the real Amphitryon and gives birth to two sons: Iphicles by Amphitryon and Heracles by Zeus.

In 1995, a young woman gave birth to diamniotic monochorionic twins, who were originally assumed to be monozygotic twins until a paternity suit led to a DNA test. This led to the discovery that the twins had different fathers.

In 2001, a case of spontaneous monopaternal superfecundation was reported after a woman undergoing IVF treatments gave birth to quintuplets after only two embryos were implanted. Genetic testing supported that the twinning was not a result of the embryos splitting, and that all five boys shared the same father.

In 2016, an IVF-implanted surrogate mother gave birth to two children: one genetically unrelated child from an implanted embryo, and a biological child from her own egg and her husband’s sperm.

In 2019, a Chinese woman was reported to have two babies from different fathers, one of whom was her husband and the other was a man having a secret affair with her during the same time.