Superfetation


Superfetation is the simultaneous occurrence of more than one stage of developing offspring in the same animal.
In mammals, it manifests as the formation of an embryo from a different menstrual cycle while another embryo or fetus is already present in the uterus. When two separate instances of fertilisation occur during the same menstrual cycle, it is known as superfecundation.
Superfetation is claimed to be common in some species of animals. In mammals, it can occur only where there are two uteri, or where the estrous cycle continues through pregnancy.

Other animals

Animals that have been claimed to be subject to superfetation include rodents, rabbits, horse, sheep, marsupials, felines, and primates. Superfetation has also been clearly demonstrated and is normal for some species of poeciliid fishes.

Humans

While proposed cases of superfetation have been reported in humans, the existence of this phenomenon in humans is deemed unlikely. Better explanations include differential growth between twins due to various reasons such as twin-to-twin transfusion syndrome. Artificially induced superfetation has been demonstrated, although only up to a short period after insemination.
In 2017, it was reported that an American woman who had agreed to act as a surrogate for a Chinese couple birthed two babies initially believed to be twins. Before the adoptive parents could return home to China, however, it was discovered that one of the babies was, in fact, the biological son of the surrogate. Doctors confirmed that the birth-mother had become pregnant with her and her partner's child roughly three weeks after becoming pregnant with the Chinese couple's child.
There have been multiple cases reported to local US doctors with a week or less difference in age of twins and women who report two surges of ovulation occurring within a few days of each other. Though rare, this condition is believed to affect as many as 0.3% of women but often one twin is lost so the true numbers are not known. Research has found 10% of women ovulate twice per month.