Superfetation

Superfetation (also spelled superfoetation – see fetus) 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 estrous 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, but is extremely rare in humans. In mammals, it can occur only where there are two uteri, or where the estrous cycle continues through pregnancy. The risk with superfetation in humans is that the second baby is often born prematurely, which can increase its odds of experiencing lung development problems.

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

Reports of superfetation occurring long after the first impregnation have often been treated with suspicion, and some have been clearly discredited. Other explanations have been given (and demonstrated) for different levels of development between twins. Artificially induced superfetation has, however, been demonstrated, although only up to a short period after insemination.

In 1960, John and Mary Tress of Baltimore MD had what the nurse mistakenly identified as twins. Dr Paul C Weinberg of Mt. Sinai hospital delivered the boys, Anthony John and Mark Francis, and realized that Anthony, born five minutes before his brother Mark, looked premature. Immediately, Dr Weinberg did X-rays of the boys' thigh bones and noticed a disparity in bone age. Anthony was a two-month premature baby born five minutes before his full-term brother Mark. Anthony was conceived a full two months after his brother Mark.

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