Superfetation

Superfetation (also spelled superfoetation – see fetus) is the simultaneous occurrence of more than one stage of developing offspring in the same animal. This phenomenon is extremely rare in humans, with only 10 confirmed cases.

In mammals, it manifests as the formation of an embryo from a subsequent menstrual cycle, while another embryo or fetus is already present in the uterus. When two separate instances of fertilization occur during the same menstrual cycle, it is known as superfecundation.

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