While proposed cases of superfetation have been reported in humans, the existence of this phenomenon in humans has been deemed unlikely. Better explanations include differential growth between twins due to various reasons, such as
twin-to-twin transfusion syndrome. In 2017, it was reported that an American woman who had agreed to act as a
surrogate for a Chinese couple bore two babies, who were 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. Research has found that 10% of women release two eggs in a cycle, but both eggs are released at the end of the same "wave" of folliculogenesis, which does not support the theory of superfetation in humans. In 2017, a woman in İzmir, Turkey, became pregnant with two babies conceived about a month apart and she gave birth to both on 7 October 2017. According to the news report this event has officially been registered in global medical records as the 12th superfetation case. ==Other animals==