ABSTRACT

The vast majority of human ovulations result in a monozygotic pregnancy as a consequence of fertilization of a single ovum at mid-cycle. Quite rarely, polyovulation and fertilization of more than one egg may occur, giving rise to a polyzygotic pregnancy. The true frequency of single, in contradistinction to double, ovulations is unknown, but it is accepted that there are many more multiple conceptions than multiple births. Hence, the usually quoted 0.8% of spontaneous polyzygotic multiple births is an underestimation of the frequency of polyovulation in the human female. In addition, this ratio between monozygotic and polyzygotic births is further changed because the frequency of polyovulation is much higher among women receiving ovulation-enhancing medications, and their number is not quantifiable.