There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case. Among fraternal twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse, either within one menstrual cycle or, even more rarely, later on in the pregnancy. This can lead to the possibility of a woman carrying fraternal twins with different fathers. This phenomenon is known as heteropaternal super fecundation. One 1992 study estimates that the frequency of heteropaternal super fecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.
Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes. The probability of this is so vanishingly small that multiples having different genders are universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different genders it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.