Twin Sisters

- A 1981 study of a deceased triploid XXX twin fetus without a heart showed that although its fetal development suggested that it was an identical twin, as it shared a placenta with its healthy twin, tests revealed that it was likely a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning. In 2003 a study argued that many cases of triploidity arise from semi-identical twinning. - In 2007, a study reported a case of a pair of living twins, one a hermaphrodite and one a phenotypical male. The twins were both found to be chimeras and to share all of their maternal DNA but only half of their father's DNA. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.
Degree of separation
The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity and amniocity (the number of sacs) of the pregnancy. Dichorionic twins either never divided (i.e.: were dizygotic) or they divided within the first 4 days. Monoamnionic twins divide after the first week.