Conjoined human oocytes observed during assisted reproduction: description of three cases and review of the literature.

Conjoined oocytes derived from binovular follicles have been suspected to play a role in producing dizygotic twins, mosaicism, tetraploidy or chimeras. This issue is discussed by presenting three examples for conjoined oocytes observed in our programme of human assisted reproduction and by including a review of corresponding cases. Our material comprises two associated immature oocytes and two cases in which an immature oocyte was attached to a mature oocyte. One of the mature oocytes was fertilized and transferred after cleavage without resulting in pregnancy. In the literature, another fifteen descriptions of conjoined oocytes were found. Here, simultaneous fertilization of both gametes occurred only once and there was no evidence for pregnancies arising from binovular follicles. These data suggest that binovular follicles do not cause dizygotic twins or genetic abnormalities.