Twin pregnancies with diploid hydatidiform mole and co-existing normal fetus may originate from one oocyte.

: BACKGROUND In twin pregnancies comprising a hydatidiform mole and a normal co-fetus, the ploidy of the mole is almost exclusively reported as diploid and very rarely as triploid. We aimed at understanding this unbalanced distribution of diploid and triploid moles in twin pregnancies by investigating the number of gametes involved. : METHODS Using polymorphic DNA markers, we compared the alleles of seven moles with those of the normal co-fetuses and deduced the number of oocytes and spermatozoa represented in each twin pregnancy. : RESULTS The genomes of all seven moles were androgenetic diploid; six moles were homozygous in all loci analyzed and one mole was heterozygous in several loci. In one homozygous mole, the paternal alleles were identical to those of the normal co-fetus in 13 non-linked informative microsatellite loci, indicating the involvement of one spermatozoon only, and thus of one oocyte only. Duplications of the paternal genome followed by abnormal cell division can explain this observation. In six moles, the paternal alleles were different from those of the normal co-fetus suggesting involvement of two (or more) spermatozoa. Overfertilization of one oocyte followed by abnormal cell division is a possibility. : CONCLUSIONS It is possible that twin pregnancies comprising a diploid mole and a normal co-fetus most often derive from one single oocyte fertilized with one or more spermatozoa. This can explain why diploid moles are far more frequent than triploid moles in twin pregnancies. [ABSTRACT FROM AUTHOR]