We report the delivery of healthy monozygous (MZ) twins in a 31 year-old nulligravida following gonadotrophin ovulation induction, ICSI, assisted hatching and fresh embryo transfer. Although a sonogram on day 35 con®rmed that two of four transferred embryos had implanted, a second transvaginal sonogram 1 week later showed each gestational sac had two conceptusesÐall four were associated with distinct amniotic compartments. Cardiac activity was observed in all four embryos. At 12 weeks and 5

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... gestation, chorionic villus sampling was performed on fetuses 1 and 2 which were euploid for chromosomes 13, 18, 21, X and Y via¯uorescence in-situ hybridization analysis. Subsequent KCl injection into sacs 3 and 4 resulted in asystole for these fetuses, while cardiac activity in sacs 1 and 2 remained unchanged after reduction. A twin vaginal delivery occurred at 36 weeks gestation, resulting in the birth of two male infants and one placenta. This case represents the ®rst known report of human quadruplet pregnancy consisting of two MZ twin sets conceived by assisted reproductive techniques. Our report reviews proposed mechanisms for explaining twinning, with special emphasis on zona pellucida micromanipulation and subsequent MZ twin induction.