ORIGINAL ARTICLE Genetics The incidence of monozygotic twinning following PGD is not increased

background: Monozygotic (MZ) twin pregnancies are associated with increased perinatal mortality and morbidity, and risk of conge-nital anomalies. The causes of MZ twinning in humans are unclear but the incidence may increase after PGD, for example, as a result of holes created in the zona pellucida. We compared the incidence of MZ twin pregnancies in ICSI cycles with PGD, versus ICSI cycles without PGD. methods: In this retrospective comparative cohort study, we analysed incidence of twin pregnancies in unselected patients undergoing ICSI and PGD (group A; 1992 cycles) with blastocyst transfer at Day 5, versus a period-matched control population of unselected patients undergoing ICSI and blastocyst transfer at Day 5 without PGD (group B; 2429 cycles) from January 2001 to December 2006. results: Clinical pregnancy per embryo transfer was established in 618/1992 (31.0%) and 947/2429 (39.0%) in group A versus B, respectively (P, 0.01). Overall MZ twin rate was 29/4421 (0.7%) per embryo transfer and 29/1565 (1.9%) per established clinical preg-nancy. The incidence of MZ twinning per established clinical pregnancy did not differ between groups (1.5 versus 2.1%, group A and B, respectively). In group A, seven MZ twins were born versus 19 MZ twins in group B. In group B, one MZ twin pregnancy resulted in two stillbirths. In group A, two MZ twins had severe congenital malformations versus none in group B. conclusions: The incidence of MZ twinning was not increased in PGD compared with regular ICSI with blastocyst transfer. This information is useful in counselling patients about potential risks of PGD