Objective: To assess the frequency of monozygotic twinning after IVF-ET. Design: A retrospective review of all in vitro fertilization procedures resulting in an embryo transfer procedure from January 1, 1997 through December 31, 2000. Materials/Methods: One thousand and sixty five IVF-ET records were reviewed. Each record was classified as to day of transfer (day 3 vs. day 5/6). Ultrasound data confirming an intrauterine gestational sac was used to determine intrauterine pregnancy (IUP). Number of documented beating hearts was compared to number of gestational sacs to indicate the presence of a monozygotic twin. Data were analyzed by Fisher’s Exact Test. Results: Six hundred and six transfers were performed on day 3, resulting in 213 (36%) IUPs. Four hundred and fifty nine transfers were performed on day 5/6, resulting in 236 (51%) IUPs. There was only one documented monozygotic twin amongst pregnancies established with day 3 transfer as compared to 16 monozygotic pregnancies resulting from day 5/6 transfer (p < .01). One of the 16 monozygotic pregnancies involved a monozygotic triplet. Day 5/6 transfers which included at least one top quality blastocyst had a higher (p < .01) pregnancy rate (112/168; 66%) as compared to day 5/6 transfers with no top quality blastocysts (102/206; 49.5%). Monozygotic twin rates tended (p < .06) to be lower when the transfer included a top quality blastocyst (4/112; 3.5%) as compared to transfers with no top quality blastocysts (11/202; 10.7%). Day 5/6 transfers with no blastocysts had a lower pregnancy rate (22/85; 26% IUP) with one (1/22;4.5%) monozygotic pregnancy. Tabled 1IVF-ETIUPMonozygotic twinningDay 3696213 (36%)1 (0.5%)Day 5/6459236 (51%)16 (6.7%) Open table in a new tab Conclusions: These data suggest that current practice involving day 5/6 embryo transfer may have a greater risk for monozygotic pregnancies. Each program should rigorously evaluate their own data in order to best counsel patients for risks associated with multiple pregnancy.