['To evaluate the effect of assisted reproductive techniques on the incidence of monozygotic twins (MZT) and the associated pregnancy outcomes.', 'This was a retrospective study of all in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) cycles with MZT pregnancies in our center from January 2001 to December 2011. The diagnosis of MZT pregnancies with their respective placental configurations was based on the results of ultrasonographic examinations performed during either the first or second trimester. The treatment characteristics and outcomes of each IVF cycle were recorded and stored in a computer database.', 'A total of 17 cycles with MZT pregnancies were identified, resulting in an overall incidence of MZT of 1.3%. The incidence of MZT for women aged <35 years and ≥35 years were 1.5% and 0.8%, respectively (p = 0.319). The incidence was not significantly different between ICSI and non-ICSI cycles (1.4% vs. 1.0%; p = 0.620). In addition, the incidence was not increased in the assisted hatching (AH) group compared to those without AH (0.9% vs. 2.1%; p = 0.103). Finally, cycles with embryo transfer at the blastocyst stage had an MZT incidence that was not significantly different from those transferred at the cleavage stage (1.4% vs. 1.3%, respectively; p = 1.000). The incidence of each type of chorionicity, dichorionic-diamniotic, monochorionic-diamniotic, and monochorionic-monoamniotic, was 33.3%, 46.7%, and 20.0%, respectively. A total of 11 of 39 (28%) monozygotic babies and 16 of 19 (84%) coexisting heterozygotic babies were born alive.'] Is monozygotic twinning after in vitro fertilization/intracytoplasmic sperm injection treatment related to advanced maternal age , intracytoplasmic sperm injection , assisted hatching , or blastocyst transfer?