Objective: To evaluate monochorionic twins conceived by assisted reproductive technology (ART). Design: We compared perinatal outcomes of monochorionic twins conceived by ART with their dichorionic counterparts and with spontaneous monochorionic twins. Setting: Referral center. Patient(s): Mothers to monochorionic and dichorionic twins conceived by ART and spontaneous monochorionic twins. Intervention(s): None. Main Outcome Measure(s): Maternal characteristics, pregnancy complications, and perinatal outcomes. Result(s): Monochorionic twin pregnancies (n ¼ 25) comprise 7.2% of all ART twins and 4.9% of all monochorionic twins in this data set. Monochorionic pairs have a significantly worse outcome compared with dichorionic sets in terms of lower gestational age and birth weight. ART appears to increase the already high risk of monochorionicity compared with spontaneous conception: odds ratio (OR), 2.9 (1.1-7.3) for preterm birth at <32 weeks and OR, 5.9 (2.5-1.49) for birth weight <1,500 g.

Both delivery routes seem equally valid, neither posing high maternal or neonatal complications' incidence.

Intensive prenatal surveillance might decrease the unexpected fetal death rates after 33 week's gestation and our data do not support elective preterm birth for uncomplicated MCBA twins.

Concordant EFWs at 21-24 weeks exclude discordant birth weight in monochorionic twins whereas discordant EFWs are poor predictors of birth weight discordance especially in dichorionic twins irrespective when the twins were born.