Background
The widespread use of assisted reproductive technologies has led to an increase in the prevalence of monozygotic twins. Twinning after blastocyst transfer results in monochorionic placentation; a form of placentation that is associated with higher risks of mortality and morbidity.

Aims
This study describes complication rates of monochorionic diamniotic (MCDA) twin pregnancies and examines whether they differ between spontaneous and assisted conceptions.

Methods
A five‐year retrospective review of 294 MCDA twin pregnancies that had no evidence of structural abnormality on ultrasound at 12 weeks’ gestation. Outcomes of spontaneous and assisted conceptions Day 3 (D3) cleavage stage embryo or Day 5 (D5) blastocyst transfer) pregnancies were compared.

Results
Two hundred and eighteen (74.1%) MCDA twin pregnancies were conceived spontaneously, whilst 14 (4.8%) resulted from D3 cleavage stage embryo and 62 (21.1%) resulted from D5 blastocyst transfer. Fetal and whole pregnancy loss rates were high, affecting 11.4% and 8.8% of cases, respectively. 16.2% of pregnancies were delivered <32 weeks' and 66% <37 weeks' gestation. 36.2% of infants were small for gestational age and selective intrauterine growth restriction (IUGR) affected 7.5% of pregnancies. There was no significant difference in the prevalence of complications between spontaneous and assisted conceptions. Conclusions Assisted conception with either D3 cleavage stage embryo or D5 blastocyst transfer does not increase the risk of complication in a MCDA twin pregnancy. Mortality in monochorionic twins remains high despite early recognition and heightened surveillance throughout pregnancy. Information describing the risks of monochorionic twinning and of subsequent complications may be of value to women undergoing assisted conception.