Approximately 22 % of all twins are monochorionic, giving a prevalence of 1 in 320 births. Data from an unselected population show that 50% of these pregnancies will be complicated. Monochorionic twins have 6x risk of miscarriage (<24 weeks) and 2x risk of preterm birth, growth restriction or intrauterine death compared to dichorionic twins. Review of a cohort referred for a tertiary opinion found that the presumed pathology was incorrect in 50% of cases. A formal protocol describing appropriate surveillance for monochorionic twins may improve the identification of complications and allow earlier referral when intervention is more likely to succeed.

Surveillance relies on early diagnosis, normal defined by assessment of the intertwin membrane at the time of the 12 week scan. At this gestation, large differences in the CRL and NT values of the two fetuses are associated with a higher risk of twin twin transfusion (TTS) and other complications. Lethality between 16 and 24 weeks gestation appears to be primarily associated with TTS, which can be treated effectively with laser ablation. The current staging system does not necessarily reflect disease progression and the inherent unpredictability of this condition makes scanning very two weeks worthwhile.

A second peak for intrauterine death is seen in the late third trimester (>34 weeks) so continued surveillance beyond 24 weeks gestation is recommended, including assessment of growth velocity. Assessment of the umbilical artery waveform and of mid-cerebral artery blood velocity may be useful in identifying a group of fetuses with evidence of unstable anastamoses.