Until recently, all twin pregnancies were treated in a similar fashion. Ultrasounds were undertaken every 3 to 4 weeks to assess serial fetal growth. Monochorionic (MC) twins comprise only 20% of all twin pregnancies yet unique complications such as twin-twin transfusion syndrome (TTTS), twin reversed arterial perfusion sequence, twin anemia-polycythemia sequence, and selective intrauterine growth restriction can occur. In addition, the in utero death of one twin of a MC pair poses significant risks for death or severe neurologic morbidity in the cotwin. With the exception of discordant growth, these complications are not seen in dichorionic twinning due to the lack of placental anastomoses. In the last two decades, new technologies such as laser photocoagulation for the treatment of severe TTTS and radiofrequency ablation and bipolar cautery for selective reduction have markedly improved outcomes for many of the complications of MC twins. Thus, stratification of "low-risk" twinning (dichorionic twins) versus "high-risk" twinning (MC twins) is paramount to improved outcomes. This can be easily and accurately accomplished with first trimester ultrasound by evaluating the interface of the intertwin membrane with the placenta. This should now be the standard of care for all multiple gestations.
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