Twin pregnancies are followed more closely than singleton pregnancies due to higher risk for complications such as twin-twin transfusion syndrome, selective fetal growth restriction, and preterm labor. Ultrasound is a non-invasive and highly useful tool for screening, diagnosis, and guiding management of these potential complications. Ultrasound monitoring protocols vary between different types of twin pregnancies.
Caption: Image by Kevin Dufendach, MD (2008). Used by permission. CC BY 3.0
Lambda or Delta Sign Indicating Dichorionic Twins
T Sign Indicating Monochorionic Twins
Uncomplicated monochorionic twin pregnancy
Uncomplicated dichorionic twin pregnancy
Note: ISUOG guidelines do include umbilical artery Doppler monitoring as part of routine surveillance | ACOG/ SMFM does not consider evidence to be sufficient for umbilical artery Doppler or antepartum testing in the case of uncomplicated dichorionic twins
Most twin pregnancies will have good outcomes. However, diligence is required, especially in the case of monochorionic twins due to risk for twin-twin transfusion syndrome (TTTS) and twin anemia polycythemia sequence (TAPS). Monochorionic twins may have potentially significant vascular anastomoses such that the twins share a common vasculature. Significant risks for dichorionic twins include preterm labor, medical complications due to increased placental mass (e.g., preeclampsia and GDM) and selective growth restriction. Different centers will have different protocols for labeling twin A vs twin B. The important point to remember is to be consistent with labeling.
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