Twin Pregnancies after Single Embryo Transfer

Mechanisms of Formation

• The American College of Radiology notes that dichorionic-diamniotic (DCDA) twin pregnancies can occur after single embryo transfer through two mechanisms: monozygotic embryo splitting, which occurs within 1-3 days of fertilization, resulting in monozygotic DCDA twins, and spontaneous conception, resulting in dizygotic DCDA twins 1, 2

Clinical Implications and Recommendations

Ultrasonographic Monitoring

• The American College of Radiology recommends early transvaginal ultrasound (5-6 weeks) to confirm the number of gestational sacs and determine chorionicity 3
• A high initial beta-HCG level after single embryo transfer may indicate a multiple pregnancy, including monozygotic twins, and a gestational sac should be visible on transvaginal ultrasound when beta-HCG is >3000 mIU/mL 3

Prognostic Significance

• The American College of Radiology states that DCDA twin pregnancies have a better prognosis than monochorionic pregnancies, which account for 20% of twin pregnancies but are responsible for 30% of all complications 1, 2
• Monochorionic-diamniotic twin pregnancies have an overall mortality rate of approximately 10%, primarily due to twin-to-twin transfusion syndrome (TTTS) 1, 2