Monochorionic monoamniotic twin pregnancies with two yolk sacs may not be a rare finding: a report of two cases.

The exact determination of amnionicity is a major issue for the clinical management of monochorionic twin pregnancies, due to the high risk of perinatal mortality and morbidity in monochorionic monoamniotic (MCMA) twins. Counting the number of yolk sacs is believed to be a good indicator of amnionicity in the early first trimester, and it has previously been suggested that the number of yolk sacs is equal to amnionicity in both MCMA and monochorionic diamniotic twin pregnancies. However, the accuracy of the relationship between number of yolk sacs and amnionicity has recently been called into question. To the best of our knowledge, no previous reports have shown two yolk sacs in MCMA twin pregnancies. We report two cases of MCMA twins with two yolk sacs on first-trimester ultrasonography, and confirmed monoamnionicity in the second trimester showing umbilical cord entanglement. Postnatal examination showed an MCMA placenta in both cases, and entangled umbilical cords confirmed monoamnionicity. The possibility of monoamnionicity must still be suspected when two yolk sacs are detected early in the first trimester on ultrasound examination in monochorionic twin pregnancies.