First trimester loss of a member of a twin gestation.
Small gestational sac with a fetus whose development lags compared to the other fetus. No fetal heart activity can be detected. The spectrum varies from a crescent-shaped sac adjacent to a normal early gestation (up to 10-14 weeks) to a well-formed but dead fetus (fetus papyraceus). Early vanishing twins should be distinguished from implantation bleeds that are not surrounded by a trophoblastic ring.
The differential diagnosis of a sonolucent area adjacent to a viable pregnancy would include an implantation bleed in the first trimester; chorioangioma and placental cysts later in pregnancy. Chorioangioma is the most important differential because of its known relationship to nonimmune hydrops as well as preeclampsia. The lack of blood flow on color Doppler would exclude the diagnosis of chorioangioma.
Non-viable pregnancy accompanying healthy co-twin
May range from crescentic gestational sac to normally formed but dead fetus later in pregnancy