Abstract
Background: Twin reversed arterial perfusion (TRAP) sequence is a rare monochorionic twin pregnancy complication. It should be suspected in the first trimester ultrasound, when one fetus has no cardiac activity in a monochorionic pregnancies. Doppler study is essential for early diagnosis of acardiac twin. This manuscrips shows the importance of reassessing cases considered as vanishing twins in early pregnancy, to avoid a late diagnosis of acardiac twin, without timely treatment.
Case Summary: In this case, the assumption of death of one fetus in the first trimester, delayed the correct diagnosis of TRAP until 25 weeks, when the case was referred to our unit. We opted conservative management with weekly ultrasound surveillance, looking for features of heart failure in the pump twin. After suspecting fetal anemia, at 32 weeks a healthy baby weighing 2100g was delivered along with a acardiac anceps with 518g. This is a case of expectant management, for lack of another option, with a successful end.
Conclusion: Despite twin reversed arterial perfusion (TRAP) sequence being a rare monochorionic twin pregnancies complication, it should be suspected in the first trimester ultrasound, when detected a monochorionic pregnancies and one embryo/fetus has no cardiac activity. Follow up in fetal death of one twin in a monochorionic pregnancies must be done with Doppler to look for acardiac twin. Because pump twin may develop a high-output cardiac failure and intrauterine fetal demise, for continuing pregnancies without poor prognostic criteria, it is important to maintain weekly ultrasound surveillance, with attention to the intervention criterias.

Key words: TRAP sequence, acardiac fetus