To summarize the ultrasound features and natural course of twin reversed arterial perfusion sequence (TRAPS) during 8-13⁺⁶ weeks of gestation in the first trimester.

A retrospective analysis was performed on 75 cases of TRAPS diagnosed by prenatal ultrasound in early pregnancy and confirmed by post-induction or postpartum pathology at Hubei Maternity and Child Health Hospital from May 2001 to December 2020 to summarize the ultrasound characteristics in early pregnancy and pregancy outcome.

Seventy of the 75 cases of TRAPS were twin pregnancies, and the other 5 cases were triple pregnancies. Among them, 5 cases misdiagnosed as singleton pregnancy were referred to our hospital at 9-10⁺³ weeks of gestation, and 4 cases were misdiagnosed as singleton pregnancy at 8-10 weeks of gestation. The characteristics of TRAPS in early pregnancy were as follows: Follow-up observation found that the size of the fetus that was diagnosed as "stopped" in the early stage of twins increased, and "single pregnancy" was diagnosed as TRAPS. The pump fetuses with matching weeks were 0⁺⁴ - 2⁺¹ weeks older than those with enlarged fetuses. Fetal movement or limb activity could be seen in enlarged fetuses. Fifty-eight cases of acardiac fetus had no heart structure or heart tube beating, and fetal structure was less with a variety of severe malformations. Seventeen cases had cardiac relics and heart tube beats, and the fetal structure was relatively well developed. Seventeen cases of acardiac fetuses had lymphatic cysts, 3 had nuchal translucency (NT) thickening, and 55 had no obvious edema. Color Doppler ultrasound revealed that 39 acardiac fetuses were perfused through a single umbilical artery. The blood flow was naturally blocked in the late stage (12⁺³-16 weeks of gestation) in 14 cases of acardiac fetuses, which showed a thin line-like structure between acardiac fetus and the placenta. Sixty-seven cases of pump fetuses had normal anatomical structures, 5 were complicated with cardiac malformations, 1 had cleft lip, 2 had edema, 5 had polyhydramnios, 2 had oligohydramnios, and 17 had cardiac insufficiency (1 case improved later). Regarding outcomes and prognosis, 15 cases of pump fetuses had a normal delivery, of which 12 survived and 3 died neonatally; 19 cases of intrauterine twin stillbirths at 12⁺⁵-19 weeks of gestation, 16 cases of cardiac insufficiency, 1 case of premature rupture of membranes at 17 weeks of gestation, and 8 cases of pump twin with malformations underwent induced labor; and 16 family members chose to undergo induced labor.

TRAPS in early pregnancy have characteristic ultrasound manifestations. The combination of abdominal and transvaginal ultrasound, color Doppler ultrasound, and dynamic observation are helpful for early diagnosis.