Evidence of unidirectional pulsatile flow in the umbilical interarterial anastomosis

Background An anastomosis between umbilical arteries (UAS), located within 1–3 cm from the placental insertion, has been previously described at delivery and in utero. However, the prenatal investigation of this vessels is limited to case reports. We report a series of antenatal functional evaluation of this vessel. Method 41 women underwent a target ultrasonography to evaluate the blood flow characteristics of UAS and UAS anastomosis. The resistance index (RI) of the anastomosis and the UAS RI before and after the anastomosis was obtained. The direction of the blood flow in the anastomosis was determined by color Doppler evaluation. Results An anastomosis between the two stems of the UAS was present in 36 cases while a fusion of the two UAS was found in the remaining 5 cases. The median (range) gestational age at diagnosis was 33.1 weeks (25.5–40.1). The median (range) diameter of the anastomosis was 2.3 mm (1.3–7.1). The blood flow in the anastomosis was pulsatile with a median (range) RI of 0.62 (0.45–0.85). The difference between UAS RI was higher after than before the anastomosis [0.07 (0–0.3) vs. 0.04 (0–0.17), P = 0.051]. The anastomosis blood flow was always unidirectional. Conclusion During fetal life, the UAS anastomosis acts as a pressure‐equalizing system between UAS and the placental lobes.