In-utero characterization of the blood flow in the Hyrtl anastomosis.

An anastomosis between the two umbilical arteries, located approximately within 3 cm from the placental insertion, has been previously described at delivery and in utero. However, the prenatal investigation of the Hyrtl anastomosis is limited to case reports. We report a series of antenatal functional evaluations of this vessel. Forty-one women underwent a target ultrasonography to evaluate the blood flow characteristics of the Hyrtl anastomosis. The resistance index of the anastomosis and the umbilical arteries resistance indices before and after the anastomosis were obtained. The direction of the blood flow in the anastomosis was determined by color Doppler evaluation. An anastomosis between the two stems of the umbilical arteries was present in 36 cases while a fusion of the two umbilical arteries was found in the remaining five cases. The median (range) gestational age at diagnosis was 33.1 weeks (25.5-40.1). The median diameter of the anastomosis was 2.3 mm (1.3-7.1). The blood flow in the anastomosis was pulsatile with a median resistance index of 0.62 (0.45-0.85) and unidirectional toward the umbilical artery with lower resistance index. The difference between the resistance indices of the two umbilical arteries was higher after than before the anastomosis [0.07 (0-0.3) versus 0.04 (0-0.17), P=0.05]. The median diameter of the Hyrtl anastomosis was significantly higher when the anastomosis was oblique (n=8) than when it was transverse (n=28) (4.8 mm [2-7.1] versus 2.3 mm [1.3-5.3], P< 0.05). In three out of the five cases with fusion of the two umbilical arteries the placental insertion was marginal or velamentous. We conclude that the Hyrtl anastomosis may act as a pressure-equalizing system between umbilical arteries. This supports the hypothesis that the Hyrtl anastomosis plays an important role when the placental territories supplied by the umbilical arteries are different in size.