Fetal umbilical artery flow velocity‐time waveforms in twin pregnancies

Abstract

The umbilical artery flow velocity-time waveforms were studied in 76 twin pregnancies. The ratio of peak systolic (A) to least distolic (B) velocity was calculated for each fetus as an index of umbilical placental flow resistance. Sets (71)of twins were studied within 14 days before delivery. In 65 cases both twins were alive at the time of study. In 32 pregnancies both fetuses were of birthweight appropriate for gestational age (AGA) and had A/B ratios within the normal singleton range. In 33 pregnancies one or both of the liveborn infants were small for gestational age (SGA) and in 78% of these at least 1 fetus had an elevated A/B ratio. Discordancy in birthweight and A/B ratio was associated with growth retardation. Clinically manifest twin-to-twin transfusions occurred in 5 of the 10 pregnancies resulting in an SGA infant (8 with discordant weight) associated with a normal and concordant A/B ratio. Two twin-to-twin transfusions were associated with perinatal death. The placentas were examined in 61 patients. In 43 dichorionic pairs the A/B ratio was elevated in 12 of the 18 where there was at least one SGA infant. There was a greater incidence of growth retardation in the monochorionic pairs (12 of 18). Only 7 of these were identified by an elevated A/B ratio. In 10 of these 18 pairs vascular anastomoses were demonstrated on placental inspection and in a further two there was evidence of twin-to-twin transfusion by Hb discrepancy. Twin pregnancy may result in the birth of a small-for-dates infant because of intrauterine growth retardation or twin-to-twin transfusion. An abnormally elevated A/B ratio identifies growth retardation. In twin-to-twin transfusion the A/B ratio of the 2 fetuses is similar (crossed circulation) yet difference in size may be seen on ultrasound measurement.