37 women (of a total of 50) with an uneventful obstetric history, normal course of a singleton pregnancy and spontaneous delivery at term (> 37 weeks, birthweight > 10th centile) were studied at two to four weeks intervals in a longitudinal design using cw-Doppler velocimetry (Doptek 9013,4MHz, SPTA 30 mWcm2, high pass filter 200 Hz, Chichester, U.K.) to establish normal reference values for Doppler flow velocity waveform indices (DI) A/B ratio, RI, PI obtained from uterine and umbilical arteries from 16 to 40 weeks of gestation. Readings from the placental (p) and non-placental (np) uterine and umbilical arteries formed the database from which percentiles (5th, 50th, 95th) were calculated for weeks 16, 20, 24, 28, 32, 36 and 40 of gestation. DI consistently decreased from week 16 to term. For the uterine arteries this decrease was significant from weeks 16 to 24 with a further but insignificant decrease from 24 weeks to term. Up to 24 weeks DI values obtained from the placental side were significantly lower than those from the non-placental side. DI values from the umbilical arteries decreased continuously from 16 weeks to term with a significantly negative correlation DI values to gestational age.
