Objective: To investigate the etiology of oligohydramnios in postterm pregnancy using Doppler velocimetry.
Methods: Renal and umbilical artery Doppler velocimetry were performed in women with singleton postterm (287 days' or more gestation) pregnancies. The renal and umbilical artery Doppler resistance index (RI) and end-diastolic velocity were measured. Stepwise logistic regression and the two-tailed t test were used to determine whether the Doppler indices correlated with oligohydramnios (amniotic fluid index less than 5 cm).
Results: We studied 147 well-dated, singleton, postterm pregnancies, of which 21 (14.3%) had oligohydramnios. For the study cohort, the mean (+/-standard deviation) gestational age at Doppler was 41.4 +/- 0.45 weeks and at delivery 41.8 +/- 0.47 weeks. Stepwise logistic regression using renal and umbilical artery Doppler indices found the renal RI to be the only significant predictor of oligohydramnios: beta = -10.4186, P <.05 (odds ratio [95% confidence interval (CI)] = 0, 0.88). The renal artery RI was significantly higher in cases with oligohydramnios (RI: mean (+/-standard error) = 0.8843 +/- 0.11 versus 0.8601 +/- 0.05, P <or= 0.05). A renal artery Doppler end-diastolic velocity below the mean for gestation significantly increased the risk of oligohydramnios: relative risk (95% CI), 1.5 (1.1, 2.0).
Conclusion: Renal artery Doppler was more predictive of oligohydramnios than the umbilical RI. The reduced renal artery end-diastolic velocity suggests that increased arterial impedance is an important factor in the development of oligohydramnios in prolonged pregnancies.