Serial assessment of amniotic fluid index in uncomplicated term pregnancies: prognostic value of amniotic fluid reduction.

OBJECTIVE: Assessment of amniotic fluid volume in association with a non-stress test is a commonly used method to monitor fetal well-being in high-risk pregnancies. The aims of our study were to determine whether oligohydramnios and the trend in amniotic fluid volume have prognostic significance in low-risk pregnancies between 40.0 and 41.6 weeks' gestation.

METHODS: Between January 1997 and December 2000, all uncomplicated gestations with a singleton non-anomalous fetus reaching 40.0 weeks' gestation underwent semi-weekly monitoring of amniotic fluid index (AFI) until delivery. Oligohydramnios was defined as an AFI of < or = 5 cm. Changes in AFI were expressed as centimeters per day, and were calculated as: [(last AFI before delivery minus first AFI at 40.0 weeks) / interval in days between the two scans]. Adverse outcome was considered the occurrence of 5-min Apgar score < 7; umbilical artery pH < 7.0; Cesarean section for fetal distress; or fetal death. Comparisons between the groups with favorable and adverse outcomes was performed with chi(2) or Fisher's exact test for categorical variables, and Student's t test for continuous variables. A two-tailed p value < 0.05 was considered significant.

RESULTS: A total of 3050 women met the study criteria, and underwent a median number of two (range 1-7) sonographic assessments of AFI after 40.0 weeks, with oligohydramnios detected in 341 women. In 1466 women at least two serial AFI determinations were obtained, allowing computation of an AFI trend. Gestations resulting in adverse perinatal outcome (n = 167, 5.5%) had a significantly higher rate of oligohydramnios (33/167, 19.8% vs. 308/2883, 10.7%, p = 0.001), but a similar rate of reduction in AFI ( -0.65 +/- 0.64 vs. - 0.66 +/- 0.66 cm/day; p = 0.85) than those with favorable outcome. The difference in rate of reduction of AFI between the two groups was not significant, even in the subset of gestations that developed oligohydramnios ( -1.08 +/- 0.87 vs. -1.26 +/- 0.89 cm/day; p = 0.27).

CONCLUSION: A sonographic diagnosis of oligohydramnios carries an increased risk of adverse perinatal outcome, even in low-risk pregnancies after 40 weeks. The trend in amniotic fluid volume reduction does not seem to have prognostic significance.