OBJECTIVE To determine whether the forewaters should be considered in the assessment of amniotic fluid (AF) volume in patients beyond 37 weeks' gestation. METHODS Sixty patients were prospectively studied and designated as having oligohydramnios or normal AF volume based upon on the standard four-quadrant AF index (AFI) of < or = 5 cm or > 5 cm, respectively. The distance between the internal os and the fetal head was measured transvaginally. This measurement was first added to the standard AFI and subsequently interchanged with the lowest of the two lower abdominal quadrant measurements. Statistical analysis included Student's t-test with p < 0.05 considered significant. RESULTS Thirty patients were classified as oligohydramnios and 30 normal AF volume. The two subgroups did not differ as to maternal age, parity, gestational age at sonographic examination, incidence of meconium-stained amniotic fluid or 5-min Apgar scores < 7, or birth weight. No significant difference was noted between the mean forewaters measurement of patients with oligohydramnios and controls (0.2 +/- 0.1 and 0.4 +/- 0.1 cm, respectively). In patients with oligohydramnios, there was no significant difference between the standard AFI and the AF volume with the forewaters in each of the methods assessed (2.7 +/- 0.3, 2.9 +/- 0.3, and 2.7 +/- 0.3 cm, respectively). CONCLUSIONS We conclude that various permutations of the AFI, which include sonographic assessment of the forewaters, do not impact on the diagnosis of oligohydramnios.