Objective:To determine in pregnant women with preterm labor the relative efficacy of the amniotic fluid index (AFI) and the two‐diameter pocket to detect abnormalities in amniotic fluid volume (AFV), and to relate these findings to pregnancy outcome.Methods:Fifty‐seven healthy women with preterm labor underwent amniocentesis in the third trimester to detect subclinical chorioamnionitis and assess fetal lung maturity. The AFV was estimated by the AFI and two‐diameter‐pocket methods, then confirmed by a dye (aminohippurate sodium)‐dilution technique. Each labor was evaluated for severe variable decelerations requiring amnioinfusion, fetal distress resulting in cesarean delivery, and a 5‐minute Apgar score below 7.Results:Using fluid volume confirmed by dye dilution, the AFI correctly diagnosed AFV as low (less than 500 mL) in only two of 23 (8.7%) patients, compared to 14 of 23 (61%) for the two‐diameter pocket (P< .001). Fetal distress requiring cesarean delivery occurred significantly more often in the hydramnios group (three of six) compared to those with normal AFV (one of 23) (P< .03), and approached significance in the oligohydramnios group (two of 21) (P= .056). There were no significant differences among the three patient groups regarding the need for amnioinfusion for severe variable decelerations or the occurrence of 5‐minute Apgar scores below 7.Conclusions:Compared to the AFI, the two‐diameter pocket is a superior sonographic measurement for the detection of oligohydramnios. In an otherwise low‐risk pregnancy with preterm labor, oligohydramnios is associated with no greater risk for an adverse outcome than is a normal AFV.(Obstet Gynecol 1994;83:959‐62)