The fetal outcome in pregnancies with isolated reduced amniotic fluid volume in the third trimester

Our aim was to assess the outcome of pregnancies where oligohydramnios, def ined by a published gestational reference range for amniotic fluid index, w as the only abnormal finding at third trimester scan,and all other ultrasou nd parameters including biometry were within normal limits at initial scan. A retrospective case-control study was performed at The Liverpool Maternity Hospital. 103 pregnancies with reduced amniotic fluid index in the third t rimester and apparently normal fetal growth profile ultrasonographically we re identified from ultrasound reports throughout 1993. Pregnancies in the t hird trimester with normal amniotic fluid index on index scan were also ide ntified from these reports and 103 were matched for parity gestational age at delivery, mode of onset of labour, presentation at labour and medical co nditions. Exclusion criteria were ruptured membranes, fetal abnormalities, estimated fetal weight below the fifth centile at index scan and multiple p regnancies. The outcome criteria were birthweight, Apgar scores at delivery , induction and emergency delivery for fetal reasons and admission to Neona tal Intensive Care Unit. Statistical analysis was performed by Fisher's exa ct test and Cart's odds ratio. Compared with controls, pregnancies in the reduced liquor group had a highe r number of babies below the 5th centile (odds ratio 5.2, 95 % confidence i nterval 1.6 to 22), a higher risk of induction for fetal reasons (odds rati o 34.4, 95 % confidence interval 5.35 to 1425.5) and admission to Neonatal Intensive Care Unit (odds ratio 9.77 95 % confidence interval 1.3 to 432). Any observed difference in the need for emergency delivery due to fetal rea sons was not clinically significant (odds ratio 2.16, 95 % confidence inter val 0.77 to 6.6). The definition used for oligohydramnios used in this study appears to ident ify a group of babies with a fourfold risk of low birthweight and a high ri sk of admission to the Neonatal Intensive Care Unit and induction of labour for fetal reasons. This would suggest that pregnancies with isolated oligo hydramnios require some form of fetal monitoring and further prospective st udies are required to determine the most appropriate method.