The Amniotic Fluid Index and Oligohydramnios: a Deeper Dive into the Shallow End

Second- and third-trimester obstetrical ultrasound examinations include an amniotic fluid volume assessment. Professional organizations' clinical guidance recommends using semiquantitative techniques, such as the single deepest vertical pocket or amniotic fluid index, for this purpose. The single deepest vertical pocket is described as the preferred method of assessing amniotic fluid volume based on fewer oligohydramnios diagnoses and labor inductions with no demonstrable difference in pregnancy outcomes compared with the amniotic fluid index. We offer an alternative interpretation of the evidence for this advice, drawn from 6 randomized clinical trials and 2 meta-analyses comparing the single deepest vertical pocket to the amniotic fluid index. Individually and collectively, these reports are underpowered to detect significant differences in maternal and perinatal outcomes by study group. Moreover, randomized clinical trials comparing maternal and perinatal outcomes resulting from a policy of labor induction at or beyond 37 weeks of gestation vs expectant care consistently favor labor induction, the very intervention paradoxically cited as favoring the single deepest vertical pocket vs the amniotic fluid index. We conclude that the amniotic fluid index should be considered a reasonable method for third-trimester amniotic fluid assessment and diagnosing oligohydramnios.