A review of amniotic fluid dynamics and the enigma of isolated oligohydramnios.

Traditionally, oligohydramnios has been implemented as a sign of potential fetal compromise and associated with an increased incidence of adverse perinatal morbidity and mortality. Decreased amniotic fluid volume is especially of concern when it occurs in conjunction with structural fetal anomalies, fetal growth restriction, postdates pregnancies, and maternal disease. Consequently, following ultrasonographic diagnosis of oligohydramnios at term, delivery is routinely advocated even in otherwise uncomplicated pregnancies with an appropriate-for-gestational-age fetus, irrespective of the presence of reassuring fetal evaluation and the absence of maternal disease. Numerous factors complicate the ultrasonographic diagnosis of oligohydramnios. These include a lack of complete detailed understanding of the physiology of the dynamics of oligohydramnios, the transient condition at times of decreased amniotic fluid volume, generally poor performance of ultrasonography in detecting oligohydramnios, an array of different ultrasound diagnostic criteria, and varying ultrasonographic thresholds. In light of the latter and the lack of prospective randomized data, is unclear that the practice of effecting delivery for isolated oligohydramnios at term is justified. This article presents physiologic dynamics of amniotic fluid, factors that may affect amniotic fluid volume, possible pitfalls in the ultrasonographic assessment of amniotic fluid volume, and the clinical significance of oligohydramnios. In addition, the literature regarding perinatal outcome associated with oligohydramnios and current available data supporting expectant noninterventional management of cases complicated by isolated oligohydramnios at term are discussed.