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Oligohydramnios

INTRODUCTION

Oligohydramnios refers to amniotic fluid volume that is less than expected for gestational age. It is typically diagnosed by ultrasound examination and may be described qualitatively (eg, mild, moderate, or severe oligohydramnios) or quantitatively (eg, amniotic fluid index [AFI] <5). Methods of amniotic fluid volume assessment are reviewed separately. (See "Assessment of amniotic fluid volume".)

An adequate volume of amniotic fluid is critical to allow normal fetal movement and growth, and to cushion the fetus and umbilical cord. Oligohydramnios, particularly when severe, inhibits these processes and can lead to fetal deformation, umbilical cord compression, and death.

INCIDENCE

Reported rates of oligohydramnios are influenced by variations in diagnostic criteria, the population studied (low or high risk, screening or indicated ultrasound examination), the threshold used (eg, mild, moderate or severe), and the gestational age at the time of the ultrasound examination (preterm, term, or postterm).

A study of 3050 uncomplicated pregnancies with singleton non-anomalous fetuses between 40 and 41.6 weeks of gestation noted oligohydramnios (defined as AFI less than 5) in 11 percent [1]. The incidence is high in laboring women, largely due to rupture of fetal membranes during or just before labor [2-4].

PATHOPHYSIOLOGY

The volume of amniotic fluid is ultimately determined by the volume of fluid flowing into and out of the amniotic sac. Fetal urination, lung fluid, and swallowing all make important contributions to fluid movement in late gestation, with minimal contributions from other sources. Fetal disorders that affect any of these processes will affect the amniotic fluid volume. As an example, growth restricted fetuses may redistribute blood flow away from their kidneys, which decreases fetal urine production, resulting in oligohydramnios [5].
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