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, normal, reduced) or quantitatively (eg, amniotic fluid index [AFI] <5). Methods of amniotic fluid volume assessment are reviewed separately.
In this topic, we use the term oligohydramnios to describe pregnancies with AFI <5 cm and borderline/low normal amniotic fluid volume to describe pregnancies with AFI 5 to 8 cm. Alternatively, some clinicians prefer the single vertical pocket (SVP) with severe oligohydramnios defined as SVP less than 1 cm and mild oligohydramnios defined as SVP 1 to 2 cm. (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 may inhibit these processes and can lead to fetal deformation, umbilical cord compression, and death.
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, 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 . The incidence is high in laboring women, largely due to rupture of fetal membranes during or just before labor [2-4].