Oligohidramnios

Oligohydramnios
Authors
Ron Beloosesky, MD
Michael G Ross, MD, MPH
Section Editors
Charles J Lockwood, MD
Deborah Levine, MD
Deputy Editor
Vanessa A Barss, MD
Disclosures
All topics are updated as new evidence becomes available and our peer review process is complete.
Literature review current through: Oct 2012. | This topic last updated: jul 9, 2012.
INTRODUCTION — Oligohydramniosrefers 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 fetalmovement 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.
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, and thegestational 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].
Homeostatic mechanisms, such as intramembranous absorption (transfer of amniotic fluid across the amnion into the fetal circulation), also exist and work to maintain amniotic fluid volume. These mechanisms appear to be more successful in limitingexcess fluid volume than in preventing reduced fluid volume. As an example, only half of fetuses with esophageal atresia, and two-thirds of fetuses with duodenal or proximal jejunal atresia develop polyhydramnios [6], whereas renal agenesis invariably results in oligohydramnios.
The physiology of normal amniotic fluid production and volume regulation are discussed separately. (See "Physiology ofamniotic fluid volume regulation".)
ETIOLOGY — Conditions commonly associated with oligohydramnios are listed in the table (table 1). The most likely etiologies of oligohydramnios vary according to severity and the trimester in which they are diagnosed. The majority of women with oligohydramnios or borderline/low normal amniotic fluid volume have no identifiable cause.
First trimester — Theetiology of first trimester oligohydramnios is often unclear. Reduced amniotic fluid prior to 10 weeks of gestation is rare because gestational sac fluid is primarily derived from the fetal surface of the placenta, transamniotic flow from the maternal compartment, and secretions from the surface of the body of the embryo.
Criteria suggested for determining reduced amniotic fluid at this gestational...