Oligohydramnios is defined as an amniotic fluid volume that is less than expected for gestational age. On ultrasound (US), oligohydramnios can be quantitatively defined as amniotic fluid index less than five cm. When there is a decreased volume of amniotic fluid, normal fetal growth and movement are restricted, and complications can arise.
Amniotic fluid is largely comprised of fetal urine. Urinary tract abnormalities, such as fetal renal agenesis (absence of one or both kidneys) or fetal genitourinary obstruction, can result in decreased fetal urine output. Decreased fetal urine output can cause oligohydramnios.
Another cause of oligohydramnios is uteroplacental insufficiency due to conditions such as preeclampsia, placental thrombosis, maternal hypertension or maternal diabetes. In these conditions, the placenta may not provide enough blood and nutrients to the fetus for appropriate recycling of amniotic fluid.
Premature rupture of membranes is defined as the rupture of the fetal membranes (amniotic sac) more than one hour before the onset of labor.
Ultrasound is currently the best way to measure amniotic fluid volume and to diagnose oligohydramnios. An amniotic fluid index (AFI) less than five cm, or the maximum deepest vertical pocket of fluid less than two cm at term found on ultrasound is diagnostic.
During an ultrasound, AFI is determined by dividing the uterus into four quadrants with the umbilicus as the midpoint, and taking the sum of deepest vertical pocket in each quadrant. An AFI less than five cm is diagnostic of oligohydramnios.
Complications of oligohydramnios may include spontaneous abortion. Oligohydramnios in the first trimester usually results in spontaneous abortion. In the third trimester, umbilical cord compression due to decreased amniotic fluid can result in fetal heart rate abnormalities and fetal distress.
Bilateral renal agenesis causing oligohydramnios results in Potter sequence. In Potter sequence, decreased amniotic fluid causes compression of the fetus by the uterine wall, resulting in features such as clubfoot, compressed facial features and pulmonary hypoplasia.
Decreased amniotic fluid may result in intrauterine restraint causing musculoskeletal abnormalities including clubfoot and facial distortion.
When there is not enough amniotic fluid, IUGR may occur. IUGR is a term used to describe a fetus that cannot reach its growth potential due to genetic or environmental factors, such as multiple gestations, placenta previa, maternal systemic disease and maternal drug abuse. IUGR is defined as weight below the tenth percentile for gestational age.
There is no effective long term treatment for oligohydramnios. Oftentimes, treatment is aimed at identifying the underlying cause and if possible, treating it.
For women in the third trimester with a viable pregnancy diagnosed with oligohydramnios, delivery is often recommended. If fetus is not determined to be viable, hydration and bed rest are recommended.
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