Oligohydramnios

Did You Know?

• Amniotic fluid helps maintain a constant temperature around the fetus.
• Untreated anhydramnios can lead to pulmonary hypoplasia or skeletal deformities.
• The opposite of oligohydramnios is polyhydramnios, an excess of amniotic fluid.

Etiology

The volume of amniotic fluid depends on the balance between its production and removal. In the first trimester, fluid comes from fetal lung secretions, transport of maternal plasma across fetal membranes, and the placental surface. By the second trimester, fetal urine becomes the main source. Oligohydramnios may be idiopathic or arise from maternal, fetal, or placental causes. Maternal conditions such as preeclampsia, chronic hypertension, collagen vascular disease, nephropathy, and thrombophilia cause uteroplacental insufficiency, reducing blood flow to the placenta and impairing fetal urine production. Medications including angiotensin converting enzyme inhibitors, prostaglandin synthetase inhibitors, and trastuzumab decrease fetal renal blood flow; nimesulide and chemotherapeutic agents have been linked to anhydramnios. Maternal dehydration from severe diarrhea, vomiting, or excessive fluid loss can contribute. TORCH infections and parvovirus B19 are infectious causes. Chromosomal abnormalities like Down syndrome, associated with gastrointestinal anomalies, and congenital abnormalities such as renal agenesis, cystic renal disease, posterior urethral valves, or urethral atresia impair urine production or cause obstruction. Bilateral renal agenesis (Potter syndrome) is the most prevalent cause of anhydramnios. Intrauterine demise, post-term pregnancy, rupture of membranes, and intrauterine growth restriction from placental insufficiency also lead to reduced amniotic fluid.

Diagnosis

Amniotic fluid volume normally increases until 36 weeks and decreases after 40 weeks in post-term gestations. Discrepancies between fundal height and gestational age may indicate an abnormality and warrant ultrasound evaluation. Symptoms of anhydramnios can include a decrease in the size of the baby's abdomen, decreased fetal movement, and uterine contractions not associated with pain. Diagnosis is made via transabdominal ultrasound using either the amniotic fluid index (AFI) or the single deepest pocket (SDP) measurement. An AFI less than 5 cm or an SDP less than 2 cm indicates oligohydramnios; an AFI of 0 cm or absent SDP indicates anhydramnios. The AFI is calculated by summing measurements from the four quadrants of the abdomen. Randomized control trials have shown that AFI can lead to more false positive diagnoses, and the SDP method is recommended instead. The SDP is the deepest pocket of amniotic fluid without fetal body parts or umbilical cord, measured from the 12 o'clock to the 6 o'clock position. In multiple gestation pregnancies, the SDP is the most accurate method for determining adequate fluid levels.

Management

After diagnosis, a thorough history and physical exam are performed, followed by diagnostic testing if indicated. Timely diagnosis and intervention for anhydramnios can improve outcomes. Treatment depends on the underlying cause and may include amnioinfusion to improve fetal lung development and reduce complications, close fetal monitoring, and adjustment of delivery timing. A maternal and family history and physical exam can identify maternal conditions or medications causing oligohydramnios. Premature prelabor rupture of membranes is ruled out with a nitrizine test, evidence of ferning, or pooling of liquid in the cervix. Sonographic evaluation of the fetus identifies anomalies, aneuploidy, growth restriction, or placental abnormalities; the National Institute of Health recommends detailed documentation of the kidneys, bladder, and umbilical cord. If ultrasound visualization is inadequate, MRI imaging can be considered. Genetic testing is useful if fetal anomalies are seen. Elevated maternal serum alpha fetal protein can indicate leaking amniotic fluid and is associated with a poor prognosis. Maternal blood or amniotic fluid testing is performed if TORCH infection is suspected. There is no permanent way to increase amniotic fluid volume, but it can be temporarily increased to allow a complete anatomy scan.