Oligohydramnios

Introduction

Oligohydramnios refers to an abnormally low volume of amniotic fluid surrounding the fetus during pregnancy.

Aetiology

Pathophysiology

Amniotic fluid lies within the amniotic membrane (or sac) that surrounds the fetus throughout pregnancy.

This clear-yellow fluid is initially composed of water produced by the mother and is gradually replaced by fetal urine. Throughout pregnancy, the amount of amniotic fluid increases via a small imbalance between the production of the fluid from the fetus’ kidneys/lungs and the removal of fluid by fetal swallowing. The amount of fluid peaks around 38 weeks of pregnancy to approximately 1000 ml and then decreases as birth approaches.

Amniotic fluid plays an important role in protecting and cushioning the fetus within the uterus as well as aiding in the development of many fetal organs including the lungs, kidneys and gastrointestinal tract.

Causes of oligohydramnios

Oligohydramnios develops when there is either increased fluid loss or decreased fluid production. There are several possible causes, with the most common being rupture of membranes.

Other causes of oligohydramnios include:¹

• Fetal growth restriction
• Maternal medical comorbidities (e.g. hypertension)
• Placental abnormalities (e.g. abruption)
• Fetal urinary tract anomalies (e.g. renal agenesis, posterior urethral valves)
• Maternal drug use (e.g. NSAIDs, ACE inhibitors)
• Post-term pregnancy

Clinical features

History

Oligohydramnios is diagnosed via ultrasound during routine pregnancy monitoring.

The history can vary depending on the cause and each woman may experience symptoms differently or may experience no symptoms at all. If the history includes clear or light pink fluid leaking from the vagina, rupture of membranes should be considered.

Clinical examination

During abdominal palpation, fetal parts may be easier to palpate and feel more firm due to the reduced amount of fluid. In addition, the uterus may appear small for dates (e.g. when measuring symphyseal-fundal height).

In the context of suspected rupture of membranes (ROM), a sterile speculum examination should be performed to assess fluid leakage as well as cervical dilatation and effacement.

Diagnosis of ROM can be confirmed with visualisation of the amniotic fluid pooling in the vaginal vault. Other diagnostics tests that may be performed include a pH test of the fluid or microscopy of the dried vaginal fluid, which would identify ferning.

Investigations

Fluid analysis

There are a number of investigations that can be used to differentiate amniotic fluid from other vaginal secretions including:

• Ferning test: cervical secretions are placed onto a slide which is then allowed to dry; amniotic fluid forms a fern-like pattern of crystals which can be viewed under the microscope.
• Amnisure: a vaginal swab to screen for the presence of placental alpha microglobulin-1 (PAMG-1) which is found in high concentration in amniotic fluid.
• Actim-PROM: a swab that screens for insulin-like growth factor binding protein-1 (IGFBP-1) which is found in high concentration in amniotic fluid.

Assessment of fluid volume

Amniotic fluid volume is estimated with ultrasonography using one of two methods:

• In the first method, the maximum vertical pocket (MVP) is identified and measured. A normal MVP is 2 cm to 8 cm.
• The second method calculates the amniotic fluid index (AFI) and involves dividing the uterus into four quadrants and then adding together the MVP from each quadrant. A normal AFI is 5 cm to 25 cm.²

Diagnosis

Oligohydramnios is defined on ultrasound by:²

• AFI <5 cm or
• MVP <2 cm

Management

Monitoring

If oligohydramnios is identified before term (i.e. <37 weeks), serial fetal testing is appropriate due to the increased risk of fetal demise and emergent delivery.¹ Testing may include biophysical profile, non-stress testing or contraction-stress testing.

Intervention

There is some evidence for therapeutic amnioinfusion, where saline or Ringer’s lactate is infused into the amniotic cavity under ultrasound guidance, but clinical effectiveness remains to be determined.³

Timing of delivery

In isolated oligohydramnios, induction of labour should be offered between 36 to 38 weeks gestation.⁴ Earlier delivery may be considered in cases with co-existing risk factors (e.g. non-reassuring fetal testing, maternal comorbidity).

In preterm rupture of membranes (PROM), conservative management is offered before 34 weeks gestation unless there is evidence of infection (i.e. chorioamnionitis) or other co-existing risk factors. In patients with otherwise normal antenatal testing, delivery is recommended at 37 weeks gestation.  

Complications

The earlier in pregnancy that oligohydramnios is diagnosed, the poorer the prognosis. If severe and early in onset (i.e. <24 weeks), mortality rates are as high as 90%.¹

Complications when diagnosed at this gestational age commonly include limb deformities (e.g. muscle contractures, talipes) due to fetal compression, which may or may not resolve with physiotherapy. Pulmonary hypoplasia may also occur and is a significant predictor of fetal mortality.

Other possible complications depend on the cause of oligohydramnios. If PROM occurs, there is an increased risk of chorioamnionitis and subsequent preterm labour. Oligohydramnios caused by fetal growth restriction or uteroplacental insufficiency also carries an increased likelihood of preterm delivery.

Additional complications to be aware of during labour include:

• Umbilical cord compression
• Meconium aspiration
• Caesarean delivery
• Non-reassuring fetal CTG traces

Key points

• Oligohydramnios refers to a reduced volume of amniotic fluid.
• The most common cause of oligohydramnios is rupture of membranes.
• Symptoms vary depending on the cause, with some patients experiencing no symptoms.
• Potential clinical findings include easily palpable fetal parts, a small-for-dates uterus, and visible leakage of fluid.
• Oligohydramnios is diagnosed using ultrasound.
• If diagnosed preterm, management includes increased fetal monitoring.
• Timing of delivery is recommended between 36-38 weeks gestation.
• The earlier oligohydramnios is diagnosed, the worse the prognosis.
• A severe complication of oligohydramnios is pulmonary hypoplasia, which can be fatal.

Reviewer

Maternal-Fetal Medicine Fellow

Editor

Dr Chris Jefferies

References

1. Payne J. Oligohydramnios. Published January 2016. Available from: [LINK]
2. Lord M, Marino S, Kole M. Amniotic Fluid Index. Published January 2021. Available from: [LINK]
3. NICE Clinical Guideline. Therapeutic amnioinfusion for oligohydramnios during pregnancy (excluding labour). Published November 2006. Available from: [LINK]
4. The American College of Obstetricians and Gynecologists [ACOG]. Medically indicated late-preterm and early-term deliveries. July 2021. Available from: [LINK]