Oligohydramnios

What is oligohydramnios?

Oligohydramnios is defined as too little amniotic fluid. There is an association between oligohydramnios and both intrauterine growth restriction and adverse perinatal outcomes.¹ Normal amniotic fluid volume changes with gestational age and ways of accurately estimating it have changed over the years. Oligohydramnios occurs where there is less amniotic fluid than would be expected for the gestational age.

Physiology

Amniotic fluid volume (AFV) increases throughout most of pregnancy, with a volume of about 30 ml at 10 weeks of gestation and a peak of about 1 L at 34-36 weeks of gestation. AFV decreases towards term, with a mean AFV of 800 ml at 40 weeks.

Amniotic fluid constantly circulates, with an estimated exchange rate as high as 3600 ml/hour. Excretion of urine by the fetus is the major source of amniotic fluid production in the second half of the pregnancy. Fluid secreted by the fetal respiratory tract also contributes to AFV.

Fetal swallowing is the major pathway of amniotic fluid clearance in the last half of gestation. Fetal skin is highly permeable in the first half of pregnancy but becomes keratinised at 22-25 weeks of gestation, significantly reducing transfer.

The mother's fluid balance (and also therefore the fetus') has a major effect on the AFV. Increased maternal fluid intake has been shown to increase the AFV in women with oligohydramnios, with oral hydration having a slightly better effect than IV hydration.²

Pathology

Oligohydramnios is secondary to either an excess loss of fluid, or a decrease in fetal urine production or excretion. Oligohydramnios is usually associated with one of the following conditions:

• Rupture of amniotic membranes (ROM).

• Congenital absence of functional renal tissue or obstructive uropathy:

  • Conditions that prevent the formation of urine or the entry of urine into the amniotic sac.

  • Fetal urinary tract malformations, including renal agenesis, cystic dysplasia and ureteral atresia.

• Decreased renal perfusion leading to reduced urine production:

  • As a sequela of hypoxaemia-induced redistribution of fetal cardiac output.

  • In growth-restricted fetuses, chronic hypoxia results in shunting of fetal blood away from the kidneys to more vital organs.

  • Anuria and oliguria lead to oligohydramnios.

• Post-term gestation:

  • The cause of decreased AFV in post-term pregnancies is unknown.

  • The decreased efficiency of placental function has been proposed as a cause but this has not been confirmed histologically.

  • Decreased fetal renal blood flow and decreased fetal urine production have been demonstrated beyond 42 weeks in pregnancies involving oligohydramnios.

Oligohydramnios epidemiology²

Oligohydramnios is more common in pregnancies beyond term, as the AFV normally decreases at term. It occurs in approximately 1.1% of pregnancies between 37 and 39 weeks of gestation. By 40 weeks that has risen to 1.7% and it is 4.4% by 42 weeks. The incidence is less than 1% in premature pregnancies.

Oligohydramnios causes (aetiology)

• Fetal causes include:

  • Chromosomal factors.

  • Congenital factors.

  • Intrauterine growth restriction.

  • Post-term pregnancy.

  • Premature ROM (PROM).

  • Fetal demise.

• Placental causes include:

  • Abruption.

  • Twin-to-twin transfusion syndrome (monochorionic twins).³

• Maternal causes include:

  • Maternal dehydration.

  • Uteroplacental insufficiency.

  • Hypertension.

  • Pre-eclampsia.

  • Diabetes (either pre-existing or gestational diabetes).

  • Chronic hypoxia.

• Drug-induced causes include indometacin and angiotensin-converting enzyme (ACE) inhibitors.

• Idiopathic.

Investigations

Maternal risk factors (including hypertension and diabetes) should be assessed.

Ultrasound

The diagnosis is confirmed by ultrasound. It may be discovered incidentally during routine scanning, or noted during antepartum surveillance for other conditions.

• Suspicion of oligohydramnios may be prompted by discrepancies in sequential fundal height measurements, or by fetal parts that are easily palpated through the maternal abdomen.

• During ultrasound, normal-appearing fetal kidneys and fluid-filled bladder may be observed to rule out renal agenesis, cystic dysplasia and ureteral obstruction.

• Fetal growth should be checked to exclude intrauterine growth restriction leading to oliguria. Doppler ultrasound could be used to assess placental insufficiency, if suspected.

Sterile speculum examination

• Sterile speculum examination should be performed in secondary care to check for ROM. Amniotic fluid may pool in the vagina and a ferning pattern may be observed when fluid from the posterior vault is dried and examined under a microscope.

• Cervical mucus may cause false positive results (as can semen and blood).

• Nitrazine paper/sticks turn blue. (The amniotic fluid is more alkaline - pH 6.5-7.0 - than normal vaginal discharge - pH 4.5.)

Oligohydramnios treatment and management

Management of oligohydramnios is based on gestational age. Planned birth in an obstetric unit is recommended.⁴ Transfer to a tertiary referral centre may be appropriate if oligohydramnios is severe. Decisions will need to be made about the timing and mode of delivery. Other treatment options include:

• Amnioinfusion (infusion of fluid directly into the amniotic cavity) - this is only done in the UK under special arrangements, including for research purposes.⁵

• Vesco-amniotic shunts, to divert fetal urine to the amniotic cavity, where an obstructive uropathy in the fetus is the cause of the oligohydramnios. The same principle applies as for amnioinfusion, as to its use in the NHS.⁶

Prognosis²

• The prognosis of oligohydramnios varies depending on the underlying aetiology, the gestational age at diagnosis, and the severity of oligohydramnios. Diagnosis of oligohydramnios during the second trimester is more likely to be associated with fetal or maternal anomalies, whereas diagnosis in the third trimester is more likely to be of unexplained origin.

• The assessment of AFV is important in pregnancies complicated by abnormal fetal growth or intrauterine growth restriction. Oligohydramnios is a frequent finding in pregnancies involving intrauterine growth restriction and is most likely secondary to decreased fetal blood volume, renal blood flow and subsequently fetal urine output. Pregnancies complicated by severe oligohydramnios have been shown to be at increased risk of fetal morbidity.

• Compared with women with normal AFI, those with isolated oligohydramnios have been found to have significantly higher rates of an infant with meconium aspiration syndrome (relative risk (RR), 2.83; 95% CI, 1.38-5.77), Caesarean delivery for fetal distress (RR, 2.16; 95% CI, 1.64-2.85) and admission to the neonatal intensive care unit (NICU) (RR, 1.71; 95% CI, 1.20-2.42).⁷ Patients with oligohydramnios and comorbidities were more likely to have an infant with low birth weight (RR, 2.35; 95% CI, 1.27-4.34).

Complications

• Potter syndrome.⁸

• Pulmonary hypoplasia.

• Fetal compression syndrome.

• Amniotic band syndrome.