Oligohydramnios (Low Amniotic Fluid)

What amniotic fluid does

Amniotic fluid cushions the baby, lets the umbilical cord float freely, protects against infection, and is essential for the development of the lungs, muscles and limbs. In the first part of pregnancy it comes mainly from the mother's body; from about 16 weeks, it is mostly the baby's urine. The baby also swallows fluid and "breathes" it into the lungs. Fluid volume peaks around 34 to 36 weeks at roughly 800 mL and falls naturally after that.

How low amniotic fluid is measured

The single deepest pocket method is now often preferred because it leads to fewer unnecessary interventions without worse outcomes. Anhydramnios means almost no fluid at all.

Causes of low amniotic fluid

In the second trimester

Low fluid before about 24 weeks is less common and more serious:

• Fetal kidney or urinary tract problems: absent kidneys (renal agenesis), polycystic or non-functioning kidneys, or a blockage of the bladder outlet, so the baby produces little or no urine.
• Early rupture of the membranes.
• Severe early placental problems and early-onset growth restriction.
• Chromosomal conditions in some cases.

In the third trimester

• Ruptured membranes: waters leaking before labor, sometimes as a slow trickle. See premature rupture of membranes.
• Placental insufficiency: the placenta delivers less blood and oxygen, so the baby diverts blood away from the kidneys and makes less urine. It is linked to fetal growth restriction, preeclampsia and chronic high blood pressure.
• Post-term pregnancy: fluid declines after 40 weeks, especially after 41.
• Medicines: ACE inhibitors and ARBs (blood pressure drugs), and NSAIDs such as ibuprofen or indomethacin, particularly after 20 weeks.
• Twin-to-twin transfusion syndrome in identical twins sharing a placenta.
• Maternal dehydration, which can lower fluid mildly.

In many cases near term, no cause is found; this is called isolated oligohydramnios.

Symptoms

Oligohydramnios usually causes no symptoms and is found on a scan. Possible clues:

• leaking fluid from the vagina
• a bump that measures small for dates
• reduced baby movements in some cases

Risks to the baby

Risks depend on how early it starts and the cause:

• Before about 24 weeks: prolonged lack of fluid can cause underdeveloped lungs (pulmonary hypoplasia), which can be fatal, as well as limb contractures and facial changes (Potter sequence).
• Third trimester: umbilical cord compression, abnormal heart rate patterns in labor, meconium in the fluid, and a higher chance of cesarean delivery.
• With growth restriction: higher risk of stillbirth, which is why monitoring is increased.

Isolated low fluid at term in an otherwise healthy, well-grown baby carries much lower risk.

How it is assessed and monitored

1. Check for ruptured membranes: a speculum exam and a test of any fluid.
2. Detailed ultrasound: the baby's kidneys, bladder and growth, and the placenta.
3. Doppler blood flow studies of the umbilical cord when growth restriction is suspected.
4. Maternal blood pressure and urine checks for preeclampsia.
5. Fetal wellbeing tests: non-stress tests (heart rate monitoring) and biophysical profiles once or twice a week.
6. Repeat fluid measurements, often twice weekly.

Treatment

There is no medicine that reliably restores amniotic fluid. Management depends on the cause and the stage of pregnancy.

• Hydration: drinking extra water (around 2 liters above usual intake) can raise fluid measurements modestly in isolated cases, though it has not been shown to improve outcomes.
• Stop causative medicines under medical advice.
• Close monitoring as above, and kick-count awareness: report any reduction in movements.
• Delivery timing: ACOG suggests delivery at 36 0/7 to 37 6/7 weeks for isolated, persistent oligohydramnios. Earlier delivery may be needed with growth restriction, abnormal Doppler results or ruptured membranes, and steroids are given to mature the baby's lungs if delivery is likely before 34 weeks.
• Amnioinfusion: saline infused into the uterus through a catheter during labor can relieve cord compression when the baby's heart rate shows repeated dips. It is not used to treat low fluid before labor.
• Specialist fetal treatment: for some bladder outlet blockages or twin-to-twin transfusion syndrome, fetal medicine centers can offer procedures in the womb.

Outlook

Most people with oligohydramnios late in pregnancy, without other problems, have healthy babies, usually after induction of labor. The outlook is more guarded when low fluid starts in the second trimester, especially with absent or non-working kidneys or very early rupture of the membranes. A fetal medicine specialist can give individual information.

Frequently asked questions

What causes low amniotic fluid?

In late pregnancy, the most common causes are leaking waters, placental problems (often with a small baby), going past the due date and certain medicines such as NSAIDs or ACE inhibitors. Earlier in pregnancy, problems with the baby's kidneys or urinary tract are a major cause.

Can drinking water increase amniotic fluid?

Drinking extra water can increase fluid measurements slightly in people with isolated low fluid, especially if they were dehydrated. It is safe and often suggested, but it does not treat the underlying cause and does not replace monitoring.

Is low amniotic fluid at 36 weeks dangerous?

It needs careful monitoring but is often manageable. If the baby is growing well and tests are reassuring, many care teams recommend delivery between 36 and 38 weeks. Low fluid with poor growth or abnormal tests may need earlier delivery.

What is a normal AFI?

An AFI between about 5 and 24 cm is generally considered normal. Values of 5 cm or less suggest oligohydramnios, and values of 24 or 25 cm or more suggest too much fluid (polyhydramnios). Many centers now use the single deepest pocket, with 2 to 8 cm considered normal.

Can low amniotic fluid go back to normal?

Sometimes. Fluid may increase with better hydration, after stopping a medicine, or when a measurement was borderline. Low fluid from ruptured membranes or placental problems usually does not recover, so monitoring continues until delivery.