Polyhydramnios (Excess Amniotic Fluid)

Amniotic fluid is produced by the baby's kidneys and cleared by fetal swallowing and absorption across the placenta. Polyhydramnios occurs when production outstrips clearance. We grade it on ultrasound:

• Deepest Vertical Pocket (DVP): greater than 8 cm defines polyhydramnios (ISUOG-preferred single-pocket method).
• Amniotic Fluid Index (AFI): greater than 24 cm.

We also describe severity, which guides how closely we watch:

• Mild: AFI 24–29.9 cm (or DVP 8–11 cm) — the large majority, usually idiopathic.
• Moderate: AFI 30–34.9 cm (or DVP 12–15 cm).
• Severe: AFI 35 cm or more (or DVP 16 cm or more) — more likely to have an underlying cause and more likely to cause symptoms.

The chance of finding a cause rises with the severity of the excess fluid:

• Idiopathic (~50–60%): No cause identified after a thorough work-up. This is the most common category, especially in mild cases.
• Gestational or pre-existing diabetes: High maternal glucose leads to high fetal glucose, increased fetal urination, and more fluid. This is one of the most common identifiable maternal causes.
• Fetal anomalies affecting swallowing: Any condition that stops the baby swallowing fluid normally—gastrointestinal blockages (oesophageal or duodenal atresia), or neurological/muscular conditions that impair the swallow reflex—leads to fluid accumulation.
• Fetal anaemia and hydrops: A baby who is anaemic or accumulating fluid in the body cavities may also develop polyhydramnios.
• Twin pregnancy complications: In identical (monochorionic) twins, twin-to-twin transfusion syndrome (TTTS) causes polyhydramnios around the recipient twin.
• Fetal macrosomia: A larger baby is associated with more fluid.

When polyhydramnios is confirmed, our work-up aims to identify a cause and check the baby thoroughly:

1. Maternal glucose testing: an oral glucose tolerance test to detect or confirm gestational diabetes.
2. Detailed anatomy ultrasound: looking especially at the face, mouth, stomach ('double bubble' can indicate duodenal atresia), chest, and for signs of a swallowing or neurological problem.
3. Assessment for fetal anaemia: middle cerebral artery peak systolic velocity (MCA-PSV) Doppler, and looking for signs of hydrops.
4. Twin assessment: in monochorionic twins, careful evaluation for TTTS.
5. Genetic counselling and testing (amniocentesis with microarray) offered when a structural anomaly is found, as some causes are associated with chromosomal or genetic conditions.
6. Infection screen where clinically relevant.

Most polyhydramnios—particularly mild and idiopathic—is managed with reassurance and monitoring. Management is tailored to the cause, severity, and symptoms:

• Treat the underlying cause: Optimising blood-sugar control in diabetes often improves the fluid. TTTS is treated with laser therapy at a specialist centre.
• Surveillance: Regular scans monitor fluid, fetal growth, and wellbeing, with closer attention to the risk of preterm labour.
• Amnioreduction: Drawing off excess fluid with a needle is reserved for severe, symptomatic cases, such as significant breathlessness or abdominal discomfort. It relieves symptoms but does not cure the cause and may need repeating.
• Medication: Indomethacin is not recommended solely to reduce amniotic-fluid volume. Its use for a separate obstetric indication is a specialist decision with appropriate fetal surveillance.
• Delivery planning: Because of malpresentation and cord risks, birth is planned in a setting equipped to manage complications, with awareness of the raised chance of postpartum haemorrhage.

The outlook depends on the cause and severity:

• Idiopathic and mild polyhydramnios usually has a good outcome, though these babies deserve a newborn check because a small number have a subtle swallowing problem not seen before birth.
• Diabetes-related cases generally do well when glucose is well controlled.
• Anomaly-related cases depend on the specific condition; identifying it before birth allows the right specialists and neonatal team to be ready.
• Practical risks of a large volume include preterm labour, an unstable lie of the baby (malpresentation), umbilical cord prolapse when the waters break, and a higher chance of postpartum haemorrhage—all of which we plan for.
• With modern surveillance, most pregnancies with polyhydramnios reach a safe delivery.

Contact your maternity team promptly if you experience:

• Sudden increase in the size of your bump, marked tightness, or breathlessness.
• Strong, painful tightenings before term (possible preterm labour).
• Your waters breaking, especially with a gush—because of the risk of cord prolapse, you should be assessed urgently.

Any confirmed excess fluid should be referred to a Maternal-Fetal Medicine unit for a cause work-up. Our MFM and fetal-imaging team carries out detailed scanning, arranges glucose and anaemia assessment, and provides a management and delivery plan aligned with current guidance.