Prevalence:

• 1 in 100 pregnancies.

Ultrasound diagnosis:

• The vertical measurement of the deepest pocket of amniotic fluid free of fetal parts is used to classify polyhydramnios into mild (8–11 cm), moderate (12–15 cm) and severe (≥16 cm).
• In about 80% of cases the polyhydramnios is mild, in 15% moderate and in 5% severe.
• Most cases of mild polyhydramnios are idiopathic, but most cases with moderate or severe polyhydramnios are due to maternal or fetal disorders.
• In most cases, polyhydramnios develops late in the second or in the third trimester of pregnancy. Acute polyhydramnios at 16–22 weeks is mainly seen in association with twin-to-twin transfusion syndrome.

Associated abnormalities:

• There are essentially two major causes of polyhydramnios:
  • Reduced fetal swallowing: due to brain abnormalities (e.g anencephaly, Dandy-Walker malformation), facial tumors, gastrointestinal obstruction (e.g. esophageal or duodenal atresia, small bowel obstruction), compressive pulmonary disorders (e.g. pleural effusions, diaphragmatic hernia, CPAM, CHAOS), narrow thoracic cage due to skeletal dysplasias), and fetal akinesia deformation sequence (due to neuromuscular impairment of fetal swallowing).
  • Increased fetal urination: maternal diabetes mellitus and maternal uremia (increased glucose and urea cause osmotic diuresis), hyperdynamic fetal circulation due to fetal anemia (e.g. red blood cell isoimmunization or congenital infection), fetal and placental tumors (e.g. sacrococcygeal teratoma, placental chorioangioma), or twin-to-twin transfusion syndrome. 

Investigations:

• Detailed ultrasound examination.
• Invasive testing for karyotyping and array if there are fetal abnormalities or growth restriction. DNA testing for the myotonic dystrophy mutation if there is abnormal posturing of the extremities.
• Glucose tolerance test if there is associated macrosomia.
• TORCH test if there are fetal features suggestive of infection.

Follow up:

• Ultrasound scans every 1-3 weeks to monitor fetal condition, amniotic fluid volume and cervical length.

Prenatal therapy:

• Maternal diabetes mellitus: good glycemic control.
• Hydrops due to dysrhythmias: antiarrhythmic medication.
• Hydrops due to fetal anemia: intrauterine blood transfusion.
• Pulmonary cysts or pleural effusions: thoracoamniotic shunting.
• Twin-to-twin transfusion syndrome: laser occlusion of placental anastomoses.
• Fetal or placental tumors: laser occlusion of feeding vessels.
• Defects resulting in reduced fetal swallowing or severe idiopathic polyhydramnios: serial amniodrainages if there is cervical shortening. However, the procedure itself may precipitate premature labor. An alternative and effective method of treatment is maternal administration of indomethacin; however, this drug may cause fetal ductal constriction, and close monitoring by serial fetal echocardiographic studies is necessary.

Delivery:

• Standard obstetric care and delivery in most cases.
• Fetal abnormalities: induction of labor at 38 weeks’ gestation in a hospital with neonatal intensive care and facilities for pediatric surgery.
• Fetal tumors: consideration for cesarean section and EXIT procedure.
• Severe polyhydramnios: controlled induction and membrane rupture at 38 weeks’ gestation to avoid risk of umbilical cord prolapse.

Prognosis:

• This depends on the cause of polyhydramnios and the gestational age at delivery.

Recurrence:

• Idiopathic: no increased risk.
• Associated maternal or fetal conditions: depends on the cause.