Under normal circumstances, a balance between production and absorption of amniotic fluid exists. After around 20 weeks of gestation, the amniotic fluid is mainly produced by fetal urination and absorption occurs via fetal swallowing. It is estimated that the fetus makes 500 to 1200 mL of urine and swallows around 200 to 800 mL of fetal fluid daily. In polyhydramnios, an imbalance occurs and fluid volume exceeds that of normal amounts. This may be caused by decreased fetal swallowing and/or increased fetal urination. 

For most mild cases, the cause of increased amniotic fluid is unknown. In moderate to severe cases, some conditions can also be associated with polyhydramnios:

• Fetal anomalies (e.g. anomalies in the gastrointestinal tract leading to obstruction)
• Maternal diabetes
• Multiple pregnancies, associated with twin-twin transfusion syndrome
• Aneuploidy (e.g. trisomy 18)
• Rh factor mismatch
• Fetal infections
• High cardiac output states
• Bartter syndrome
• Neuromuscular disorders

In most cases, patients are asymptomatic. However in severe cases wherein the increase volume and pressure becomes extreme, the following signs and symptoms may be felt:

• Shortness of breath
• Abdominal discomfort
• Uterine irritability and contractions
• Acid reflux or heartburn
• Constipation
• Peeing more often
• Swelling in your vulva (external genitals), legs and feet

Suspicion of polyhydramnios is also raised when uterine size measures larger than expected gestational age (fundal height [in cm] that exceeds the weeks of gestation by >3). 

Polyhydramnios is diagnosed via sonographic visualization of increased amniotic fluid volume (Single deepest pocket ≥ 8cm or Amniotic fluid index ≥ 24cm). If diagnosed, additional tests for congenital disorders, diabetes and other conditions may be requested such as fetal ECG, amniocentesis, and glucose tests.

Treatment varies based on severity of polyhydramnios and its etiology. Most cases, mild and idiopathic, only need close monitoring and rarely require treatment. For severe cases, the underlying condition must be treated. Other options include:

• Draining excess amniotic fluid (Reductive amniocentesis) for symptomatic patients with significant respiratory complaints
• Inducing labor (between 37 and 39 weeks of pregnancy)

Polyhydramnios cannot be fully prevented however some interventions or precautions may be done to allow for early detection or to prevent complications. These includes:

• Management of maternal diabetes
• Prenatal tests, including ultrasound evaluation for anatomic abnormalities and such

References:

Beloosesky R, Ross MG. (2022). Polyhydramnios: Etiology, diagnosis, and management. UptoDate. Available from: https://www.uptodate.com/contents/polyhydramnios-etiology-diagnosis-and-management

Cleveland Clinic. [Updated August 2022]. Polyhydramnios. Available from: https://my.clevelandclinic.org/health/diseases/17852-polyhydramnios 

Hwang DS, Mahdy H. Polyhydramnios. [Updated 2022 Sep 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK562140/