What does polyhydramnios (increased amniotic fluid) in the third trimester mean?

Polyhydramnios in the Third Trimester

Increased amniotic fluid (polyhydramnios) in the third trimester is an independent risk factor for stillbirth and perinatal mortality that requires investigation for underlying causes—most commonly maternal diabetes and fetal anomalies—and warrants enhanced surveillance based on severity. 1

Definition and Diagnostic Criteria

Polyhydramnios is defined by ultrasound measurements as either:

• Maximal vertical pocket (MVP) ≥8 cm, OR
• Amniotic fluid index (AFI) ≥25 cm or >95th percentile for gestational age 1, 2

The MVP measurement may be preferable to AFI as it results in fewer false-positive diagnoses and unnecessary interventions while maintaining equivalent detection of adverse outcomes. 1

Clinical Significance and Mortality Risk

Polyhydramnios carries substantial perinatal risk regardless of whether it occurs in isolation:

• Independent risk factor for stillbirth with odds ratios ranging from 1.8 to 5.8 depending on severity and presence of other anomalies 1
• In one large cohort of over 200,000 singleton births, polyhydramnios was independently associated with stillbirth (OR 1.8; 95% CI 1.4-2.2) 1
• Another study comparing 4,001 normal pregnancies with 210 cases of polyhydramnios found perinatal mortality odds ratio of 5.8 (95% CI 3.68-9.11) 1
• Isolated polyhydramnios (without fetal anomalies) has lower perinatal mortality than cases with additional fetal abnormalities, but risk remains elevated 1

Common Etiologies Requiring Investigation

The two most common pathologic causes are maternal diabetes mellitus and fetal anomalies, though 60% of cases remain idiopathic. 3, 2, 4

Maternal Causes:

• Gestational diabetes mellitus (most common maternal cause) 3, 5, 2
• Pre-existing diabetes 2

Fetal Causes:

• Fetal anatomical anomalies affecting swallowing (gastrointestinal obstruction, esophageal atresia, duodenal atresia) 3, 2
• Neurological abnormalities affecting swallowing mechanisms 2
• Genetic syndromes and chromosomal abnormalities 3, 2
• Congenital infections 3, 2
• Fetal anemia and alloimmunization 2

Twin-Specific Causes:

• Twin-twin transfusion syndrome (TTTS): The recipient twin develops polyhydramnios (MVP >8 cm) while the donor develops oligohydramnios (MVP <2 cm) 1
• This represents Stage I TTTS at minimum and requires specialized evaluation 1

Maternal Symptoms and Complications

Polyhydramnios causes maternal symptoms through mechanical effects:

• Respiratory discomfort and dyspnea from diaphragmatic pressure 3, 6
• Abdominal pain and distension 3, 6
• Premature uterine contractions 3
• Preterm labor with rates up to 22% 3, 4
• Premature rupture of membranes (PPROM) 3
• Abnormal fetal presentation 3
• Cord prolapse risk 3
• Postpartum hemorrhage 3

Recommended Evaluation

When polyhydramnios is identified, systematic evaluation should include:

• Detailed fetal anatomic survey focusing on gastrointestinal tract, central nervous system, and cardiac structures 2
• Maternal glucose tolerance testing (if not already performed) to exclude gestational diabetes 3, 5, 2
• Fetal echocardiography if structural anomalies suspected 2
• Consideration of genetic testing/amniocentesis for karyotype and microarray if anomalies present 2
• Testing for congenital infections (TORCH titers) when indicated 2
• Assessment for fetal anemia (middle cerebral artery Doppler peak systolic velocity) 2

Management and Surveillance

Mild Idiopathic Polyhydramnios:

• Antenatal fetal surveillance is NOT required for mild idiopathic polyhydramnios alone 2
• Allow spontaneous labor at term; if induction planned, should not occur before 39 weeks without other indications 2
• Mode of delivery determined by usual obstetric indications 2

Severe Polyhydramnios:

• Delivery should occur at a tertiary center due to significant possibility of fetal anomalies 2
• Amnioreduction should be considered ONLY for severe maternal discomfort or dyspnea in severe polyhydramnios 2
• Indomethacin should NOT be used solely to decrease amniotic fluid volume 2

Twin Pregnancies with Polyhydramnios:

• If TTTS suspected (polyhydramnios in one twin with oligohydramnios in co-twin), surveillance should occur at least every 2 weeks starting at 16 weeks, with more frequent monitoring if pathology develops 1
• Concerns including isolated polyhydramnios, discordant fluid volumes, or Doppler abnormalities should prompt more frequent monitoring 1

Important Clinical Pitfalls

• Do not assume mild polyhydramnios is benign: Even isolated cases carry increased stillbirth risk, though lower than cases with anomalies 1
• Polyhydramnios may be transient: In gestational diabetes, fluid volumes can normalize with glycemic control, but may recur 5, 6
• Avoid routine indomethacin use: While historically used to reduce amniotic fluid, current guidelines recommend against this practice due to potential fetal risks including premature ductus arteriosus constriction 2
• In twin pregnancies, polyhydramnios requires immediate assessment for TTTS, which has stage-dependent prognosis and may require specialized intervention 1