Why do they induce labor in a pregnant woman at 37 weeks of gestation with polyhydramnios (excessive amniotic fluid)?

Why Induction is Performed at 37 Weeks with Polyhydramnios

Induction at 37 weeks for polyhydramnios is performed to prevent serious maternal and fetal complications, including stillbirth, cord prolapse, placental abruption, abnormal fetal presentation, and postpartum hemorrhage—risks that outweigh the benefits of continuing pregnancy once term gestation is reached. 1, 2, 3

Understanding the Risks of Polyhydramnios

Polyhydramnios carries substantial perinatal mortality risk that justifies earlier delivery:

• Stillbirth risk is independently elevated with odds ratios ranging from 1.8 to 5.8, even in isolated cases without fetal anomalies 1
• In a cohort exceeding 200,000 singleton births, polyhydramnios independently increased stillbirth risk (OR 1.8; 95% CI 1.4-2.2) 1
• Preterm delivery occurs in up to 66% of polyhydramnios cases, with preterm labor before 37 weeks in 48.8% of severe cases 1

Specific Complications That Drive Early Delivery

The excessive amniotic fluid volume creates mechanical and physiologic problems:

• Cord prolapse risk increases dramatically due to the fluid cushion allowing the umbilical cord to slip past the presenting part 2, 3
• Abnormal fetal presentations (breech, transverse, face, brow) occur more frequently, with non-vertex presentations seen in 7.8% versus 1% in controls 4
• Premature rupture of membranes (PPROM) is more common, potentially leading to sudden decompression and placental abruption 2, 3
• Postpartum hemorrhage occurs due to uterine overdistension causing atony after delivery 2, 3

Why 37 Weeks Specifically

The timing balances fetal maturity against escalating risks:

• 37 weeks represents early term gestation where neonatal respiratory morbidity is minimal compared to earlier gestational ages 5
• The FDA label for oxytocin specifically indicates induction "at or near term, when delivery is in the best interest of mother and fetus" for conditions including maternal complications 6
• Continuing pregnancy beyond 37 weeks with polyhydramnios increases cumulative exposure to stillbirth risk without meaningful fetal benefit 1

The Delivery Management Approach

When polyhydramnios reaches 37 weeks, the management strategy focuses on controlled delivery:

• Cesarean section rates are already elevated in polyhydramnios (45.4% versus 8% in controls), even after excluding induced labor and abnormal presentations 4
• Controlled amniotomy during induction allows gradual fluid release, reducing the risk of cord prolapse and placental abruption compared to spontaneous rupture 4
• Delivery at a tertiary center is recommended for severe polyhydramnios due to the significant possibility of undetected fetal anomalies and need for immediate neonatal resuscitation 3

Critical Distinction: Mild vs Severe Polyhydramnios

The approach differs based on severity:

• Mild idiopathic polyhydramnios (without identified cause or fetal anomalies) may be managed expectantly with spontaneous labor at term, with induction not recommended before 39 weeks absent other indications 3
• Severe polyhydramnios or cases with identified pathology warrant earlier intervention at 37 weeks due to the compounding risks 1, 3

Common Pitfall to Avoid

Do not delay delivery waiting for spontaneous labor when severe polyhydramnios is present at 37 weeks—the cumulative stillbirth risk increases with each additional week, and the mechanical complications (cord prolapse, abruption) can occur suddenly and catastrophically 1, 2. The Society for Maternal-Fetal Medicine explicitly recommends against routine expectant management in these cases 3.