A 28-year-old woman, gravida 2 para 1, at 24 weeks gestation comes to the office for a routine prenatal visit. She has had increasing shortness of breath but no cough or chest pain. The patient has had no vaginal bleeding, leakage of fluid, or contractions. Fetal movement is normal. She has no chronic medical conditions, and her only prior surgery is a cesarean delivery with her first pregnancy. The patient had a fetal anatomy ultrasound at 18 weeks gestation, but not all structures were well visualized. Blood pressure is 116/70 mm Hg and pulse is 68/min. Fundal height is 32 cm. Ultrasound reveals an amniotic fluid index of 40 cm (normal: 8-24 cm). The fetus is in breech presentation and has an estimated fetal size consistent with 24 weeks gestation. An isolated tracheoesophageal fistula is visualized. The placenta is located posteriorly. This patient is at increased risk for which of the following obstetric complications?
Amniotic fluid index | ||
Oligohydramnios (AFI <5 cm) | Polyhydramnios (AFI ≥24 cm) | |
Causes |
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Complications |
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AFI = amniotic fluid index; NSAIDs = nonsteroidal anti-inflammatory drugs. | ||
Amniotic fluid volume is maintained via amniotic fluid production (ie, fetal urination) and removal (ie, fetal swallowing); a normal volume is required for fetal development (eg, lung maturity). This patient has polyhydramnios, an excessive amniotic fluid index (≥24 cm). In this case, polyhydramnios is due to an isolated fetal tracheoesophageal fistula that impairs fetal swallowing and removal of amniotic fluid. Most patients with polyhydramnios are asymptomatic and have a uterine size-larger-than-dates discrepancy (eg, fundal height 32 cm at 24 weeks gestation); others may have dyspnea due to insufficient maternal lung expansion from an enlarged uterus.
Polyhydramnios, especially with increasing severity, can cause obstetric complications due to uterine overdistension and increased intraamniotic pressure. The increased tension of the fetal membranes makes them more susceptible to rupture, placing these patients at higher risk for preterm prelabor rupture of membranes. Uterine overdistension may also cause inflammation, prostaglandin release, uterine irritability, and an increased risk of preterm labor. Additional complications include fetal malpresentation (eg, breech), umbilical cord prolapse, and postpartum uterine atony. Patients with symptomatic polyhydramnios may benefit from amnioreduction (ie, amniotic fluid removal by amniocentesis).
(Choice A) The risk of placenta accreta is increased in patients with a prior cesarean delivery and an anterior placenta due to abnormal placental attachment at the uterine scar. This patient's placenta is posterior.
(Choice B) The risk of postterm pregnancy is increased in nulligravid patients and those with a history of postterm delivery. Polyhydramnios increases the risk of preterm delivery.
(Choice C) Preeclampsia is associated with oligohydramnios (ie, a uterine size-less-than-dates discrepancy) due to uteroplacental insufficiency, fetal growth restriction, and decreased fetal urine output. This patient has polyhydramnios.
(Choice E) The risk of shoulder dystocia is increased in fetuses with macrosomia, another common cause of a uterine size-greater-than-dates discrepancy. This patient's estimated fetal size on ultrasound is equal to the gestational age indicating normal growth, not macrosomia.
Educational objective:
Polyhydramnios, or excessive amniotic fluid index (≥24 cm), can occur due to impaired fetal swallowing (eg, tracheoesophageal fistula). Patients with polyhydramnios are at higher risk of obstetric complications (eg, preterm prelabor rupture of membranes) due to uterine overdistension and increased intraamniotic pressure.