bubble_chart Overview

The volume of amniotic fluid during normal pregnancy increases with gestational age, begins to gradually decrease in the last 2–4 weeks, and reaches approximately 1000ml (800–1200ml) at full-term pregnancy. Polyhydramnios is defined as an amniotic fluid volume exceeding 2000ml at any stage of pregnancy, with the maximum reaching up to 20000ml. Most cases involve a slow increase in amniotic fluid over an extended period, termed chronic polyhydramnios, while a few pregnant women experience a rapid accumulation within days, known as acute polyhydramnios. The incidence of polyhydramnios is reported in the literature as 0.5–1%, but it can rise to 20% in pregnancies complicated by diabetes. The appearance and characteristics of the amniotic fluid in polyhydramnios are no different from those in normal conditions.

bubble_chart Etiology

Through radionuclide tracer measurements, it has been demonstrated that amniotic fluid undergoes continuous exchange between the fetus and the mother, maintaining a dynamic equilibrium. The fetus exchanges fluid through swallowing, breathing, urination, as well as via the pre-keratinized skin, umbilical cord, and other pathways. When the exchange of amniotic fluid becomes imbalanced, polyhydramnios or oligohydramnios occurs. The exact cause of polyhydramnios is not entirely clear, but it is clinically observed in the following conditions.

1. Fetal abnormalities: Among pregnant women with polyhydramnios, approximately 20–50% have fetal abnormalities, with central nervous system and upper digestive tract malformations being the most common. In cases of anencephaly, encephalocele, or spina bifida, the exposed meninges and proliferation of choroid membrane tissue lead to increased fluid exudation, resulting in polyhydramnios. Anencephalic fetuses and those with severe hydrocephalus lack central swallowing function and the swallowing reflex, as well as antidiuretic hormone, leading to increased urine output and polyhydramnios. Esophageal or small intestine atresia and pulmonary hypoplasia prevent the fetus from swallowing and absorbing amniotic fluid, which can also cause polyhydramnios due to abdominal mass accumulation.

2. Multiple pregnancies: The incidence of polyhydramnios in multiple pregnancies is 10 times higher than in singleton pregnancies, particularly in monozygotic twins, and often occurs in the larger fetus. This is due to the interconnected blood circulation in monozygotic twins, where the dominant fetus has increased blood circulation and urine output, leading to polyhydramnios.

3. Maternal and fetal diseases: Conditions such as diabetes, ABO or Rh blood type incompatibility, severe fetal edema, pregnancy-induced hypertension, acute hepatitis, and severe maternal anemia can contribute to polyhydramnios. In diabetic pregnancies, the fetal blood sugar levels rise, causing polyuria and increased fluid excretion into the amniotic fluid. In cases of maternal-fetal blood type incompatibility, the placenta is often enlarged, with reports indicating that when the placenta weighs over 800g, 40% of cases are associated with polyhydramnios. Chorionic edema disrupting fluid exchange is the pathological basis.

4. Placental and umbilical cord abnormalities: Placental chorioangioma and velamentous cord insertion can sometimes lead to polyhydramnios.

5. Idiopathic polyhydramnios: This accounts for about 30% of cases and is not associated with any maternal, fetal, or placental abnormalities, with the cause remaining unknown.

bubble_chart Clinical Manifestations

Symptoms usually appear when the amniotic fluid volume exceeds 3000ml.

(1) Acute polyhydramnios: Mostly occurs between 20–24 weeks of pregnancy. Due to the rapid increase in amniotic fluid, the uterus enlarges significantly within a few days, resembling the size of a full-term or twin pregnancy. Within a short period, the extreme enlargement of the uterus elevates the diaphragm, leading to dyspnea, inability to lie flat, and even cyanosis. The pregnant woman appears distressed, experiences abdominal tension and pain, reduced appetite, and constipation. The enlarged uterus compresses the inferior vena cava, impairing venous return and causing edema and varicose veins in the lower limbs and vulva. The pregnant woman has difficulty walking and can only lie on her side.

(2) Chronic polyhydramnios: Accounts for about 98% of cases and mostly occurs between 28–32 weeks of pregnancy. The amniotic fluid gradually increases over several weeks, showing moderate and slow growth. Most pregnant women can adapt, and it is often detected during prenatal checkups when the uterine height and abdominal circumference exceed those of women at the same gestational stage. On physical examination, the abdomen appears more distended than expected for the gestational age. The pregnancy chart may show the uterine height curve exceeding the normal percentile range. The abdominal skin appears shiny and thin, with high tension on palpation and a sense of fluid tremor. Fetal position is unclear, and sometimes parts of the fetus may feel floating. Fetal heart sounds are distant or inaudible. Pregnant women with polyhydramnios are prone to complications such as pregnancy-induced hypertension, abnormal fetal positions, and premature labor. After membrane rupture, the sudden reduction in uterine size may cause placental abruption, and the umbilical cord may slip out with the amniotic fluid, leading to umbilical cord prolapse. Postpartum, the overly enlarged uterus may result in uterine atony, causing postpartum metrorrhagia.

bubble_chart Auxiliary Examination

(1) B-mode ultrasound examination: The method of measuring amniotic fluid volume by the single largest vertical depth of the amniotic fluid dark area (AFD) shows an increased distance between the fetus and the uterine wall. A measurement exceeding 7 cm may indicate polyhydramnios (some scholars suggest that a measurement exceeding 8 cm is required for diagnosis). If the amniotic fluid index (AFI) method is used, where the pregnant woman lies supine with the head elevated at 30°, the abdomen is divided into four quadrants using the umbilicus and linea alba as reference points, and the maximum amniotic fluid dark areas in each quadrant are summed, domestic data indicate that an AFI >18 cm suggests polyhydramnios. Phelan, however, considers an AFI >20 cm necessary for diagnosis. Comparative studies show that the AFI method is significantly superior to the AFD method. In cases of polyhydramnios, the fetus occupies only a small portion of the uterine cavity, with limbs in a free posture, floating in the amniotic fluid. Concurrent fetal malformations or twins may also be detected.

(2) Amniotic sac contrast imaging and fetal imaging: To assess for fetal gastrointestinal malformations, 20–40 ml of 76% meglumine diatrizoate is first injected into the amniotic cavity, followed by imaging after 3 hours. A reduction in contrast agent in the amniotic fluid and its presence in the fetal intestines indicate normal gastrointestinal function. Subsequently, 20–40 ml of 40% iodized oil (adjusted based on amniotic fluid volume) is injected into the amniotic cavity, and the patient is turned side-to-side several times. Due to the high affinity of lipid-soluble contrast agents for fetal vernix, imaging is performed at 30 minutes, 1 hour, and 24 hours post-injection. The fetal body surface, including the head, trunk, limbs, and external genitalia, can then be visualized. Amniotic sac contrast imaging may induce preterm labor or intrauterine infection, and the contrast agent and radiation pose certain risks to the fetus, so it should be used cautiously.

(3) Detection of neural tube defects in fetuses: These fetal malformations are often associated with polyhydramnios. In addition to B-mode ultrasound, the following detection methods are available:

1) Measurement of alpha-fetoprotein (α-FP) levels in amniotic fluid and maternal blood: In cases of open neural tube defects, α-FP leaks into the amniotic cavity along with cerebrospinal fluid. When pregnancy is complicated by a neural tube defect, the amniotic fluid α-FP value exceeds the mean of normal pregnancies at the same gestational age by more than 3 standard deviations, while maternal serum α-FP exceeds the mean by more than 2 standard deviations.

2) Measurement of the maternal urine estrogen/creatinine (E/C) ratio: In pregnancies complicated by neural tube defects, the E/C ratio is more than 1 standard deviation below the mean for normal pregnancies at the same gestational age.

3) Rapid adherent cell testing in amniotic fluid, amniotic fluid acetylcholinesterase gel disc electrophoresis, amniotic fluid Jack Bean lectin A, and a triple-sandwich solid-phase immunoradiometric assay using anti-α-FP monoclonal antibodies can all detect neural tube defects. Combining multiple methods can compensate for the limitations of B-mode ultrasound and α-FP testing.

bubble_chart Diagnosis

The diagnosis can be made based on clinical manifestations and laboratory tests.

bubble_chart Treatment Measures

The management of polyhydramnios primarily depends on the presence of fetal abnormalities and the severity of maternal symptoms.

1. Polyhydramnios with fetal abnormalities: The principle of management is to terminate the pregnancy promptly.

(1) For chronic polyhydramnios in pregnant women with stable general condition and no obvious cardiopulmonary compression symptoms, transabdominal amniocentesis can be performed to drain an appropriate amount of amniotic fluid, followed by the injection of 50–100 mg of Rivanol to induce labor.

(2) Use a high-level membrane rupture device to puncture the fetal membrane 15–16 cm upward from the cervical os, allowing amniotic fluid to drain slowly at a rate of 500 ml per hour to avoid a sudden decrease in intrauterine pressure leading to placental abruption. Monitor blood pressure, pulse, and vaginal bleeding during the drainage process. After draining the amniotic fluid, place a sandbag or apply an abdominal binder to prevent shock. If no uterine contractions occur within 12 hours after membrane rupture, antibiotics should be administered. If contractions still do not occur within 24 hours, appropriate use of sodium prasterone sulfate to promote cervical ripening or oxytocin and prostaglandins for labor induction may be considered.

(3) Perform abdominal puncture first to partially drain the amniotic fluid and reduce pressure before artificial membrane rupture to avoid placental abruption.

2. Polyhydramnios with a normal fetus: The management approach should be determined based on the severity of polyhydramnios and gestational age.

(1) For severe symptoms intolerable to the pregnant woman (gestational age less than 37 weeks), amniocentesis should be performed. Use a 15–18-gauge lumbar puncture needle to drain amniotic fluid at a rate of 500 ml per hour, with a single drainage volume not exceeding 1500 ml, until maternal symptoms are relieved. Draining polyhydramnios may induce premature labor. The procedure should be performed under B-ultrasound monitoring to prevent injury to the placenta and fetus. Strict sterilization should be maintained to prevent infection, and sedatives or tocolytics may be used as needed to prevent premature labor. The procedure can be repeated after 3–4 weeks to reduce intrauterine pressure.

(2) Prostaglandin inhibitor—indomethacin therapy: Indomethacin has an antidiuretic effect and is used to inhibit fetal urination and treat polyhydramnios. The specific dosage is 2.0–2.2 mg/(kg·d), administered for 1–4 weeks, and can be repeated if polyhydramnios recurs. Weekly B-ultrasound monitoring is required during treatment. In the advanced stage of pregnancy, amniotic fluid is mainly formed by fetal urine, and indomethacin can be detected in fetal blood within 15 minutes after maternal administration. Due to the side effect of indomethacin causing closure of the ductus arteriosus, its use should be limited.

(3) If the pregnancy is near 37 weeks and fetal maturity is confirmed, artificial membrane rupture can be performed to terminate the pregnancy.

(4) For mild symptoms, the pregnancy can be continued with rest, a low-salt diet, and sedatives as needed, while closely monitoring changes in amniotic fluid volume.

Regardless of the method used for amniotic fluid drainage, the fetus should be fixed in a longitudinal position abdominally, uterine contractions should be closely monitored, and attention should be paid to signs of placental abruption and umbilical cord prolapse, while preventing postpartum hemorrhage.