Polyhydramnios and Oligohydramnios

The amniotic fluid that bathes the fetus is necessary for its proper growth and development. It cushions the fetus from physical trauma, permits fetal lung growth, and provides a barrier against infection. Normal amniotic fluid volume varies. The average volume increases with gestational age, peaking at 800-1000 mL, which coincides with 36-37 weeks' gestation. An abnormally high level of amniotic fluid, polyhydramnios, alerts the clinician to possible fetal anomalies. An inadequate volume of amniotic fluid, oligohydramnios , results in poor development of the lung tissue and can lead to fetal death.

Polyhydramnios occurs in 1% of pregnancies,[1]  whereas oligohydramnios occurs in about 11% of pregnancies.[2]  No age variables are recognized.

In pregnancies affected by polyhydramnios, approximately 20% of neonates are born with a congenital anomaly of some type; therefore, the delivery of these newborns in a tertiary care setting is preferred. This article presents the causes, outcomes, and treatments of polyhydramnios and oligohydramnios, as well as their effects on the developing fetus and neonate.

Rupture of the membranes is the most common cause of oligohydramnios. However, because the amniotic fluid is primarily fetal urine in the latter half of the pregnancy, the absence of fetal urine production or a blockage in the fetus's urinary tract can also result in oligohydramnios. Fetal swallowing, which occurs physiologically, reduces the amount of fluid, and an absence of swallowing or a blockage of the fetus's gastrointestinal tract can lead to polyhydramnios.

A near term fetus produces 500-1200 mL of urine and swallows between 210 and 790 mL of amniotic fluid per day.[3]

Polyhydramnios

The underlying cause of the excessive amniotic fluid volume is obvious in some clinical conditions and is not completely understood in others. Causes include the following:

• Twin gestation with twin-to-twin transfusion syndrome (increased amniotic fluid in the recipient twin and decreased amniotic fluid in the donor) or multiple gestations
• Fetal anomalies, including esophageal atresia (usually associated with a tracheoesophageal fistula), tracheal agenesis, duodenal atresia, and other intestinal atresias
• Central nervous system abnormalities and neuromuscular diseases that cause swallowing dysfunction
• Congenital cardiac-rhythm anomalies associated with hydrops, fetal-to-maternal hemorrhage, and parvovirus infection
• Poorly controlled maternal diabetes mellitus (Oligohydramnios may also be seen if severe vascular disease is present.)
• Chromosomal abnormalities, most commonly trisomy 21, followed by trisomy 18 and trisomy 13.
• Fetal akinesia syndrome with absence of swallowing

In a study by Kollmann et al of 860 singleton pregnancies complicated by polyhydramnios, 68.8% of the polyhydramnios cases were idiopathic, whereas maternal diabetes was found in 19.8% of cases; congenital anomalies, in 8.5%; and a positive TORCH (toxoplasmosis, other [such as syphilis, varicella-zoster, parvovirus B19], rubella, cytomegalovirus, herpes infection) serology, in 2.9%.[4]

Oligohydramnios

Causes of oligohydramnios include the following:

• Fetal urinary tract anomalies, such as renal agenesis, polycystic kidneys, or any urinary obstructive lesion (eg, posterior urethral valves ^[5] )
• Premature rupture of membranes and chronic leakage of the amniotic fluid;  chorioamnionitis is an additional important maternal complication from oligohydramnios due to rupture of the membranes, which has an incidence of 21-74%. The earlier chorioamnionitis occurs in pregnancy, the greater the fetal risk of bronchopulmonary dysplasia, neurologic complications, pulmonary hypoplasia, and, in severe cases, respiratory failure in the neonate.
• Placental insufficiency, as seen in pregnancy-induced hypertension, maternal diabetes, or postmaturity syndrome when the pregnancy extends beyond 42 weeks' gestation
• Maternal use of prostaglandin synthase inhibitors or angiotensin-converting enzyme

If polyhydramnios is not associated with any other findings, the prognosis is usually good. According Desmedt et al, there is a 61% perinatal mortality in polyhydramnios associated with a fetal or placental malformation.[6]  About 20% of infants with polyhydramnios have some type of an anomaly; in these cases, the prognosis depends on the severity of the anomaly. Yefet et al reported that even with normal detailed prenatal ultrasonographic evaluation, polyhydramnios (amniotic fluid index [AFI] >24 cm) is associated with an increased rate of fetal malformations, genetic syndromes, neurologic disorders, and developmental delay, conditions that may only be diagnosed postnatally.[1]

In a study of 163 women with idiopathic polyhydramnios, resolution was more likely when it was diagnosed early in pregnancy and when the AFI was low.[7] When idiopathic polyhydramnios persisted across the pregnancy, there was an increased risk of macrosomia and preterm delivery.

Studies show that as the severity of polyhydramnios increases, the likelihood of determining the etiology increases. In cases of mild polyhydramnios, the likelihood of finding a significant problem is only about 16.5%; this should be communicated to the parents. 

In the setting of oligohydramnios with renal agenesis, mortality is 100%. Milder forms of renal dysplasia or obstructive uropathy can be associated with a mild to severe degree of pulmonary hypoplasia and long-term renal failure. In cases of pulmonary hypoplasia, the effectiveness of many treatments such as the administration of surfactant, high-frequency ventilation, and nitric oxide has not been established. The prognosis in these cases is related to the volume of amniotic fluid and the gestational age at which oligohydramnios develops.

Borderline oligohydramnios (AFI 5.1-8 cm) does not appear to be a risk factor for adverse perinatal outcomes in uncomplicated, late preterm pregnancies.[8]

Morbidity/mortality

When Chamberlin used ultrasonography to evaluate the perinatal mortality rate (PMR) in 7562 patients with high-risk pregnancies, the PMR of patients with normal fluid volumes was 1.97 deaths per 1000 patients.[9]  However, the PMR increased to 4.12 deaths per 1000 patients with polyhydramnios and 56.5 deaths per 1000 patients with oligohydramnios.

In twin gestation with twin-to-twin transfusion, polyhydramnios may occur in the recipient twin, and oligohydramnios may occur in the donor. This complication is associated with high morbidity and mortality rates.

Polyhydramnios

Idiopathic polyhydramnios appears to increase the risk of prolonged first stage of labour, nonvertex presentation, and cesarean delivery.[10] Preterm labor and delivery occurs in approximately 26% of mothers with polyhydramnios. Other complications include premature rupture of the membranes (PROM), abruptio placenta, malpresentation, and postpartum hemorrhage.[11]

Studies show an increased risk of associated fetal anomalies in more severe forms of polyhydramnios. In a series, 20% of cases of polyhydramnios involved associated fetal anomalies, including problems of the gastrointestinal system (40%), central nervous system (26%), cardiovascular system (22%), or genitourinary system (13%).[12]  Among these cases of polyhydramnios, multiple gestations occurred in 7.5%, 5% were due to maternal diabetes, and the remaining 8.5% were due to other causes. However, at least 50% of the patients had no associated risk factors.

Oligohydramnios

The mortality rate in oligohydramnios is high. The lack of amniotic fluid allows compression of the fetal abdomen, which limits movement of its diaphragm. In addition to chest wall fixation, the lack of amniotic fluid flowing in and out of the fetal lung leads to pulmonary hypoplasia.

Oligohydramnios is also associated with meconium staining of the amniotic fluid, fetal heart conduction abnormalities, umbilical cord compression, poor tolerance of labor, lower Apgar scores, and fetal acidosis. In cases of intrauterine growth restriction (IUGR), the degree of oligohydramnios is often proportional to growth restriction, is frequently reflective of the extent of placental dysfunction, and is associated with a corresponding increase in the PMR.

Maternal body mass index (BMI) does not appear to be associated with oligohydramnios in late gestation.[13]  However, there is an increased risk of primary cesarean delivery with increasing maternal BMI.

Complications

Polyhydramnios

Risks and complications of amnioinfusion include amniotic fluid embolism, maternal respiratory distress, increased maternal uterine tone, and transient fetal respiratory distress. An increase in the risk of maternal or fetal infection is not substantiated.

Risks of amniocentesis include fetal loss (1-2%). Other complications are placental abruption, preterm labor, fetal-maternal hemorrhage, maternal Rh sensitization, and fetal pneumothorax. The risk of fetal infection is slightly increased.

Oligohydramnios

The primary complications of oligohydramnios are those related to fetal distress before or during labor. The risk of fetal infection is increased in the presence of prolonged rupture of the membranes.

Amniotic fluid

The volume of the amniotic fluid is evaluated by visually dividing the mother's abdomen into four quadrants. The largest vertical pocket of fluid is measured in centimeters. The sum of these measurements is the amniotic fluid index (AFI).

Polyhydramnios is usually defined as an AFI of more than 24 cm or a single pocket of fluid at least 8 cm in depth that results in an amniotic fluid volume of more than 2000 mL.[14]

Oligohydramnios is ultrasonographically defined as an AFI less than 7 cm or the absence of a fluid pocket 2-3 cm in depth.

Polyhydramnios

Visual inspection may reveal a rapidly enlarging uterus in the pregnant mother. Multiple gestations are associated with polyhydramnios. 

Fetal abnormalities associated with polyhydramnios include neonatal macrosomia, fetal or neonatal hydrops with anasarca, ascites, pleural or pericardial effusions, and gastrointestinal tract obstruction (eg, duodenal atresia, tracheoesophageal fistula).

Skeletal malformations can also occur, including congenital hip dislocation, clubfoot, and limb reduction defect.

Abnormalities in fetal movement are suggestive of primary neurologic abnormalities or may be in association with a genetic syndrome, such as polyploidy.

Oligohydramnios

When the oligohydramnios is associated with renal agenesis or dysgenesis, symptoms/signs include a marked deformation of the fetus due to of intrauterine constraint (Potter syndrome). Obstructive uropathies cause similar deformations, including external compression with a flattened facies and epicanthal folds, hypertelorism, low-set ears, a mongoloid slant of the palpebral fissure, a crease below the lower lip, and micrognathia. Thoracic compression may also occur.

Oligohydramnios adversely affects fetal lung development, resulting in pulmonary hypoplasia that typically leads to death from severe respiratory insufficiency. Other fetal deformations include bowed legs, clubbed feet, a single umbilical artery, gastrointestinal atresias, and a narrow chest secondary to external compression. Infants are typically small for their stated gestational age (SGA).[15]  When an abdominal mass is found on examination of the infant in this clinical setting, it often represents multicystic-dysplastic kidney, enlarged urinary bladder, or prune-belly syndrome.

In the setting of polyhydramnios or oligohydramnios, testing of the infant is recommended, depending on the results of postnatal evaluation of the infant. Such evaluation may include chromosome testing, testing for evidence of congenital infection, ultrasonography of the genitourinary tract, and appropriate radiologic evaluation of the gastrointestinal tract. Electrocardiography and echocardiography may also be indicated.

Histologic examination of the placenta may be helpful in determining the cause of the polyhydramnios or oligohydramnios.

If premature delivery is anticipated with either oligohydramnios or polyhydramnios, the amniotic fluid lamellar body count, lecithin-sphingomyelin (L:S) ratio, and phosphatidylglycerol (PG) concentration are helpful in determining the maturity of the fetal lungs and, therefore, in assessing the likelihood of respiratory distress syndrome.

Polyhydramnios

Obtain glucose tolerance testing for mothers with suspected type 2 diabetes mellitus.

If fetal hydrops is present, immunologic and fetal infection need to be investigated. This should include screening for maternal antibodies to D, C, Kell, Duffy, and Kidd antigens to determine maternal antibody production against the fetal red blood cells. Infections of the fetus include cytomegalovirus (CMV), toxoplasmosis, syphilis, and parvovirus B19. The investigation should include the following studies:

• Venereal Disease Research Laboratories (VDRL) test to screen for syphilis

• Immunoglobulin G (IgG) and IgM titers to evaluate for exposure to rubella, CMV, toxoplasmosis, and parvovirus

• A test for congenital viruses in the amniotic fluid using polymerase chain reaction (PCR)

• Kleihauer-Betke test to evaluate fetal-maternal hemorrhage

• Hemoglobin Bart in patients of Asian descent (who may be heterozygous for alpha-thalassemia)

• Fetal karyotyping for trisomy 21, 13, and 18

Oligohydramnios

Premature rupture of membrane (PROM)  is the most common cause of oligohydramnios; therefore, obtain available tests to confirm or exclude this condition. Maternal substance abuse and certain medications (eg, nonsteroidal anti-inflammatory drugs [NSAIDs], angiotensin-converting enzyme inhibitors [ACEI]) can also cause oligohydramnios.

Test for systemic lupus erythematosus (SLE), which causes immune-mediated infarcts in the placenta as well as placental insufficiency.

Evaluate for pregnancy-induced hypertension (PIH) and hemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome. Test for elevated blood pressure, proteinuria, elevated uric acid levels, increased liver function test results, and low platelet count.

Prenatal ultrasonography and polyhydramnios

Evaluate fetal swallowing. A decrease in fetal deglutition occurs in anencephaly, trisomy 18, trisomy 21, muscular dystrophy, and skeletal dysplasia.

Evaluate the fetal anatomy; assess for diaphragmatic hernia, lung masses, and the absence of the stomach bubble (which is associated with esophageal atresia). The double-bubble sign or a dilated duodenum suggests the possibility of duodenal atresia.

Test for fetal arrhythmias and malformations that result in cardiac failure and hydrops.

An abnormally large abdominal circumference may be observed with ascites and hydrops fetalis.

A macrosomic fetus is observed in association with poorly controlled maternal diabetes.

Assess the blood flow velocity in the fetal middle cerebral artery for fetal anemia.

Prenatal ultrasonography and oligohydramnios

Perform serial measurements of the amniotic fluid index during the pregnancy. If the mother is in the third trimester and if the volume is less than 8 cm, suspect oligohydramnios. Levels below 5 cm indicate significant oligohydramnios.

Visualize the fetal kidneys, collecting system, and bladder. If these are normal, suspect the chronic leakage of amniotic fluid or pregnancy-induced hypertension.

Assess fetal growth. If premature rupture of the membrane or urinary tract anomalies are absent, consider placental insufficiency and intrauterine growth restriction.

Uterine artery Doppler study findings may aid in the diagnosis of placental insufficiency.

Postnatally, evaluate organ systems likely to be involved on the basis of the pregnancy history and results of other prenatal evaluations. For more information, see Oligohydramnios Imaging.

Color Doppler ultrasonography

In a study that used color Doppler ultrasonography in addition to grayscale ultrasonography to evaluate 428 women for detection of amniotic fluid disturbances and perinatal outcomes, investigators did not find an increased detection of pregnancies at risk for adverse intrapartum or perinatal outcomes with this added evaluation.[16] However, color Doppler ultrasonography did appear to result in lower estimates of amniotic fluid volume (ie, overdiagnosis of low amniotic fluid volume).

Polyhydramnios

Reductive amniocentesis may be performed and has contributed to prolonged pregnancy in patients who are severely affected by hydramnios.[17] This procedure can reduce the risk of preterm labor, premature rupture of the membrane (PROM), umbilical cord prolapse, and placental abruption. However, if too much fluid is removed, placental abruption may occur. Other risks of the procedure include infection, bleeding, and trauma to the fetus.

Laser ablation of placental vessels may be effective in cases of twin-to-twin transfusion syndrome, performed at highly specialized centers.

Oligohydramnios

The transabdominal instillation of indigo carmine may be used to evaluate for PROM.

The transcervical instillation of isotonic sodium chloride solution (ie, amnioinfusion) at the time of delivery reduces the risk of cord compression, fetal distress, and meconium dilution. It also reduces the potential need for cesarean delivery.

In women with polyhydramnios or oligohydramnios, note the following:

• Consider hospitalizing and thoroughly evaluating the mother in cases diagnosed after 26-33 weeks' gestation.
• If the fetus does not have an anomaly, delivery should be performed if the biophysical profile is nonreassuring.
• The instillation of isotonic sodium chloride solution in the second trimester may be of benefit in some patients. Use transabdominal amnioinfusion to instill 400-600 mL, which may improve visualization for ultrasonography and increase the amniotic fluid volume.
• In cases associated with postmaturity, review the pregnancy dating. If the gestation is truly longer than term, deliver the fetus by means of either induction or cesarean delivery.
• If meconium is present during labor, administer amnioinfusion therapy to reduce the potential for fetal distress and prenatal aspiration.

Transfer these pregnant women to a tertiary center when they have a high likelihood of maternal illness, preterm delivery, or infant problems that may require the resources of a tertiary care facility.

The first step is identifying the etiology of the abnormal volume of amniotic fluid. Medical care includes the use of steroids to enhance fetal lung maturity if preterm delivery is anticipated.

Polyhydramnios

Patients with polyhydramnios tend to have a higher incidence of preterm labor secondary to overdistention of the uterus. Schedule weekly or twice weekly perinatal visits and cervical examinations.

Place patients on bed rest to decrease the likelihood of preterm labor.

Perform serial ultrasonography to determine the amniotic fluid index (AFI) and document fetal growth.

In cases of polyhydramnios associated with fetal hydrops secondary to fetal anemia, the direct intravascular transfusion of erythrocytes (or infusion into the fetal abdomen) may improve the fetal hematocrit and fetal congestive heart failure, thereby allowing prolongation of the pregnancy and improving survival.

In cases of polyhydramnios in which maternal diabetes is suspected, perform a glucose tolerance test. If the test results are positive, treat the mother with an American Diabetes Association (ADA) diet. Insulin is rarely needed.

Oligohydramnios

Maternal bed rest and hydration promote the production of amniotic fluid by increasing the maternal intravascular space. Bed rest may also help when pregnancy-induced hypertension (PIH) is present, allowing prolongation of the pregnancy.

Studies show that oral hydration, by having the women drink 2 liters of water, increases the AFI by 30%. In singleton pregnancies where oligohydramnios is present without maternal and fetal complications, evidence exists that either oral or intravenous maternal hydration (1500-2500 mL/day) is associated with a 20-30% improvement in AFI and a reduction in cesarean delivery.[18]  Further studies are necessary to determine if a corresponding improvement in fetal and neonatal well-being can be substantiated with such "forced hydration" therapy.

A specialist in maternal-fetal medicine should be consulted when significant polyhydramnios or oligohydramnios is present, especially when the condition is unexplained, involves hydrops fetalis, or is associated with congenital malformations.

Genetic counseling may be helpful in cases in which congenital anomalies are identified.

Consult a neonatologist, pediatric surgeon, pediatric cardiologist, pediatric nephrologist, pediatric infectious disease specialist, or other genetics specialists as required to care for the infant.

Most cases of polyhydramnios respond in the first week of treatment with indomethacin.[19, 20, 21] This approach appears to be highly effective (90-100% in some studies), provided that the cause is not hydrocephalus or a neuromuscular disorder that alters fetal swallowing.

Class Summary

When administered to pregnant women with polyhydramnios, these drugs can reduce fetal urinary flow, decreasing the volume of amniotic fluid.

Indomethacin (Indocin)

Rapidly absorbed; metabolism occurs in liver by demethylation, deacetylation, and glucuronide conjugation; inhibits prostaglandin synthesis.