Giancarlo Mari, MD

Polyhydramnios, or hydramnios, is an abnormal increase in the volume of amniotic fluid. Identification of polyhydramnios should prompt a search for an underlying etiology. The two most common pathologic causes of polyhydramnios are maternal diabetes mellitus and fetal anomalies. When no etiology for the excess amniotic fluid is identified, polyhydramnios is termed “idiopathic,” or unexplained. Idiopathic polyhydramnios accounts for approximately 60% to 70% of cases of polyhydramnios in published series and is identified in nearly 1% of pregnancies. This summary of SMFM Consult Series #46 reviews major considerations in evaluation and management of polyhydramnios (Table 1).

Q| How is amniotic fluid volume assessed? 

After 20 weeks’ gestation, amniotic fluid volume is assessed by using 

either the deepest vertical pocket (DVP) or the amniotic fluid index (AFI). In multiple gestations, the DVP is used. When the single DVP is used, polyhydramnios is defined as a measurement ≥ 8.0 cm. When AFI is used, the threshold is generally considered to be 24 cm or 25 cm, depending on whether a 95th or 97th percentile is selected. 

Polyhydramnios is most often identified in the third trimester. Idiopathic polyhydramnios is usually detected in the third trimester, at a mean gestational age of 31 to 36 weeks across various series. 

The degree of polyhydramnios is frequently categorized as mild, moderate, or severe, based on an AFI of 24.0 to 29.9 cm, 30.0 to 34.9 cm, and ≥ 35 cm, or a DVP of 8 to 11 cm, 12 to 15 cm, or ≥ 16 cm, respectively. Mild polyhydramnios accounts for approximately 65% to 70% of cases, moderate polyhydramnios for 20%, and severe polyhydramnios for < 15%.    

Q| What are the underlying causes of polyhydramnios? 

When an etiology of polyhydramnios is identified, it is most commonly a fetal anomaly or maternal diabetes. Other potential causes of apparently isolated polyhydramnios in a structurally normal fetus include alloimmunization and congenital infection. Physiologically, the fluid increase in many of these cases can be attributed to either (1) impaired fetal swallowing or (2) overproduction of fetal urine due to a high-output cardiac state, renal abnormality, or osmotic fetal diuresis. In general, any anomaly severe enough to cause nonimmune hydrops fetalis (NIHF) may result in polyhydramnios, as these entities are often associated with each other. With maternal diabetes, maternal hyperglycemia leads to fetal hyperglycemia, with subsequent osmotic diuresis into the amniotic fluid compartment. 

Alloimmunization can lead to fetal anemia with resultant NIHF and polyhydramnios. Congenital infections, such as parvovirus, cytomegalovirus, or syphilis, can lead to polyhydramnios by a variety of mechanisms, including anemia or cardiac dysfunction.

Q| What evaluation should be performed when polyhydramnios is detected?

Idiopathic polyhydramnios is a diagnosis of exclusion. While the cause may be unexplained during pregnancy, the underlying risk that a structural or genetic abnormality is identified after birth in a pregnancy associated with apparently idiopathic polyhydramnios is 9% in the neonatal period to as high as 28% when infants were followed up to age 1 year.

 Initial evaluation for polyhydramnios involves targeted ultrasonography to assess for fetal abnormalities. It is important to assess fetal growth because idiopathic polyhydramnios may be associated with macrosomia, and fetal growth restriction associated with polyhydramnios presents a high risk for an underlying fetal abnormality, including trisomy 13 or 18. 

Not all of the abnormalities associated with polyhydramnios are detectable by ultrasound. Fetal esophageal atresia and tracheoesophageal fistula are among the most common polyhydramnios-associated abnormalities and may be difficult to diagnose by ultrasound. Disorders associated with apparently isolated polyhydramnios also include genetic syndromes for which there may be no sonographic findings or no screening or diagnostic test available.

Diabetes, alloimmunization, and congenital infection should be considered in a structurally normal fetus with mild polyhydramnios. Routine prenatal care includes screening for diabetes and alloimmunization, as well as testing for syphilis. Congenital infection usually presents with additional sonographic findings, such as NIHF, hepatomegaly, splenomegaly, or placentomegaly. In cases of polyhydramnios associated with NIHF or additional sonographic features, evaluation for fetal anemia and congenital infection is recommended. 

Severe polyhydramnios presenting earlier in gestation should raise a greater concern for an underlying etiology. In severe cases, especially early in gestation, it is important to review the medical and family history, in addition to obtaining a detailed ultrasound examination. Genetic counseling and consideration of testing for neurologic disorders such as congenital myotonic dystrophy should be considered, especially with decreased fetal movement.

Q| How is a pregnancy with polyhydramnios managed?

Polyhydramnios severe enough to cause maternal respiratory compromise, significant discomfort, or preterm labor often has an underlying etiology, whereas idiopathic polyhydramnios, because it is usually mild and does not present until the mid-third trimester, does not typically require treatment. In selected cases, however, amnioreduction may be considered in an effort to relieve maternal dyspnea or discomfort. Overall, in cases of severe polyhydramnios that results in maternal respiratory compromise such that amnioreduction is considered, an underlying fetal abnormality is usually present. In addition, polyhydramnios usually recurs after amnioreduction, making its efficacy somewhat limited. 

Indomethacin, which is often used for tocolysis, decreases fetal urine production. Due to reported neonatal complications, and in the absence of data on improved maternal or neonatal outcomes, we recommend that indomethacin not be used for the sole purpose of decreasing amniotic fluid in the setting of polyhydramnios.

Likelihood of an underlying fetal abnormality is significantly higher with greater degrees of polyhydramnios, and progression of the condition is suggestive of an underlying etiology. In pregnancies with an identified underlying etiology, the degree of polyhydramnios is associated with an increased likelihood of preterm birth (PTB), a small-for-gestational age infant, macrosomia, and perinatal mortality. 

Rates of PTB are not generally increased with idiopathic polyhydramnios (which is usually mild) but PTB is associated with more severe polyhydramnios. Reported data on whether perinatal mortality is increased with idiopathic polyhydramnios have been inconsistent. Idiopathic polyhydramnios is associated with infant birthweight > 4000 g in approximately 15% to 30% of cases. 

The most recent guidance from the American College of Obstetricians and Gynecologists on antepartum fetal surveillance does not specifically address isolated polyhydramnios or list it as an indication for surveillance. Although antepartum surveillance is often performed in this setting, there are no data to suggest that such assessment decreases perinatal mortality. The role and frequency of follow-up ultrasonography is unclear, but the imaging may be warranted in cases where there is concern for progression of polyhydramnios or to evaluate fetal growth. 

Regarding timing of delivery, there are no data to suggest that induction of labor or PTB are associated with an improved outcome in the setting of mild idiopathic polyhydramnios. Labor should be allowed to occur spontaneously at term and the mode of delivery should be determined based on usual obstetric indications. If an induction is planned, it should not occur at < 39 weeks’ gestation in the absence of other indications.  

Rates of fetal nonvertex presentation are reported to increase as severity of polyhydramnios increases. Clinical or sonographic determination of the fetal presenting part should be performed upon presentation in labor. External version for nonvertex fetal presentation may be considered if there are no contraindications to this procedure.

Rates of dysfunctional labor are increased in the presence of polyhydramnios. Studies have also demonstrated that women with pregnancies complicated by idiopathic polyhydramnios are significantly more likely to undergo a cesarean delivery for failure to progress. Rates of caesarean delivery for women with pregnancies complicated by idiopathic polyhydramnios range from 35% to 55%. An increased risk of operative vaginal delivery in the presence of polyhydramnios has also been reported. Some investigators report a higher frequency of nonreassuring fetal heart rate tracings and postpartum hemorrhage. 

Planning for care of the neonate is necessary in cases of polyhydramnios. As noted previously, there is an increased rate of structural abnormalities or genetic syndromes in the neonate following a gestation complicated by polyhydramnios. Idiopathic polyhydramnios has also been associated with an increased risk of neonatal intensive care unit admission. Pediatric support should be available at delivery for women with mild, idiopathic polyhydramnios; for women with severe polyhydramnios, delivery at a tertiary center is recommended. 

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This summary of SMFM Consult Series #46 reviews major considerations in evaluation and management of polyhydramnios.

Evaluation and management of polyhydramnios

Ventriculomegaly, or dilation of the fetal cerebral ventricles, is a relatively common ï¬nding on prenatal ultrasound. This summary of SMFM Consult Series #45 reviews key points for diagnosis, evaluation, and management of mild fetal ventriculomegaly.

Fetal cerebral ventriculomegaly is defined as an atrial diameter  ≥ 10 mm on prenatal ultrasound. Ventriculomegaly is typically categorized in one of two ways: mild (10–15 mm) or severe (> 15 mm); or as mild (10–12 mm), moderate (13–15 mm), or severe (> 15 mm). Because the chance of an adverse outcome and potential for other abnormalities are higher when the ventricles measure 13–15 mm vs 10–12 mm, the latter classification method should be used for the purposes of patient counseling. The incidence of mild to moderate fetal ventriculomegaly is approximately 1%. 

Q| What are the causes of ventriculomegaly?

The differential diagnosis of ventriculomegaly is extensive and includes a normal variant as well as disorders associated with severe impairment. A thorough evaluation is critical to make the correct diagnosis and to provide an accurate prognosis.

Fetuses with a ventricular atrial diameter of 10–12 mm are found to have a normal postnatal evaluation in > 90% of cases. The chance that mild ventriculomegaly represents a normal variant decreases with increasing degrees of dilation. Mild ventriculomegaly is likely to represent a normal variant if no other structural abnormalities are noted and if aneuploidy screening or diagnostic genetic testing results are normal. Because it is not possible to determine with certainty that mild ventriculomegaly is truly isolated during pregnancy, normal variation is a diagnosis of exclusion that cannot be made with certainty until after birth.

Structural abnormalities that can lead to dilation or enlargement of the lateral ventricles include agenesis of the corpus callosum, Dandy-Walker malformation, neural tube defects, cortical defects, and migrational abnormalities or heterotopia. Some structural central nervous system (CNS) anomalies, such as holoprosencephaly, hydranencephaly, porencephaly, or schizencephaly, and cystic lesions, such as arachnoid cysts, result in abnormal fluid collections in the fetal brain that may be misdiagnosed as ventriculomegaly, although these anomalies do not truly represent dilation of the ventricular system. In rare cases, a tumor or choroid plexus papilloma may result in overproduction of cerebrospinal fluid with resultant ventriculomegaly. Large isolated choroid plexus cysts may transiently dilate the fetal cerebral ventricles.

The most common cause of severe ventriculomegaly is aqueductal stenosis, which results from narrowing of the cerebral aqueduct of Sylvius located between the third and fourth ventricle, leading to progressive dilatation of the lateral and third ventricles. Aqueductal stenosis can be genetic or can result from fibrosis secondary to fetal infection (e.g., cytomegalovirus [CMV], toxoplasmosis, or Zika virus) or bleeding (e.g., intraventricular hemorrhage). A mass or congenital tumor can also lead to compression of the aqueduct with resultant ventriculomegaly. In many cases, the cause of aqueductal stenosis is unknown.

Approximately 5% of cases of mild to moderate ventriculomegaly are reported to result from congenital fetal infections, including cytomegalovirus (CMV), toxoplasmosis, and Zika virus. Many cases of ventriculomegaly associated with congenital infection demonstrate other sonographic features, including fetal growth restriction; periventricular, hepatic, and other intra-abdominal calcifications; echogenic fetal bowel; hepatosplenomegaly; ascites; meconium peritonitis; polyhydramnios, and microcephaly. However, these features may not be evident until later in gestation, and not all infected fetuses will have other sonographic signs.

Approximately 5% of fetuses with apparently isolated mild to moderate ventriculomegaly have an abnormal karyotype, most commonly trisomy 21. Another 10% to 15% have abnormal findings on chromosomal microarray. Although hydrocephalus is a component of several congenital syndromes, there are relatively few genetic causes of isolated ventriculomegaly or hydrocephalus. Syndromes typically associated with more severe ventriculomegaly as well as additional abnormalities that may be identified sonographically or by fetal magnetic resonance imaging (MRI) include Walker-Warburg, Bardet-Biedl, Meckel, Joubert, and hydrolethalus syndromes. 

NEXT: How should a fetus with mild or moderate ventriculomegaly be evaluated?

Q| How should a fetus with mild or moderate ventriculomegaly be evaluated?

When mild or moderate ventriculomegaly is detected, further evaluation is indicated. Such evaluation is focused on determining whether additional structural (CNS and non-CNS) anomalies, genetic abnormalities, or congenital infection are present.

Incidence of additional CNS and non-CNS sonographic abnormalities identified in fetuses with mild or moderate ventriculomegaly ranges from 10% to 76% but appears to be < 50% in most studies. When ventriculomegaly is identified, a detailed ultrasound should be performed by a practitioner experienced in diagnosis of fetal anomalies. Careful attention should be given to intracranial anatomy including the lateral, third, and fourth ventricles; corpus callosum; thalami; germinal matrix region; cerebellum; and the cerebellar vermis. The fetal heart should be carefully examined, and fetal biometry should be assessed for evidence of growth restriction. Finally, a thorough inspection should be performed for signs of fetal infection, including intracranial or extracranial calcifications, hepatosplenomegaly, ascites, and fetal growth restriction.

Diagnostic testing (amniocentesis) with chromosomal microarray should be offered when ventriculomegaly is detected. Cell-free DNA screening can be considered for women who decline diagnostic testing after counseling about the limitations of this approach. Women with prior normal screening test results, including cell-free DNA, should still be offered diagnostic testing due to the higher diagnostic yield.

The woman’s history should be reviewed for symptoms suggestive of CMV infection, and exposure to potential sources of toxoplasmosis (e.g., outdoor cats, gardening, consumption of undercooked meat) and Zika virus should be assessed. A detailed travel history should be included in the evaluation. For women with a history of travel to any Zika-endemic area, testing according to the Centers for Disease Control and Prevention guidelines is recommended. Testing for CMV and toxoplasmosis is recommended when ventriculomegaly is detected, regardless of known exposure or symptoms. Testing can include maternal serology or polymerase chain reaction (PCR) on amniotic fluid. For women who decline amniocentesis, serum testing for CMV includes IgG and IgM, as does screening for toxoplasmosis

Fetal MRI can be useful in evaluation of ventriculomegaly because this modality can identify significant abnormalities not easily detected by ultrasound, such as cortical malformations and migrational abnormalities. The most common abnormality detected on MRI but missed on fetal ultrasound is agenesis of the corpus collosum. MRI may also be of benefit in assessing the extent of destructive injury in fetuses with known infection, hemorrhage, or ischemia, and when other sonographically evident CNS malformations, such as agenesis of the corpus callosum or Dandy-Walker malformation, are present.

MRI is most useful at > 22–24 weeks’ gestation, as milestones of CNS development become more evident with advancing gestation. The added value of MRI depends in part on the degree of ventricular dilation, as well as on the quality of the original ultrasound and whether a detailed neurosonography examination was performed by a provider with specific expertise. MRI can be considered in cases of mild or moderate fetal ventriculomegaly when this modality and expert radiologic interpretation are available. It is likely to be of less value if the patient has had a detailed ultrasound exam performed by an individual with specific experience and expertise in sonographic imaging of the fetal brain.

Q| What is the appropriate antenatal management of a pregnancy after mild to moderate ventriculomegaly is detected? 

Follow-up ultrasound after initial detection of fetal ventriculomegaly is helpful to assess progression, stability, or resolution. The optimal timing and frequency of follow-up ultrasound examinations in the setting of mild to moderate ventriculomegaly is dependent on the initial gestational age at diagnosis as well as other clinical factors.

Women should receive counseling from a health care provider with specific expertise in prenatal diagnosis and prognosis of fetal ventriculomegaly. They should be informed that the prognosis varies widely based on the exact findings of the complete prenatal and postnatal evaluation. If ventriculomegaly is progressive, consultation with a pediatric neurosurgeon may be useful, as some neonates may require postnatal surgical intervention, such as ventriculoperitoneal shunting. Overall, the likelihood of mild to moderate ventriculomegaly requiring surgical intervention after birth is low.

Antepartum fetal testing is not likely to be beneficial in the setting of mild to moderate ventriculomegaly, as this abnormality is not typically associated with placental insufficiency, unless other abnormalities such as fetal growth restriction or amniotic fluid abnormalities are present.

Q| What is the optimal timing and mode of delivery for fetuses with ventriculomegaly?  

There is no evidence that preterm or cesarean delivery improves maternal or neonatal outcomes in the setting of mild to moderate ventriculomegaly. Timing and mode of delivery should therefore be based on standard obstetric indications. Given the potential for mild to moderate ventriculomegaly to be associated with long-term adverse neurodevelopmental outcomes, the primary pediatrician should be made aware of this prenatal finding.

Q| What is the prognosis for infants with mild ventriculomegaly? 

The prognosis for infants with mild to moderate ventriculomegaly is widely variable and depends on presence or absence of structural or genetic abnormalities, fetal infection, and severity of ventricular dilation. After a complete evaluation, if the ventriculomegaly is mild and isolated, the outcome is most commonly normal. With isolated moderate ventriculomegaly of 13–15 mm, after a complete evaluation, the outcome is likely to be favorable, but there is an increased risk of neurodevelopmental disabilities. In the setting of mild to moderate ventriculomegaly with associated abnormalities, the prognosis primarily depends on the specific abnormality rather than the degree of ventricular dilation.

Progression is also a factor in outcomes. In cases in which ventriculomegaly progresses, the rate of adverse outcomes is reported to be as high as 44%, while outcomes are normal in > 90% of cases in which ventriculomegaly improves. In most cases, risk of recurrence of isolated ventriculomegaly in future pregnancies is low. In cases with an underlying cause, such as a chromosomal or genetic condition, risk of recurrence will depend on the specific diagnosis. 

Ventriculomegaly, or dilation of the fetal cerebral ventricles, is a relatively common ﬁnding on prenatal ultrasound. This summary of SMFM Consult Series #45 reviews key points for diagnosis, evaluation, and management of mild fetal ventriculomegaly.

Giancarlo Mari, MD

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Evaluation and management of polyhydramnios

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