INTRODUCTION — Hydrops fetalis is a condition of excess fluid accumulation in the fetus that results in significant fetal demise and neonatal mortality. It was first described by Ballantyne in a case series of 65 human fetuses and newborn infants, in which he suggested there were a variety of etiologies [1]. In 1943, Potter defined two forms of hydrops fetalis based upon etiology [2]:
●Immune-mediated − In immune-mediated cases, hydrops fetalis is a result of severe fetal anemia due to destruction of fetal red blood cells by maternal immunoglobulin G (IgG) antibodies. Before the introduction of antenatal Rh(D) (rhesus D antigen) immune globulin prophylaxis in the 1960s, the most frequent cause of hydrops fetalis was Rh incompatibility between the mother and fetus. (See "Alloimmune hemolytic disease of the newborn: Postnatal diagnosis and management".)
●Nonimmune − Nonimmune hydrops fetalis (NHF) is associated with numerous disorders that include cardiac, pulmonary, infectious, and genetic etiologies (table 1). NHF accounts for almost 90 percent of current hydropic cases in neonates.
The postnatal care of hydrops fetalis will be reviewed here. The antenatal care of Rhesus alloimmunization and nonimmune hydrops fetalis is discussed separately. (See "RhD alloimmunization in pregnancy: Management" and "Nonimmune hydrops fetalis".)
DIAGNOSIS — The diagnosis of hydrops fetalis is based upon antenatal ultrasonography or postnatal evaluation that demonstrates two or more of the following findings:
●Skin edema
●Pleural effusion
●Pericardial effusion
●Ascites
●Polyhydramnios
(See "Nonimmune hydrops fetalis", section on 'Diagnosis'.)
EPIDEMIOLOGY AND ETIOLOGY — In developed countries, since the implementation of antenatal Rh(D) immune globulin prophylaxis, the incidence of immune-mediated hydrops fetalis has drastically fallen from 80 percent of cases in 1970 to less than 10 percent in 2007 [3,4]. However, in some parts of the world immune causes remain a significant cause of hydrops fetalis [5].
Non-immune hydrops fetalis — The reported incidence of nonimmune hydrops fetalis (NHF) ranges from 1 in 1500 to 3800 births. The wide variation is based upon differences in population (eg, increased incidence of alpha thalassemia in Southeast Asia) and thoroughness of evaluation, and whether late pregnancy terminations were included in the analysis. (See "Nonimmune hydrops fetalis", section on 'Prevalence'.)
In a review from a large national data set from the United States that identified 598 live-born infants with hydrops fetalis from a cohort of 253,651 infants discharged from a neonatal intensive care unit (NICU), the following were the most common conditions associated with hydrops fetalis [4].
●Congenital heart problems (14 percent)
●Heart rate abnormalities (10 percent)
●Twin-to-twin transfusions (9 percent)
●Noncardiac congenital anomalies (9 percent)
●Chromosomal abnormalities (8 percent)
●Congenital viral infections (7 percent)
●Congenital anemia (5 percent)
●Congenital chylothorax (3 percent)
A systematic review of the literature that identified over 1300 patients with non-immune hydrops fetalis classified etiologies into 14 classifications [6]:
●Cardiovascular disease (20 percent)
●Lymphatic dysplasia (15 percent)
●Hematologic disease (9 percent)
●Chromosomal abnormalities (9 percent)
●Infections (7 percent)
●Syndromic causes without a known genetic defect (6 percent)
●Twin-to-twin transfusions and placental vascular abnormalities (4 percent)
●Thoracic malformations (2 percent)
●Inborn error of metabolism (1 percent)
●Congenital anomalies of the kidney and urinary tract (1 percent)
●Thoracic tumors (1 percent)
●Miscellaneous (4 percent)
●Idiopathic (20 percent)
In an autopsy study of 429 neonates from a tertiary center in the United States, 32 cases (7 percent) had hydrops fetalis, of which 16 infants had cardiovascular disease and 6 had congenital infections [7].
In Asia, the most common causes of hydrops fetalis are homozygous alpha-thalassemia and cardiac disease [8-10].
The etiologies of NHF are discussed in greater detail separately (table 1). (See "Nonimmune hydrops fetalis", section on 'Etiology and prenatal management of disorders associated with hydrops'.)
PATHOGENESIS — Understanding the basic mechanisms that underlie the development of hydrops fetalis is important in directing both antenatal and postnatal treatment of hydrops fetalis.
Normally, the formation of interstitial fluid from the vascular space is balanced by the return of interstitial fluid back to the central venous system through the lymphatic system. The process is governed by Starling's Law and is dependent upon the four following components:
●Capillary hydrostatic pressure
●Capillary oncotic pressure
●Interstitial hydrostatic pressure
●Interstitial osmotic pressure, which is dependent on lymphatic return
The fetus is particularly vulnerable to increased interstitial fluid accumulation because of its greater capillary permeability, compliant interstitial compartments, and increased susceptibility to impaired lymphatic flow.
In cases of hydrops fetalis, accumulation of interstitial fluid occurs because the production of interstitial fluid greatly exceeds the lymphatic return. Although the pathogenesis is not clearly understood, hydrops fetalis appears to be multifactorial due to mechanisms that produce elevated central venous pressure, impair lymphatic return, and increase capillary leakage.
Elevated venous pressure — Elevated central venous pressure (CVP) increases capillary hydrostatic pressure and impairs lymphatic return to the vascular space [11,12].
Both low (eg, cardiac arrhythmia, congenital structural heart disease, and myocarditis) and high-output cardiac failure (eg, severe anemia, arterial-venous malformations, and twin-twin transfusion syndrome) cause elevated CVP that may result in hydrops fetalis [4,13-21]. Elevation of CVP appears to be a critical component in the pathogenesis of hydrops fetalis because animal studies have demonstrated that severe anemia alone does not cause hydrops fetalis, but rather only causes hydrops fetalis in the presence of increased CVP [22,23].
CVP elevation may also be caused by obstruction of either the superior and/or inferior vena cavas as demonstrated in cases of congenital cystic adenomatoid malformation, omphalocele, cervical or mediastinal teratoma, or congenital high airway obstruction syndrome [24-28]. Compression of the vascular system impedes lymphatic return and results in edema formation.
Impaired lymphatic drainage — Lymph return to the central venous system is reduced either by structural abnormality or, as previously discussed, by functional impairment due to increased CVP.
Anomalous development results in structural dysgenesis of the lymphatic network and is seen in the following conditions: chylothorax, cervical hygroma, congenital lymphedema, and cystic lymphangiectasia [4,29-32]. These structural anomalies are often associated with chromosomal abnormalities, such as Turner syndrome [33]. (See "Enlarged nuchal translucency and cystic hygroma".)
Capillary oncotic pressure — In theory, reduced capillary oncotic pressure should contribute to hydrops fetalis. However, data from animal studies demonstrated that hypoproteinemia does not result in nonimmune hydrops fetalis (NHF) [34].
Hypoxia — Animal studies have shown that severe fetal hypoxia causes hydrops fetalis. In these studies, there is sustained activation of the renin-angiotensin system with elevation of renin in the absence of renal injury or elevation of CVP [35,36]. In addition, hypoxia results in endothelial injury and interferes with nitric oxide and cyclic guanosine monophosphate production [37,38], which may increase capillary leakage, resulting in greater movement of fluid from the vascular to the interstitial space.
MANAGEMENT — In developed countries, most cases of hydrops fetalis will be diagnosed prior to delivery because of the routine use of antenatal ultrasound [39]. Antenatal evaluation and management are discussed separately. (See "Nonimmune hydrops fetalis".)
The delivery of a fetus with hydrops fetalis requires close collaboration between the perinatologist and neonatologist. The care of a known affected case should take place at a tertiary center that can offer intrauterine intervention including prenatal exchange transfusion, expertise in neonatal resuscitation and pediatric surgery, and postnatal mechanical ventilatory support and exchange transfusion.
Postnatal management is divided into initial resuscitation directed towards stabilizing the cardiopulmonary status in the delivery room, assessment to identify the underlying cause, and if possible, directed therapy focused on treating the underlying cause.
Initial resuscitation — At the time of delivery, hydrops fetalis increases the risk of birth trauma due to soft tissue dystocia, postpartum hemorrhage, and retained placenta. In utero aspiration of the pleural fluid or ascites prior to delivery may reduce the risk of dystocia and facilitate neonatal resuscitation. Cesarean birth should be reserved for routine obstetrical indications; however, the high frequency of nonreassuring fetal heart rate patterns and dystocia associated with hydrops fetalis increases the likelihood of cesarean delivery. (See "Nonimmune hydrops fetalis", section on 'Prognosis'.)
Although prenatal ultrasound should provide guidance upon the level of resuscitation required, delivery room management should anticipate the needs of the most severely affected infant. Preparation should anticipate the need to drain fluid if there is significant ascites. Pleural effusions or pericardial effusions compromise ventilation or cardiac output, especially if the fluid has not been drained prior to delivery. If severe anemia is anticipated, unmatched type O, RhD-negative packed red blood cells should be available.
For initial neonatal resuscitation, we suggest using the guidelines jointly developed by the American Heart Association, American Academy of Pediatrics, and International Liaison Committee on Resuscitation that focus on the following three steps. (See "Neonatal resuscitation in the delivery room".)
●Initial steps (provide warmth, position head, clear airway, dry, and stimulate)
●Breathing (ventilation)
●Circulation
Breathing — Most affected infants require endotracheal intubation because of respiratory depression. Intubation may be difficult because of edema of the head, neck, and oropharynx; thus, the most experienced available clinician should perform the procedure.
Drainage of pleural effusions and ascites by needle aspiration may be necessary, if adequate ventilation is not achieved after intubation. Paracentesis and/or thoracentesis are performed using an 18- to 20-gauge angiocatheter attached to a three-way stopcock and syringe. After entry into the chest or abdominal cavity, the needle is removed and the plastic catheter remains in place to prevent damage to either the underlying lung or abdominal organs. Fluid is gently aspirated by syringe. In some cases, cardiocentesis may be required if there is evidence of cardiac tamponade.
Hydropic infants frequently require mechanical ventilatory support, even following drainage, as many will have pulmonary hypoplasia or respiratory distress syndrome [39].
If the infant is born at or before 30 weeks gestation, the initial respiratory support is provided by a nasal continuous airway pressure rescue and administration of surfactant, as these infants are at-risk for respiratory distress syndrome. (See "Management of respiratory distress syndrome in preterm infants".)
Circulation — Pallor, tachycardia, and tachypnea are findings suggestive of either cardiogenic shock or severe symptomatic anemia. Central venous lines are necessary for monitoring pressures and obtaining blood for testing, and if necessary, administration of fluids and medications, and for partial exchange transfusion. Catheterization of the umbilical vein provides quick central venous access.
Although most hydropic infants are normovolemic, fluid resuscitation may be required in infants near cardiovascular collapse. Inotropic support (eg, dopamine) may be required to improve cardiac output, especially in infants with low-output cardiac failure.
If known or presumed fetal anemia is suspected as the underlying cause of hydrops fetalis, a simple careful transfusion or an isovolemic partial exchange transfusion with packed group O, RhD-negative red blood cells should be performed. Isovolemic exchange is preferred, as these infants will have normal or increased vascular volume, and elevated central venous pressure [40].
Assessment — Once the cardiopulmonary systems are stabilized, diagnostic evaluation is performed to determine the underlying cause of hydrops fetalis. Determining the etiology of the hydrops fetalis may direct further evaluation, guide treatment, and provide information for both the prognosis of the affected infant and genetic counseling for future pregnancies.
Assessment includes examination of the infant and the placenta, and diagnostic testing.
Examination of the infant and placenta — The assessment begins with examination of the infant and the placenta.
Neonatal physical findings that are useful in directing the evaluation and management of the affected child include the following:
●Persistent cyanosis that is unresponsive to oxygen suggests the presence of structural heart disease or cardiac myopathy [16-19,41,42]. (See "Diagnosis and initial management of cyanotic heart disease in the newborn".)
●Evidence of dermal hematopoiesis and hepatomegaly, with or without splenomegaly, is suggestive of intrauterine congenital viral infections (eg, cytomegalovirus and rubella) [43,44]. (See "Overview of TORCH infections".)
●Hypotonia may be seen in hydropic infants with congenital myopathy [45-47] or inherited error of metabolism including storage diseases, such as Gaucher disease [48-50]. In addition, cases of congenital hypothyroidism, which is associated with hypotonia, have been reported in hydropic infants [51,52]. (See "Congenital myopathies" and "Gaucher disease: Pathogenesis, clinical manifestations, and diagnosis".)
●In addition to hypotonia, other clinical features of metabolic storage diseases may include hepatomegaly, facial dysmorphism, and cardiomyopathy [53-57].
●Findings suggestive of a chromosomal abnormality may be present, such as congenital lymphedema of the hands and feet, webbed neck, nail dysplasia, high palate, and short fourth metacarpal, as seen in infants with Turner syndrome. Other chromosomal abnormalities associated with hydrops fetalis include Trisomies 21, 18, and 12, and triploidy [4]. (See "Clinical manifestations and diagnosis of Turner syndrome" and "Down syndrome: Clinical features and diagnosis" and "Congenital cytogenetic abnormalities".)
●Other anomalies associated with hydrops fetalis, such as omphalocele and intrathoracic masses, may be detected on physical examination.
The placenta should be examined for evidence of chorioangioma [58], congenital infection [59], and, in the case of twin pregnancies, twin-to-twin transfusion [20,21].
Diagnostic evaluation — Initial studies include blood gas measurements, complete blood count, and, if anemia is suspected, type and cross match.
●Blood gas measurements provide information on the infant's cardiopulmonary status and guide further interventions.
●Complete blood count includes the hematocrit, which can assess whether anemia is present, and if so, the severity of anemia. Examination of the blood smear can determine whether there is an ongoing hemolytic process and may provide clues to the diagnosis of the underlying cause.
Further diagnostic studies include the following:
●Cardiac evaluation − In hydropic infants suspected to have cardiac abnormalities (table 2), we perform an electrocardiogram to detect arrhythmias and echocardiogram to detect cardiac structural and functional abnormalities.
●Thoracic and abdominal evaluation − Radiographs are useful to ascertain the presence and size of pleural effusions, and as a screen for potential mass lesions, and pulmonary or cardiac disease. If there is a suspicion for an intrathoracic or abdominal abnormality, other imaging modalities, such as computerized tomography, ultrasonography, or magnetic resonance imaging provide better resolution to identify the abnormality (eg, cystic adenomatoid malformation [4,27], leiomyosarcoma [25], or pulmonary sequestration [60]) (table 3). Identification of surgically correctable lesions is important as there are case reports of improved outcome with surgical intervention [25,61].
●Evaluation for infection − Both congenital bacterial and viral infections are associated with cases of hydrops fetalis (table 4) [4,62]. Intrauterine infection of the fetus may arise from pathogens that ascend from the vagina and cervix, or are transferred from the maternal circulation through the uteroplacental circulation. Individual infections have characteristic histopathologic findings within the placenta. Thus, placental examination by a pathologist should be performed when infection is considered as a causal or contributing factor for hydrops fetalis.
Congenital viral infections are the most common infectious agents associated with hydrops fetalis [4]. Currently, diagnostic testing for congenital viral infections includes serologic testing and cultures of different body fluids, and molecular techniques, such as polymerase chain reaction. The choice of tests depends upon the organism. (See "Overview of TORCH infections", section on 'Approach to the infant with suspected intrauterine infection'.)
Hydrops fetalis associated with cytomegalovirus (CMV) [4,7], parvovirus [4,7], syphilis [59,63,64], and toxoplasmosis [65] is generally due to anemia and the resultant high-output cardiac failure. There are also cases of hydrops fetalis due to myocarditis from CMV [66], parvovirus [7], and coxsackie virus [67].
●Evaluation of anemia − As discussed previously, prompt intervention with transfusion should be performed in infants with severe symptomatic anemia. Once the cardiopulmonary status of the infant is stabilized, the cause of anemia should be determined or confirmed if there was a predetermined antenatal cause. Anemia can be due to hemolysis, such as isoimmune hemolytic anemia or inherited disorders of red blood cells (eg, homozygous alpha thalassemia or hereditary spherocytosis), blood loss from fetal to maternal hemorrhage [68,69], or reduced fetal red blood cell production (eg, parvovirus B12 infection) (table 5). An unusual form of anemia (transient abnormal myelopoiesis or transient leukemia) is found in 10 percent of newborn infants with Trisomy 21. (See "Overview of hemolytic anemias in children" and "Down syndrome: Clinical features and diagnosis", section on 'Transient myeloproliferative disorder'.)
Examination of the blood smear will determine whether hemolysis is the cause of anemia. If hemolysis is present, further testing includes the infant's blood type, Coombs test, and tests to identify inherited red blood cell disorders. (See "Overview of hemolytic anemias in children".)
If fetal maternal hemorrhage is suspected, Kleihauer-Betke test, which measures the percentage of fetal red blood in the maternal blood by flow cytometry, can be used to support the diagnosis. (See "RhD alloimmunization: Prevention in pregnant and postpartum patients", section on 'Testing for fetomaternal bleeding'.)
●Chromosomal microarray analysis and whole exome sequencing − Chromosomal microarray analysis (CMA) is an array-based molecular cytogenic method of testing for a genetic disorder. Like the conventional prenatal genetic test known as Giemsa(G)-banding of metaphase chromosomes or karyotyping, CMA is now recommended as the first-line genetic test in pregnancies and in infants showing abnormalities on an ultrasound screen or by physical examination. CMA can identify major chromosomal defects such as Down syndrome, as can karyotyping, but it is associated with a higher diagnostic yield and faster turnaround time. CMA can be performed in samples that are obtained through amniocentesis, chorionic villus, or fetal blood sampling. Antenatal CMA provides useful information when reviewing antenatal management options with the parents/caregivers, such as termination of the pregnancy or invasive therapeutic interventions. (See "Nonimmune hydrops fetalis", section on 'Postdiagnostic evaluation' and "Prenatal diagnosis of chromosomal imbalance: Chromosomal microarray".)
If fetal CMA has not been performed, then CMA should be performed regardless of the outcome of the infant because hydrops fetalis is associated with an increased risk of a chromosomal abnormality, including metabolic storage disease and mutations associated with cardiac disease [56,70-74]. Genetic counseling should be provided to parents/caregivers if a chromosomal abnormality is detected to direct the management of future pregnancies.
Whole exome sequencing (WES) is increasingly used to identify single gene causes of hydrops that cannot be diagnosed by conventional testing such as familial hemophagocytic lymphohistiocytosis due to mutation in the gene that encodes for Munc13-4 [75,76].
●Evaluation for inborn errors of metabolic disease − Metabolic disorders result from the absence or abnormality of an enzyme or its cofactor, leading to either accumulation or deficiency of a specific metabolite (table 6) [53-57,77-79]. The outcome for infants/fetuses with hydrops fetalis and an inborn error of metabolic disease is poor. In addition, the recurrence rate for future pregnancies is high, as most of these diseases are due to an inherited genetic defect. As a result, the diagnosis is important for future family planning [74]. The evaluation of inborn errors of metabolism is discussed separately. (See "Metabolic emergencies in suspected inborn errors of metabolism: Presentation, evaluation, and management".)
Further management — As noted previously, mechanical ventilation is often required because of lung hypoplasia. In addition, continued drainage of persistent pleural effusions and ascites may be needed.
Additional management is directed towards the underlying cause of hydrops fetalis and includes:
●Correction of anemia and treatment of hyperbilirubinemia (see "Alloimmune hemolytic disease of the newborn: Postnatal diagnosis and management" and "Initial management of unconjugated hyperbilirubinemia in term and late preterm newborns")
●Correction of cardiac arrhythmias with cardioversion, medication, or pacing
●Surgical excision of resectable masses [25]
●Administration of appropriate antimicrobial agents in infants with infections
OUTCOME
Survival — Despite intensive prenatal and postnatal interventions, live-born infants with hydrops fetalis have a 50 percent survival rate [4,8,39,80-83]. Survival is dependent upon the underlying cause of hydrops fetalis.
In the previously mentioned review of 598 live-born infants with hydrops fetalis, the mortality rate was highest in infants with congenital anomalies (58 percent) and lowest in infants with isolated congenital chylothorax (6 percent) [4]. In a logistic regression analysis, risk factors associated with decreased survival included lower gestational age, five minute Apgar score of 3 or less, and need for greater supportive care defined by high concentration of inspired oxygen and high-frequency ventilatory support.
Neurodevelopment outcome — In survivors of hydrops fetalis, poor neurodevelopment outcome appears to be the most significant morbidity. This was illustrated in the following case series:
●In one case series, 19 of 51 live-born infants with hydrops fetalis survived beyond one year of life [84]. At one year follow-up, 13 patients had normal development, two had mild developmental delay, one had intellectual disability, and three had severe psychomotor retardation, of which two were very low birth weight infants (birth weight less than 1500 g).
●A more recent study from a single center reported the outcomes of 92 pregnancies with hydrops fetalis [85]. Forty five percent died in utero. Of the 33 patients that survived to one year, 15 were developmentally normal.
●In a retrospective study of 11 infants with hydrops fetalis due to fetal tachycardia, of which 10 received intrauterine treatment, neonatal cranial ultrasonography demonstrated periventricular echogenicity in three patients (including one patient with a pseudocyst), and porencephalic cyst suggestive of an antenatal parenchymal hemorrhage in another patient [86]. The other seven infants had normal studies. Ten patients were discharged from the neonatal intensive care unit (NICU) and were assessed for cognitive and neurologic function between 6 months and 12 years of age. Eight patients had normal neurologic and cognitive function, one patient had mild hemiplegia with normal cognitive ability, and one had cognitive developmental delay.
●Prognosis is generally poor in patients with untreated twin-twin transfusion because they are frequently delivered prior to 30 weeks gestation. In one case series of 25 pregnancies with twin-twin transfusion, hydrops fetalis presented in 10 cases [21]. Two-thirds of hydropic infants died and, of the survivors, half had cerebral palsy. Outcomes for infants treated with laser photocoagulation are considerably better [87].
Because of the findings in these studies, neurodevelopmental evaluation should be provided to all survivors of hydrops fetalis [81].
Recurrence — The risk of recurrence depends upon the underlying etiology. The recurrence rate is greatest in families with infants who have a chromosomal abnormality. Therefore, every effort should be made to determine the cause of hydrops including autopsies and chromosomal microarray analysis (CMA) of non-surviving infants and non-viable fetuses.
SUMMARY AND RECOMMENDATIONS
●Hydrops fetalis is defined as the presence of two or more of the following fetal findings: skin edema, pleural effusion, pericardial effusion, ascites, and polyhydramnios. It may be caused by immune or nonimmune-mediated processes.
●With the widespread use of antenatal Rh(D) immune globulin prophylaxis, nonimmune causes account for 90 percent of the cases of hydrops fetalis. The causes of nonimmune hydrops fetalis are heterogeneous and include cardiac, pulmonary, metabolic, hematologic, and infectious etiologies (table 1). (See "Nonimmune hydrops fetalis", section on 'Etiology and prenatal management of disorders associated with hydrops'.)
●Although the pathogenesis is not clearly understood, hydrops fetalis appears to be multifactorial due to mechanisms that produce elevated central venous pressure, impair lymphatic return, and increase capillary leakage. (See 'Pathogenesis' above.)
●In developed countries, with the routine use of antenatal ultrasound, most cases of hydrops fetalis will be diagnosed prior to delivery. Intrauterine evaluation and management provided at a tertiary center may improve perinatal outcome. (See "Nonimmune hydrops fetalis".)
●The delivery of a fetus with hydrops fetalis requires close collaboration between the perinatologist and neonatologist. We suggest that delivery should be at a tertiary center with expertise in neonatal resuscitation, pediatric surgery, exchange transfusion, and mechanical ventilation (Grade 2C).
●Postnatal management is divided into initial resuscitation, assessment to identify the underlying cause, and treatment of the underlying cause, if possible. (See 'Management' above.)
●Initial resuscitation is directed towards stabilizing the cardiopulmonary status of the infant. We suggest following guidelines jointly developed by the American Heart Association, American Academy of Pediatrics, and International Liaison Committee on Resuscitation for initial neonatal resuscitation (Grade 2C). (See "Neonatal resuscitation in the delivery room".)
●In addition, delivery room management includes preparation to drain fluid if significant ascites and/or pleural effusions compromise ventilation, and the availability of unmatched type O, RhD-negative packed red blood cells for transfusion in cases of severe anemia. (See 'Initial resuscitation' above.)
●Once the cardiopulmonary system of the infant is stabilized, diagnostic evaluation is performed to determine the underlying cause of hydrops fetalis. Determining the etiology of the hydrops fetalis may direct further evaluation, guide treatment, and provide information on prognosis of the affected infant and genetic counseling for future pregnancies. (See 'Assessment' above and 'Further management' above.)
●The mortality rate for live-born infants with hydrops fetalis is approximately 50 percent and varies depending on the underlying cause of hydrops fetalis. (See 'Outcome' above.)
