Fetal Ascites

Fetal ascites is the collection of free fluid in the fetal peritoneal cavity, found on ultrasound as an anechoic rim around the bowel and liver. It may be isolated or part of hydrops fetalis. Causes include infection (parvovirus, cytomegalovirus, syphilis, toxoplasma), urinary tract obstruction, meconium peritonitis, anaemia, heart disease and chromosomal abnormality. It matters because the cause decides treatment and outcome, which ranges from full recovery to fetal death.

• Isolated ascites with normal other findings has a better outlook.
• Risk factors: maternal infection, red cell antibodies, fetal anomalies.
• Tense ascites can cause dystocia and lung compression.

Women usually have no symptoms. Fundal height may be larger than dates from fluid or polyhydramnios, with reduced fetal movements in advanced disease.

• Large abdomen for dates.
• Rash or fever in infection.
• Differential diagnosis: hydrops fetalis, urinary ascites from posterior urethral valves or bladder rupture, meconium peritonitis, fetal anaemia, cardiac failure, chylous ascites, intestinal perforation, ovarian cyst, normal small peritoneal fluid.
• Complications: hydrops fetalis, pulmonary hypoplasia, abdominal distension causing dystocia, stillbirth, neonatal death.

The approach is a detailed scan with tests for the commonest causes.

• Ultrasound: volume of fluid, echogenic bowel, calcification (meconium peritonitis), bladder size and kidney appearance, other effusions, skin oedema, heart size; middle cerebral artery peak systolic velocity for anaemia.
• Maternal tests: blood group and antibody screen, syphilis, HIV, TORCH serology, parvovirus B19 IgM and IgG, indirect Coombs test, Kleihauer test, haemoglobin electrophoresis.
• Fetal echocardiography for structural heart disease and arrhythmia.
• Amniocentesis for karyotype, cytomegalovirus PCR and parvovirus PCR where available.

The principle is to find and treat the cause, with drainage for tense ascites that compromises the fetus.

• Supportive: refer to a fetal medicine unit; serial scans every 1 to 2 weeks; daily fetal movement counts; deliver in a unit with neonatal intensive care.
• Pharmacological: syphilis: benzathine benzylpenicillin 2.4 million units IM once weekly for 3 weeks (or procaine benzylpenicillin 1.2 million units IM daily for 10 days); cytomegalovirus infection: valaciclovir 2 g oral 6-hourly; parvovirus or alloimmune anaemia: intrauterine transfusion with betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days; tachyarrhythmia: digoxin 0.5 mg oral 8-hourly for 3 doses then 0.25 mg oral 12-hourly; toxoplasma infection: spiramycin 1 g oral 8-hourly until delivery.
• Procedures: ultrasound guided paracentesis for tense ascites that threatens lung development or delivery (drain enough fluid to relieve pressure); vesicoamniotic shunt for obstructive uropathy; delivery at term, earlier for hydrops that worsens, with paracentesis of the newborn for respiratory distress.
• Red flags: hydrops, pleural effusion, polyhydramnios, reduced fetal movements, rapid fluid increase, maternal rash or fever.
• Follow-up: scan every 1 to 2 weeks; weekly Doppler if anaemia is suspected; newborn abdominal ultrasound and renal function test at birth; genetic counselling.