When your baby has fluid buildup, our team works through a careful, step-by-step evaluation to find the cause and guide your care.
Hydrops fetalis is the presence of abnormal fluid collections in two or more fetal body compartments, detected by ultrasound. The word "hydrops" comes from the Greek word for water.
The fluid can appear in any combination of these four areas:
Fluid around the lungs
Fluid around the heart
Fluid in the abdomen
Fluid under the skin (>5mm)
Additional ultrasound findings may include a large placenta (placentomegaly) and excess amniotic fluid (polyhydramnios). Non-immune hydrops fetalis (NIHF) means the fluid buildup is not caused by blood type incompatibility between mother and baby — this is the most common type, accounting for about 90% of all hydrops cases.
Hydrops is not a disease itself — it is a sign of an underlying problem. Finding that cause is the most important goal of the evaluation. There are many possible causes, grouped into the categories below.
Conditions like Down syndrome (trisomy 21), Turner syndrome (monosomy X), Noonan syndrome, and other single-gene disorders (RASopathies).
Structural heart defects, fast heart rates (SVT, atrial flutter), or slow heart rates (heart block) that impair the heart's ability to pump blood effectively.
Severe anemia from inherited conditions (like alpha-thalassemia), parvovirus infection, or fetal-maternal bleeding leads to heart failure and fluid accumulation.
Parvovirus B19 is the most common infectious cause. Others include cytomegalovirus (CMV), toxoplasmosis, syphilis, and varicella.
Large lung masses like a congenital pulmonary airway malformation (CPAM) or bronchopulmonary sequestration can compress the heart and block venous return.
In identical twin pregnancies, twin-to-twin transfusion syndrome (TTTS) can cause one twin to become severely fluid-overloaded.
Placental tumors (chorioangiomas) can act as high-flow shunts, causing the baby's heart to work too hard and leading to high-output cardiac failure.
Abnormal development of the lymphatic system prevents proper fluid drainage. Often associated with genetic syndromes like Noonan or Milroy disease.
Use this tool to understand what evaluation steps are typically recommended based on your baby's findings. Select all that apply to your situation.
The evaluation for hydrops fetalis follows a structured algorithm recommended by the Society for Maternal-Fetal Medicine (SMFM). Think of it as a systematic search to identify the underlying cause.
Your doctor reviews your family history (for genetic conditions), medical history, and all prior lab results. Blood type and antibody screening (Indirect Coombs test) are repeated. A complete blood count and hemoglobin electrophoresis help screen for inherited anemias like thalassemia.
A detailed anatomic survey examines every organ system. A fetal echocardiogram (specialized heart ultrasound) is performed to evaluate the heart's structure and rhythm. The placenta, umbilical cord, amniotic fluid, and fetal growth are also assessed.
The Middle Cerebral Artery (MCA) Doppler measures blood flow velocity in a key brain artery. When a baby is anemic, the heart pumps faster to compensate, and this shows up as an elevated MCA Doppler value (≥1.5 MoM). This is a non-invasive way to screen for fetal anemia before considering more invasive testing.
A small sample of amniotic fluid is collected by a thin needle under ultrasound guidance. This fluid is used for: Chromosomal Microarray Analysis (CMA) to detect genetic abnormalities, and PCR testing for infections like parvovirus, CMV, and toxoplasmosis. SMFM recommends this testing for all pregnancies with one or more fetal effusions.
If the MCA Doppler is elevated (≥1.5 MoM), a procedure called Percutaneous Umbilical Blood Sampling (PUBS) may be recommended. This directly measures the baby's blood count and can confirm the cause and severity of anemia. If severe anemia is found, an intrauterine transfusion can be performed at the same time.
If the chromosomal microarray and other tests do not find a cause, SMFM recommends offering exome or genome sequencing. This advanced genetic test looks at the protein-coding regions (exome) or the entire genetic code (genome) to identify single-gene disorders — such as RASopathies, inborn errors of metabolism, or other rare conditions — that are not detectable by standard chromosomal testing.
Outcomes in hydrops fetalis vary widely and depend heavily on the underlying cause, gestational age, and response to treatment. The following table summarizes how the prognosis differs by cause.
| Underlying Cause | Prognosis | Notes |
|---|---|---|
| Tachyarrhythmia (SVT) | More Favorable | Often treatable with medications given to the mother |
| Parvovirus Infection | More Favorable | Anemia often transient; intrauterine transfusion highly effective (~85% survival) |
| Lymphatic Dysplasia | More Favorable | Better long-term neurodevelopmental outcomes reported |
| Lung Masses (CPAM) | Variable | Prognosis worsens with larger lesion size (CPAM volume ratio ≥1.6) |
| Chromosomal Aneuploidy | Variable | Depends on specific diagnosis (e.g., trisomy 21 vs. trisomy 13/18) |
| Trisomy 13 or 18 | Guarded | Associated with high risk of morbidity and mortality |
| Inborn Errors of Metabolism | Guarded | Cardiac anomalies and metabolic disorders carry higher risk |
Overall, among live-born neonates with NIHF, survival ranges from 36% to 68% in published series. Your MFM specialist will provide individualized counseling based on your specific situation.