Progressive non-immune hydrops fetalis associated with RASA1 mutation: prenatal imaging and genomic insights

Hydrops fetalis (HF) is the pathological accumulation of fluid in two or more fetal compartments. While immune-mediated HF was historically predominant, non-immune hydrops fetalis (NIHF) is now increasingly common. Advances in genetic testing have revealed monogenic causes, including RASopathies (a group of genetic syndromes caused by dysregulation of the RAS–mitogen-activated protein kinase signalling pathway). We report a young primigravida, referred at 20+3 weeks for unilateral fetal pleural effusion. Serial ultrasound scans showed rapid progression to bilateral effusion, hepatomegaly, polyhydramnios and NIHF. First-trimester screening and non-invasive prenatal testing were low-risk, Toxoplasmosis, Other agents, Rubella, Cytomegalovirus, Herpes simplex serologies and PCR ruled out infection. Whole exome sequencing identified a likely pathogenic heterozygous frameshift mutation in the RASA1 gene (c.723dupT; p.Gly242TrpfsTer23), associated with capillary malformation–arteriovenous malformation type 1 (CM-AVM1). The mother had a subtle congenital capillary haemangioma on her left hand and revealed the same heterozygous variant of RASA1 gene, indicating variable expressibility. This case highlights the importance of considering rare monogenic causes like RASA1-related CM-AVM1 in NIHF, especially when early findings precede florid hydrops and classical RASopathy features are absent.