Non-immune hydrops fetalis was rare in Sweden during 1997-2015 but cases were associated with complications and poor prognosis.

The study was designed to document the incidence of non-immune hydrops fetalis (NIHF) at birth and characterise associated outcomes and obstetric complications. Data on more than 1.9 million births were extracted from the Swedish Birth Register for 1997-2015. Pregnancies not affected by NIHF served as controls. National registers on mortality and hospitalisations provided follow-up information. There were 309 cases of NIHF at birth corresponding to an incidence of 1.6 per 10,000, lower than in previous studies. NIHF was more frequent in mothers aged ≥ 35 years and with a history of stillbirth. Preterm delivery occurred in 77.7% in the NIHF group, including 31.7% before 32 weeks of gestation. Multiple births and Caesarean sections were reported more frequent in the NIHF group. NIHF was associated with poor outcome with 14.6% stillbirths and in 26.5% early neonatal death. Overall, 58.7% of live born children with NIHF were alive at 12 months compared with 99.7% of controls. The most common causes of death were cardiovascular diseases and thoracic abnormalities. NIHF at birth is associated with obstetric complications and poor prognosis for the neonate related to underlying disease. The low incidence of NIHF observed in this study may reflect well-developed antenatal care.