Early identification of fetal hydrothorax: investigating causes and potential significance as a new sonographic marker for adverse pregnancy outcomes

Fetal pleural effusion is an uncommon anomaly caused by the buildup of fluid in the thoracic cavity. The outcome of fetal pleural effusion, when detected during the 2nd or 3rd trimester, can differ greatly, ranging from natural resolution to perinatal mortality. The improvement of prenatal diagnosis methods allowed the early identification of this anomaly from the first trimester. The purpose of this study is to investigate the causes of pleural effusion discovered in the first weeks of pregnancy. This case-series study included six patients in whom, during the first trimester ultrasound, hydrothorax was discovered. Karyotyping was performed in all these cases. The serial ultrasound evaluations that followed after initial evaluation demonstrated the cessation of the pregnancy in all cases, as well as the subsequent resorption of the liquid. All pregnancies miscarried by 9 weeks. Karyotype was abnormal in 5 of 6 cases: 3 cases were Trisomy 18, 1 case was Turner syndrome and 1 case was a triploidy. It may be worthwhile to investigate the potential significance of embryonic/fetal pleural effusion as a novel sonographic indicator for predicting adverse pregnancy outcomes. Thus, we propose a new ultrasound sign, “the crescent sign”, which when discovered raises a high suspicion of non-viable pregnancy. Our findings related to fluid resorption after termination of pregnancy could indicate that the incidence of the anomaly is higher than expected.