A rare case of fetal sirenomelia malformation in the third trimester with its ultrasound appearance and review of the literature.

Sirenomelia is a very rare congenital anomaly and is the most severe form of fetal caudal agenesis, characterised by partial or total fusion of the lower limbs, generally associated with severe abnormalities of the urinary and digestive systems, including disappearance of the kidneys, which can be fatal. Our aim was to diagnose this malformation as early as possible in pregnancy, in order to distinguish it from caudal regression syndrome, and to monitor the obstetric complications associated with this pathology. Given that the patient decided to carry the pregnancy to term, we succeeded in monitoring the complications of this malformative pathology. We present a case of sirenomelia diagnosed in the first trimester at 29 weeks' amenorrhoea and a quarter days, in a patient with no particular pathological, toxic or psychological history, no consanguinity and in whom the clinical examination noted no particular physical or biological abnormality. We discuss the criteria for ultrasound diagnosis, common malformative associations and the importance of studying umbilical vascularisation using colour Doppler. Sirenomelia is a very rare congenital anomaly and is the most severe form of fetal caudal agenesis incompatible with extrauterine life. Sirenomelia should be diagnosed at the second trimester ultrasound at the latest.

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Conflict of interest statement The authors declare having no conflicts of interest for this article.