Diagnosis of sirenomelia in the first trimester.

Sirenomelia or "mermaid syndrome" is a rare congenital abnormality with an incidence of 1 in 60,000. We report a case diagnosed in the first trimester using two-dimensional, three-dimensional, and color Doppler ultrasound. With increasing emphasis on early diagnosis of fetal abnormalities, this case highlights the importance of looking for anomalies in the first trimester itself.