Ultrasound measurement of placental thickness to detect pregnancies affected by homozygous alpha-thalassaemia-1.

Homozygous alpha-thalassaemia-1, a common cause of hydrops fetalis, is usually diagnosed invasively. We measured placental thickness by ultrasound at 10-21 weeks' gestation in 231 at-risk pregnancies. 60 were affected (by DNA analysis). The sensitivity and specificity in detecting affected pregnancies at cut-off of mean placental thickness plus 2 SD before 12 weeks' gestation were 0.72 (95% CI 0.52-0.93) and 0.97 (0.9-1.0). After 12 weeks sensitivity increased to 0.95 (0.89-1.0) and by 18 weeks reached 1.0 (0.60-0.99) without change in specificity. Selection of pregnancies at risk by measurement of placental thickness will reduce the number of invasive diagnostic procedures.