Sampling pure fetal blood by fetoscopy in second trimester of pregnancy.

A technique for fetal blood-sampling in the second trimester of pregnancy (between 16 and 22 weeks' gestation) combining fetoscopy with real-time ultrasound was used in 48 attempts at fetal blood-sampling. Specimens containing fetal red cells with or without amniotic fluid or maternal blood, and adequate for diagnosing haemoglobinopathies, were obtained in 45 of the 48 fetoscopies. Sampling was successful in all 18 patients with a posterior placenta, and in 27 of the 30 with an anterior placenta. In 22 of the last 27 consecutive fetoscopies pure fetal blood was taken; the placenta was anterior in 16 and posterior in six. Out of 17 cases sampled between 18 and 22 weeks' gestation pure fetal blood was obtained in 16. The volume of the samples varied from 50 to 500 microliter. The ability to obtain pure fetal blood consistently even when the placenta is anterior will increase knowledge of fetal physiology and the scope of prenatal diagnosis.