Ultrasound placental site in relation to bloody taps in midtrimester amniocentesis.

In an unselected, consecutive series of 137 midtrimester amniocenteses for prenatal genetic diagnosis, it was found that the incidence of blood contamination of amniotic fluid samples was similar regardless of placental site as determined by ultrasound localization. Three (5.8%) patients with anterior placental location versus 5 (9.1%) patients with posterior placentas had gross blood in the amniotic fluid by visual inspection. These results support the view that placental penetration during amniocentesis is not the major cause of bloody taps. Ultrasound studies done prior to midtrimester amniocentesis should, perhaps, be more specifically evaluated in relation to pregnancy information obtained rather than solely as an aid to improve technical performance of amniocentesis.