Thirty-two patients with Rh-immunized pregnancies had amniocenteses between 16 and 20 weeks' gestation. Serial amniotic fluid values of delta optical density at 450 nm were studied to predict the severity of Rh disease. A value greater than 0.15 indicated severe Rh immunization. Values less than 0.09 indicated mild or no Rh disease; delta optical density at 450 nm between 0.09 and 0.15 require further evaluation to determine the trend. Midtrimester amniocentesis is a simple technique that helps to determine which patients may or may not require fetal blood sampling.

Spectrophotometric analysis of amniotic fluid has become the standard for assessing the fetal condition in Rh-immunized pregnancies. Serial amniocentesis is usually started at 28 to 29 weeks of gestation unless the antibody titer or history indicates it should be done earlier. This study presents the values from 14 to 20 weeks in normal pregnancies, which will also assist in evaluating pathologically elevated values.