Nucleated red blood cells (NRBCs) isolated by a triple density gradient from 100 ml peripheral blood samples of 100 pregnant women and 30 women postpartum were subjected to morphological analysis and PCR quantitation. The number of NRBCs steadily increased from 5.3 (frequency: 2.4 x 10(-7)) in early gestation to 98.2 (frequency: 4.2 x 10(-6)) near term. The number of male cells increased from 6 (frequency: 2.7 x 10(-7)) in early gestation to a peak of 31 (frequency: 1.48 x 10(-6)) in the second trimester, and slightly decreased to 27 (frequency: 1.31 x 10(-6)) near term. Both NRBCs and male fetal cells rapidly disappeared after delivery. The result implies that a significant proportion of NRBCs in maternal blood are of fetal origin before 24 weeks of gestation while in late gestation the majority of NRBCs may be of maternal origin.