OBJECTIVE As part of an exercise in establishing normograms of hematological parameters in pregnancy, we studied the red cell distribution width (RDW) in healthy pregnant women. METHODS A longitudinal study of RDW measurements in 121 pregnant women at 16 and 34 weeks gestation and during labor and at Days 3 and 7 postpartum. All the women had uncomplicated pregnancies, minimum hemoglobin (Hb) of 11.0 g/dl at recruitment and took iron supplements from 16 weeks of gestation and until 7 days after delivery. All subjects went into spontaneous labor, 110 achieving a normal vaginal delivery while the remaining 11 were delivered by cesarean section. Two-way analysis of variance was used to study the changes in RDW between any given gestations to test the variability between and within subjects. RESULTS RDW increased significantly (P < 0.0001) between 34 weeks of gestation and the onset of labor. No significant changes occurred between 16 and 34 weeks gestation, or during the 7 days postpartum. CONCLUSIONS This is the first longitudinal study analyzing the between and within women changes in RDW with progression of pregnancy. The unexpected rise in the RDW during the last 4-6 weeks leading up to the onset of labor suggests increased bone marrow activity. The stimulus is unknown, but as RDW changes are highly significant there may well be a useful indicator of impending parturition.