Maternal serum C-reactive protein (CRP) has been studied extensively as an adjunct in the diagnosis of subclinical infection among pregnant women with preterm labor or preterm rupture of membranes. However, before the utility of CRP can be studied in pregnancies with these complications, the effects of normal pregnancy and labor on maternal serum CRP levels must be established. We determined CRP levels serially from 22 weeks' gestation until delivery in healthy pregnant women without antepartum complications. Median CRP values for women not in labor ranged from 0.7-0.9 mg/dL, depending on gestational age; 95% of the values were 1.5 mg/dL or lower. No consistent change in CRP levels with gestational age was found among serially sampled women not in labor. The median CRP value for women in labor at term was 1.3 mg/dL, and 32% of values were over 1.5 mg/dL. Median CRP values in normal pregnancies appear to be higher than standardized values for nonpregnant individuals, and CRP values are further elevated in labor. Understanding the physiology and temporal course of the increase in CRP in normal pregnancy and labor may help to clarify the appropriate use of CRP in complicated pregnancies.