Objective: Comparison of fetal and maternal carboxyhemoglobin (COHb) levels at cordocentesis and the relationships of the two variables among groups with different endogenous CO production and exogenous CO exposure.

Methods: Whole blood COHb was measured by gas chromatography in 40 control, 12 Rhisoimmunized and 13 smoker maternal-fetal pairs at cordocentesis. In the smoker group, plasma cotinine, a nicotine metabolite, was measured by radioimmunoassay.

Results: Fetal and maternal COHb levels were 3- to 4-fold higher in smokers compared to control and Rh isoimmunized groups (P < 0.0001). Fetal COHb levels were higher than maternal levels in all groups, this difference achieving statistical significance only for the Rh-isoimmunized group (P = 0.005). Fetal and maternal COHb levels correlated in the control and smoker groups (r = 0.52, P = 0.0002, and r = 0.72, P = 0.003, respectively), but not in the Rh-isoimmunized group. The slopes of the fetal versus maternal COHb regression plots were steepest for the smoker group (1.21 ± 0.33), flattest for the control group (0.38 ± 0.92) and intermediate for the Rh isoimmunized group (0.91 ± 0.47). In the smokers, fetal plasma cotinine levels correlated strongly with fetal and maternal COHb (r = 0.88,P < 0.0001, and r = 0.81 P = 0.0003, respectively).

Conclusions: Differences between groups in antepartum fetal with maternal COHb concentrations are likely the result of the endogenous CO production and exogenous CO exposure. COHb and plasma cotinine among smokers are equivalent indicators of cigarette exposure.