Abstract Background. Smoking during pregnancy is associated with reduced birthweight; this relation can be reversed by smoking cessation. Some but not all previous studies have shown that smoking reduction (measured as cigarettes/day or urinary cotinine) may also improve birthweight. The relationship between maternal and spouses' expired air carbon monoxide (CO) concentrations (EACO) on fetal growth has not yet been evaluated. Methods. Eight hundred fifty-six smoking and nonsmoking pregnant women were followed during their pregnancy. Their EACO was determined in the first trimester and during delivery. The spouses' EACO were also measured at delivery. The main outcome measure was the infants' birthweight. Secondary measures included head circumference, Apgar score, and heart rate at delivery. Cord blood fetal carboxyhemoglobin (FCOHb) served as internal control. Results. Birthweight dose-dependently and significantly decreased with increasing level of maternal (0–5: 3406 ± 32; 6–10: 3048 ± 57; 11–20: 2858 ± 54; >20 ppm: 2739 ± 34 g, P < 0.0001) or spouses' EACO (0–5: 3546 ±25; 6–10: 3484 ± 51; 11–20: 3309 ± 47; >20 ppm: 3190 ± 57 g, P < 0.0001). Even the birthweight of newborns whose mother had EACO between 6 and 10 ppm was significantly lower than the birthweight of newborns whose mother had an EACO between 0 and 5 ppm. Spouses' EACO of delivering women with EACO of 0–5 ppm showed similar effect. Head circumference, Apgar score, and normal term gestational age decreased also significantly with increasing maternal or spouses' EACO. Conclusions. Both maternal and spouses' EACO measured during delivery, a proxy of EACO during pregnancy, were dose-dependently and inversely associated with fetal growth. Even low maternal (6 to 10 ppm) or spouses' (11 to 20 ppm) EACO may be associated with significantly lower birthweight.