Objectives: To measure quantitatively and objectively the maternal and fetal tobacco exposure during pregnancy and its neonatal effects.
Design: Tobacco exposure was assessed from maternal serum samples, obtained during the first half of pregnancy and from umbilical serum samples obtained at delivery, by measuring the concentration of nicotine metabolite, cotinine. Data on the respective pregnancies and neonates were collected from the Finnish Medical Birth Registry.
Subjects: One thousand two hundred and thirty-seven pregnancies and newborns, representing all pregnancies resulting in a liveborn infant during one week in one country.
Main outcome measures: Gestational age, birthweight and crown-heel length of newborns.
Results: Cotinine (> 6 micrograms/l) was detected in either maternal or umbilical serum in 300 pregnancies, and these mothers and newborns were classified as exposed. Important differences occurred between measured exposure and reported smoking behaviour. Of the exposed mothers, 38% were nonsmokers and 3.4% of the nonexposed mothers were smokers. Tobacco exposure was associated with shorter gestational age, reduced birthweight and shorter crown-heel length of the newborns. After correction for parity, gender, and gestational age, the exposed newborns were on average 188 g (95% confidence interval (CI) 123-253 g) lighter and 10 mm (95% CI 7-13 mm) shorter than the nonexposed newborns. One micrograms/ml of cotinine in maternal serum resulted in a mean decrease of 1.29 g (95% CI 0.55-2.02 g) in birthweight and in a mean decrease of 0.059 mm (95% CI 0.035-0.083 mm) in birth length. Maternal cotinine concentrations better explained the neonatal findings than the reported smoking habits.
Conclusions: There is a quantitative dose and effect relation between tobacco exposure and a decrease in the gestational age at birth and size of the neonate. The smoking habit reported by mothers themselves is not an accurate measure of fetal tobacco exposure.