AbstractObjectives To compare pregnancy outcomes between women who stopped smoking in early pregnancy and those who either did not smoke in pregnancy or continued to smoke.Participants 2504 nulliparous women participating in the Screening for Pregnancy Endpoints (SCOPE) study grouped by maternal smoking status at 15 (1) week’s gestation.Main outcome measures Spontaneous preterm birth and small for gestational age infants (birth weightResults 80% (n=1992) of women were non smokers, 10% (n=261) had stopped smoking, and 10% (n=251) were current smokers. We noted no differences in rates of spontaneous preterm birth (4%, n=88 v 4%, n=10; adjusted odds ratio 1.03, 95% confidence interval l0.49 to 2.18; P=0.66) or small for gestational age infants (10%, n=195 v 10%, n=27; 1.06, 0.67 to 1.68; P=0.8) between non smokers and stopped smokers. Current smokers had higher rates of spontaneous preterm birth (10%, n=25 v 4%, n=10; 3.21, 1.42 to 7.23; P=0.006) and small for gestational age infants (17%, n=42 v 10%, n=27; 1.76, 1.03 to 3.02; P=0.03) than stopped smokers.Conclusion In women who stopped smoking before 15 weeks’ gestation, rates of spontaneous preterm birth and small for gestational age infants did not differ from those in non smokers, indicating that these severe adverse effects of smoking may be reversible if smoking is stopped early in pregnancy.IntroductionA causal and dose dependent relationship has been established between smoking during pregnancy and the risk of having a small for gestational age infant and of spontaneous preterm birth.1 2 Women who smoke also have raised risks of pregnancy loss with increased rates of miscarriage, ectopic pregnancy, stillbirth, and neonatal death.2 3 4 The costs of maternal and infant complications due to smoking during pregnancy are enormous, with costs for the United States estimated to exceed $1.4bn (1bn, 1.1bn) in 1995.5 More than 90% of the costs for infants are attributable to care for those with low birth weight in the neonatal period and the first year of life.6 From a population perspective, smoking is the most modifiable risk factor for adverse pregnancy outcomes in developed countries.Although stopping smoking in pregnancy has been known for some time to reduce the rate of low birthweight infants and total preterm births,7 very few studies have addressed whether there is a critical gestation time by which smoking must stop to prevent smoking induced complications of pregnancy.8 9 Surprisingly, no studies to date have specifically addressed whether stopping smoking in early pregnancy reduces the rate of spontaneous preterm birth or small for gestational age infants.