Severe adverse effects can be avoided if smoking is stopped early The harm to infants of in utero exposure to tobacco smoke from maternal smoking is well established,1 2 and a quarter of a century has passed since it was first shown in a clinical trial that stopping smoking during pregnancy could avoid the adverse effects of smoking on birth weight.3 The findings from the linked cohort study by McCowan and colleagues (doi:10.1136/bmj.b1081) provide new observational evidence suggesting that for healthy women having their first baby, quitting smoking early in pregnancy (before 15 weeks’ gestation) can significantly reduce rates of spontaneous preterm birth, small for gestational age, and complicated pregnancies compared to those of non-smokers.4 Continuing to smoke was associated with an almost threefold increase in the rate of spontaneous preterm birth (4%, 4%, and 10% for non-smokers, stopped smokers, and current smokers, respectively) and a nearly twofold increase in the rate of small for gestational age infants (10%, 10%, and 17%, respectively). In the United States, up to 40% …