To determine the association between smoking in pregnant teenagers and baby birthweight.

A retrospective population-based study of women aged < 20 years who gave birth to liveborn singletons in Australia between January 2001 and December 2004. Data were drawn from the National Perinatal Data Collection.

Maternal smoking, birthweight, low birthweight (LBW).

The prevalence of LBW in babies born to teenage smokers was 9.9%, compared with 6.0% in babies born to teenage non-smokers (odds ratio [OR], 1.72 [95% CI, 1.57–1.90]). On average, babies born to teenage smokers were 179.8 g lower in birthweight than babies born to teenage non-smokers (95% CI, 165.5 –194.1 g; t = 24.6, P < 0.001). Smoking, Indigenous status, Socio-Economic Indexes for Areas category and parity were independently associated with LBW (all ORs > 1.3; P < 0.001) after adjusting for maternal age group. Teenagers smoking > 10 cigarettes a day had babies with lower birthweight that those who smoked ≤ 10 cigarettes a day, demonstrating a dose–response relationship. The babies of teenage smokers who stopped smoking before 20 weeks’ gestation had birthweights similar to those of babies born to teenage non-smokers. One in 15 teenage smokers stopped smoking during pregnancy.

Babies whose mothers smoked during pregnancy were more likely to have LBW than babies whose mothers did not smoke. Mothers who continue to smoke in the second half of pregnancy increase their baby’s risk of LBW. There is significant scope to improve the quitting rate, and health professionals need to target smoking cessation at all contacts with pregnant women who continue to smoke.