Timing of Prenatal Smoking Cessation or Reduction and Infant Birth Weight: Evidence from the United Kingdom Millennium Cohort Study

Objectives: Smoking during pregnancy is a key preventable risk contributor to poor infant health. Our study presents a full dynamic relationship between the timing of prenatal smoking cessation or reduction and infant birth weight. Methods: Using a large representative dataset of birth cohort in the United Kingdom, we apply multiple linear regressions to examine how smoking cessation or reduction at different stages especially different months of pregnancy affects infant birth weight. For robustness checks, a rich set of covariates are controlled and a series of variable selection procedures are used. Results: We find robust evidence that mothers who quit smoking before the fourth month of pregnancy have infants of the same weight as those of nonsmokers. However, cessation after the fourth month or smoking beyond this threshold month is associated with substantially lower infant birth weights. Two third of the total adverse smoking impact on infant birth weight occurs in the second trimester. Our study also shows heavy persistent smokers should cut smoking intensity before the fourth month during pregnancy, in order to deliver infants as heavy as those born to persistent light smokers. Conclusions: Overall, our research suggests that the cost effectiveness of prenatal smoking cessation services can be significantly improved, if they can encourage more pregnant women to quit or reduce smoking by the end of the first trimester. Promoting timely smoking cessation or reduction can help policy makers and medical practitioners reduce the huge healthcare costs of poor birth outcomes attributable to prenatal smoking. Key Words: Prenatal Smoking; Timing of Smoking Cessation; Timing of Smoking Reduction; Infant Birth Weight