Timing of Prenatal Smoking Cessation and Infant Birth Weight: Evidences on the Threshold

Purpose: This study examines the threshold month when pregnant women must quit smoking to nullify the adverse impact of prenatal smoking on infant birth weight.

Significance: This research is important because first it contributes to the heated epidemiological debate on the critical period over pregnancy when women should stop smoking to produce healthy babies. Second, its finding can significantly improve the cost-effectiveness of any prenatal smoking cessation program. Third, since in the long run heavier babies have higher educational achievements and better labor market performance, this study implies that public interventions at the “right” timing over pregnancy can lead to lasting lifetime benefits for the infants.

Method: Multiple regression analyses are applied into a large UK cohort data of 15,351 infants which contains detailed information on the timing of maternal smoking cessation. The study starts with the effect of smoking cessation over different trimesters on birth weight, and then examines the effect month by month. A rich set of covariates are used to control for confounding factors. A series of principal component analyses and model selection are also applied.

Results and Implications: The fourth month is an important threshold such that smoking cessation prior to it can nullify the negative impact of prenatal smoking. There is a rapid accumulation of the adverse smoking impact in the fourth month, accounting for about 57 percent of the total impact of smoking throughout a 9 month pregnancy. This result implies a huge gain in stopping mothers from smoking no later than the fourth month.