Maternal smoking accounts for 20%-30% of low birth weight (BW). Second-Hand Smoke (SHS) also negatively affects BW. This cohort study explored the differential effect of smoking patterns during pregnancy on infant BW. Smoking status for 652 self-reported smokers attending public ante-natal clinics was assessed at baseline (V1 first ante-natal visit), 28-32 weeks (V2) and one week after birth (V3). Multivariable generalised linear regression models tested smoking patterns (continuing to smoke, sustained quitting, partial quitting) on BW adjusting for household smoking and other co-variates. Total quitting showed a median increase of 288 g in BW (95% CI (confidence intervals): 153.1-423 g, p < 0.001), compared to partial quitting (147 g, (95% CI: 50-244 g), p < 0.003). In partial quitters, increased BW was observed only in females 218 g, (95% CI: 81-355 g), p = 0.002). Household SHS showed a specific negative influence on pre-term but not term BW. This study suggests that, for low-income women, quitting or partial quitting during pregnancy both have a positive influence on infant BW. Whether others in the household smoke is also important.