Risks of Preterm Delivery and Small for Gestational Age Infants: Effects of Nondaily and Low-Intensity Daily Smoking During Pregnancy

Background: Few studies have examined the effects of nondaily smoking or low-intensity daily smoking and infant outcomes. We examined the associations between preterm delivery and small for gestational age (SGA) infants in relation to both nondaily and daily smoking.

Methods: We used population-based data on women who delivered live singleton infants using the 2009-11 Pregnancy Risk Assessment Monitoring System. Women's smoking status in the last 3 months of pregnancy was categorised as nonsmokers, quitters, nondaily smokers (<1 cigarette/day), and daily smokers. Controlling for maternal age, maternal race/ethnicity, education, marital status, prepregnancy body mass index (BMI), trimester of prenatal care entry, parity, and alcohol use, we estimated adjusted prevalence ratios (PR) for the outcomes of preterm delivery (<37 weeks' gestation) and SGA.

Results: Of the 88 933 women, 13.1%, 1.7%, and 9.6% of the sample were quitters, nondaily smokers, and daily smokers, respectively, in the last 3 months of pregnancy. While nondaily smoking was not associated with preterm delivery, daily smoking was. However, we found no dose-response relationship with the number of cigarettes smoked per day. Risk of delivering a SGA infant was increased for both nondaily and daily smokers (PR 1.4, 95% CI 1.1, 1.8 and PR 2.0, 95% CI 1.9, 2.2 respectively).

Conclusions: Nondaily smoking in the last 3 months of pregnancy was associated with an increased risk of delivering a SGA infant. Pregnant women should be counselled that smoking, including nondaily and daily smoking, can adversely affect birth outcomes.