(HealthDay)—Quitting smoking, especially early in pregnancy, is associated with a reduced risk for preterm birth, even for high-frequency cigarette smokers, according to a study published online April 19 in JAMA Network Open.

Samir Soneji, Ph.D., from the Dartmouth Geisel School of Medicine in Lebanon, New Hampshire, and Hiram Beltrán-Sánchez, Ph.D., from the University of California in Los Angeles, analyzed information on 25,233,503 expectant mothers who delivered live neonates and had known prepregnancy and trimester-specific cigarette smoking frequency. The authors sought to examine the correlation between the probability of preterm birth (<37 weeks of gestation) and cigarette smoking cessation throughout pregnancy, after the first trimester, after the second trimester, and during the third trimester regardless of first and second trimester smoking.

The researchers found that the earlier smoking cessation occurred in pregnancy, the more the probability of preterm birth decreased. For example, the probability of preterm birth was 9.8 percent among 25- to 29-year-old, non-Hispanic white, primigravida, and primiparous expectant mothers who smoked one to nine cigarettes per day prior to pregnancy and maintained this frequency throughout their pregnancy. The probability of preterm birth was 9.0 percent if a woman stopped smoking at the start of the second trimester (an 8.9 percent relative decrease) and 7.8 percent if she stopped smoking at the start of pregnancy (a 20.3 percent relative decrease).

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