AbstractTo determine the effects of changes in maternal cigarette use during pregnancy on birthweight, a sample of Caucasian births free of major malformations (n = 9,943) was examined. Births were stratified by level of maternal smoking in the first trimester and subdivided according to whether the mother continued at the same level, reduced, or quit by the second trimester. Birthweights were adjusted statistically for extraneous variables. As expected, second and third trimester cigarette use was associated with birthweight, but so too was cigarette use during the first trimester, and the effect of quitting varied significantly with the level of first trimester smoking. Among moderate and light smokers, who comprise the majority of smokers, quitting before the second trimester is associated with heavier infants. However, infants of heavy smokers who quit by the second trimester did not weigh significantly more than infants of mothers who continued to smoke heavily throughout pregnancy, and weighed significantly less than infants of nonsmokers or other smokers who quit. Thus, quitting by the end of the first trimester may not completely negate the effect of heavy first trimester smoking, and the adaptive value of qutting is unequal among different levels of first trimester smoking. Further research on prenatal growth should take cigarette smoking in all trimesters into account. © 1994 Wiley‐Liss, I