Low birthweight at term and the timing of fetal exposure to maternal smoking.
OBJECTIVE
This study was undertaken to evaluate the risk of small-for-gestational-age birth for women who stop smoking or begin to smoke during pregnancy.


METHODS
Women with term singleton pregnancies from a hospital-based cohort of 11,177 were classified as (1) nonsmokers; (2) smoked throughout pregnancy; (3) smoked during first trimester only; (4) smoked during first and second trimesters only; and (5) smoked during second and third trimesters or during third trimester only. Risk of small-for-gestational-age birth according to smoking category was estimated and adjusted for confounding factors by logistic regression.


RESULTS
Women who stopped smoking by the third trimester were not at increased risk of small-for-gestational-age birth compared with nonsmokers. Women who began smoking during the second or third trimester had an elevated risk of small-for-gestational-age birth (odds ratio [OR] = 1.83; 95% confidence interval [CI] = 1.25, 2.67) similar to that for women who smoked throughout pregnancy (OR = 2.20; 95% CI = 1.90, 2.54). Risk of small-for-gestational-age birth increased with the number of cigarettes smoked during the third trimester.


CONCLUSIONS
It is during the third trimester that smoking retards fetal growth, presenting a compelling opportunity for smoking cessation interventions. Programs must emphasize the importance of not resuming smoking late in pregnancy.
Department of Obstetrics and Gynecology, Brigham and Women's Hospital, Harvard Medical School, Chestnut Hill, Mass.
E
E
Lieberman
I
I
Gremy
J M
JM
Lang
A P
AP
Cohen
D000328:Adult; D001724:Birth Weight / Q000187:drug effects*; D015331:Cohort Studies; D005260:Female; D005317:Fetal Growth Retardation / Q000139:chemically induced*; D005333:Fetus / Q000187:drug effects; D006801:Humans; D007231:Infant, Newborn; D007236:Infant, Small for Gestational Age; D016014:Linear Models; D016015:Logistic Models; D011247:Pregnancy; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects*; D016540:Smoking Cessation; D013997:Time Factors