AJOG Reviews
A systematic review and meta-analysis of prospective studies on the association between maternal cigarette smoking and preterm delivery

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Abstract

We have attempted to quantify the most up-to-date estimate of the association between cigarette smoking by the mother and preterm delivery. Studies were selected for inclusion in this review if they were prospective, reported data stratified across at least two levels of maternal smoking, and defined preterm delivery on the basis of gestational age. In a meta-analysis we combined results from multiple studies that reported on preterm delivery and maternal smoking during pregnancy. Pooled odds ratios were computed for various strata of smoking intensity with the Mantel-Haenszel fixed-effects model. Twenty studies met all inclusion criteria and were included in meta-analysis. The pooled point estimate from 20 prospective studies on any maternal smoking versus no maternal smoking was 1.27 (95% confidence interval, 1.21-1.33). Subgroup analyses stratifying maternal smoking on number of cigarettes per day suggest a dose-response relationship at low to moderate levels of smoking, which was not further increased at high levels of smoking. A nonsignificant level of publication bias appears to exist in the smoking–preterm delivery literature. Cigarette smoking is a preventable risk factor that is associated with preterm delivery. Consistent results across many study populations and research designs and evidence of a dose-response relationship support its causal role in preterm delivery. (Am J Obstet Gynecol 2000;182:465-72.)

Section snippets

Material and methods

Studies and review articles relating preterm deliveries and maternal smoking were identified with MEDLINE (1966-1997) and by searches of references cited in identified articles. MEDLINE was searched with the subject headings premature labor and smoking , as well as the text words preterm delivery and preterm birth , which resulted in the identification of 104 citations. Any articles containing original data were retrieved in their entirety, with study type ascertained when the full text of the

Results

A total of 64 published articles were identified that reported original data on the association between maternal smoking and preterm delivery. Twenty studies in 21 reports8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28 met all the inclusion criteria and were included for analysis (see Table I). Four studies reported outcomes only in the form of mean gestational age changes for children born to smokers versus nonsmokers. Whereas these data support the association

Comment

A truncated dose-response trend was observed when studies were separated into 3 groups of low, medium, and heavy smoking. It is possible that heterogeneity in design and execution of studies masked a dose-response relationship that, whereas it was apparent within individual studies, was less evident in the pooled groups. The heaviest smokers had an association between smoking and preterm delivery that was essentially the same as that for moderate smokers. An alternate explanation lies in the

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    Reprint requests: Michael B. Bracken, PhD, MPH, Department of Epidemiology and Public Health, Yale University, 60 College St, Suite 415, PO Box 208034, New Haven, CT 06520-8034.

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