Research Articles
Effects of smoking during pregnancy: Five meta-analyses

https://doi.org/10.1016/S0749-3797(98)00089-0Get rights and content

Abstract

Background:

The purpose of this study was to estimate, using meta-analysis, pooled odds ratios for the effects of smoking on five pregnancy complications: placenta previa, abruptio placenta, ectopic pregnancy, preterm premature rupture of the membrane (PPROM), and pre-eclampsia.

Methods:

Published articles were identified through computer search and literature review. Five criteria were applied to those studies initially identified to determine those eligible for the meta-analysis. A random effects model was applied to derive pooled odds ratios for the eligible studies for each pregnancy complication. Meta-analyses were repeated on subsets of the studies to confirm the overall results.

Results:

Smoking was found to be strongly associated with an elevated risk of placenta previa, abruptio placenta, ectopic pregnancy, and PPROM, and a decreased risk of pre-eclampsia. All pooled odds ratios were statistically significant. The pooled ratios ranged from 1.58 for placenta previa to 1.77 for ectopic pregnancy. The pooled odds ratio for pre-eclampsia was 0.51 and all subset analyses confirmed this seemingly protective effect.

Conclusions:

Smoking during pregnancy is a significant and preventable factor affecting ectopic pregnancy, placental abruption, placenta previa, and PPROM. The findings of smoking’s apparently protective effect on pre-eclampsia should be balanced with these harmful effects. In addition, the biological linkage between smoking and pre-eclampsia is not yet well understood. Pregnant women should be advised to stop smoking in order to reduce the overall risk of pregnancy complications as well as any risk of adverse impact on the unborn child.

Introduction

Despite widespread evidence that smoking during pregnancy adversely impacts the health of the newborn, a self-reported 19% of American women continue to smoke during pregnancy.1 The true prevalence of tobacco use by pregnant women may be even higher: Studies that biochemically validate tobacco exposure suggest that pregnant women may either fail to disclose or underreport their use of tobacco during pregnancy.2, 3 The risks associated with this behavior are well documented. The literature provides examples of numerous studies that link tobacco use during pregnancy to an increased risk of low birthweight, perinatal mortality, and sudden infant death syndrome (SIDS).4, 5 Moreover, the impact of smoking during pregnancy may occur well before the birth of the child: Research indicates that use of tobacco can affect a woman’s ability to carry a pregnancy to term without complications.

Full information on the extent of smoking’s risk to the newborn and the pregnant mother is vital for motivating physician efforts to encourage women of reproductive age to quit smoking. Unfortunately, much of the research on the impact of tobacco use on the health of the pregnancy itself has been published piecemeal, failing to provide physicians with an overall view of the risks posed by smoking to the mother. Methodologic limitations within some studies in this area may additionally undermine the use of such research by health care practitioners. For example, a number of studies evaluating the impact of smoking on pregnancy complications have failed to control for additional factors believed to be associated with the event. Examples include research on ectopic pregnancy not adjusting for a history of pelvic inflammatory disease6, 7, 8; research on placenta previa and placental abruption not controlling for parity9, 10; and research on pre-eclampsia not adjusting for body mass index.11 Failure to adjust for potential confounders may lead to variation in effect sizes from study to study and devalue the results on effect size for use by health care practitioners.

In an effort to substantiate and summarize research on the association between smoking during pregnancy and the risk of major maternal pregnancy complications, we undertook meta-analyses on the association between smoking and five adverse pregnancy events. Using meta-analyses, we derived summary measures (odds ratio) of the association of cigarette smoking during pregnancy with abruptio placenta, placenta previa, ectopic pregnancy, pre-eclampsia, and preterm premature rupture of the membrane (PPROM). Meta-analysis is a statistical method that allows researchers to combine data from different studies on the same topic. This method increases the statistical power beyond that of individual studies and can provide robust and unbiased summary measures of association. In incorporating summary measures of effect for all major pregnancy complications found to have an association with smoking within one paper, we are able to present a comprehensive picture of the risks that smoking poses to a pregnancy.

Section snippets

Study selection

Using the MEDLINE and Current Contents databases for 1966 through May 1995, we searched for published research examining the association between smoking during pregnancy and pregnancy complications. We retrieved all relevant publications, including review articles, and examined their bibliographies for additional references on the topics. We included only English-language North American or Western European studies because we assumed that these populations would be more homogeneous in their

Results

The 34 studies used in the meta-analyses, their study design, and their determined odds ratios are listed in Table 1A, Table 1B, Table 1C, Table 1D, Table 1E. The pooled odds ratios and 95% confidence intervals for each pregnancy complication are presented in Table 2. We note that the CIs presented here are larger than they would be if we had used the fixed effects model and reflect variation due to possible heterogeneity among studies.

Preterm premature rupture of the membranes (PPROM)

Simple PROM is the premature rupture of the (amniotic) membranes before spontaneous onset of labor. With expectant management of PROM, 9 of 10 patients will progress into labor. More troublesome is preterm PROM, or PPROM, which is PROM that occurs before 37 weeks of gestation. In the United States, PPROM occurs in 0.7% to 2.0% of pregnancies34 and approximately 30% of preterm births.35 The measure of effect derived in the meta-analysis for this condition was an odds ratio of 1.70 (95% CI = 1.18

Discussion

Our meta-analyses showed that cigarette smoking during pregnancy is strongly associated with an elevated risk for abruptio placenta, ectopic pregnancy, and PPROM, and a decreased risk for pre-eclampsia. Although the overall placenta previa meta-analysis also showed a significant association between smoking during pregnancy and previa, none of the previa “subset” analyses (those that analyzed subsets of studies defined by certain study design characteristics)resulted in statistically significant

References (48)

  • A Handler et al.

    The relationship of smoking and ectopic pregnancy

    Am J Public Health

    (1989)
  • A Kalandidi et al.

    Induced abortions, contraceptive practices and tobacco smoking as risk factors for ectopic pregnancy in Athens

    Br J Obstet Gynecol

    (1991)
  • A Stergachis et al.

    Maternal cigarette smoking and the risk of tubal pregnancy

    Am J Epidemiol

    (1991)
  • A.G Thomas et al.

    The effect of placenta previa on blood loss in second-trimester pregnancy termination

    Obstet Gynecol

    (1994)
  • A Spinillo et al.

    Factors associated with abruptio placentae in preterm deliveries

    Acta Obstet Gynecol Scan

    (1994)
  • H Klonoff-Cohen et al.

    Cigarette smoking and pre-eclampsia

    Obstet Gynecol

    (1993)
  • R DerSimonian et al.

    Meta-analysis in clinical trials

    Cont Clin Trials

    (1986)
  • M.D Kramer et al.

    Maternal smoking and placenta previa

    Epidemiology

    (1991)
  • P.H Shiono et al.

    Smoking and drinking during pregnancytheir effects on preterm birth

    JAMA

    (1986)
  • R Fraser et al.

    Bleeding during the latter half of pregnancy

  • M Krohn et al.

    Correlates of placental abruption

    Br J Obstet Gynaecol

    (1987)
  • A.L Brink et al.

    Risk factors for abruptio placentae

    S Afr Med J

    (1987)
  • M.J Darby et al.

    Placental abruption in the preterm gestationan association with chorioamnionitis

    Obstet Gynecol

    (1989)
  • Cited by (351)

    • The environmental risk factors associated with ectopic pregnancy: An umbrella review

      2023, Journal of Gynecology Obstetrics and Human Reproduction
      Citation Excerpt :

      In total, six meta-analyses were included that were eligible for inclusion in this umbrella review (Figure 1). These six eligible articles [13–17] included nine meta-analyses with 55,755 EP cases with 860,623 subjects. Meta-analyses were based on cohort, cross-sectional or case-control design.

    • Airway pollution and smoking in reproductive health

      2022, Best Practice and Research: Clinical Obstetrics and Gynaecology
    View all citing articles on Scopus
    View full text