Prenatal smoking linked to gestational diabetes mellitus

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A new study of nearly a quarter million women suggests that smoking during pregnancy increases risk of gestational diabetes mellitus (GDM). The findings, published in Obstetrics & Gynecology, do not prove a causal relationship but provide support for counseling smoking cessation among prenatal patients.

Methods
For the research, the authors conducted a secondary analysis of data from the Pregnancy Risk Assessment Monitoring System from 2009 to 2015. Conducted by the Centers for Disease Control and Prevention, the system is a surveillance project to monitor maternal attitudes and experiences before, during, and shortly after pregnancy. The population for the substudy was 222,408 mothers with singleton births for whom data were available on GDM and prenatal smoking status.

All of the participants were asked whether they had smoked any cigarettes in the past 2 years. Those who had were also asked how many cigarettes they smoked in an average day in the 3 months prior to conception and in the last 3 months of pregnancy. Based on the responses, the women were classified as having no smoke for more than 2 years before conception, quit smoking before pregnancy, smoked before pregnancy but quit, smoked during pregnancy but fewer cigarettes, and continued to smoke and smoked more cigarettes.

Confounders considered by the researchers included maternal age, race-ethnicity, prepregnancy body mass index and gestational weight gain. The independent association between prenatal smoking and GDM status, overall and stratified by gestational weight gain and prepregnancy body mass index (BMI). Of the participants 5.3% had GDM, 48.1% had normal pregnancy BMI, and 43.1% had excess gestational weight gain.

Findings
Two-thirds (75%) of the women had not smoked for more than 2 years before conception, almost 3% quit before pregnancy, 11.8% quit smoking while pregnant, and 6.7% smoked fewer cigarettes during pregnancy. Most of the participants were aged 25 to 34, non-Hispanic white, had normal BMIs, and had excessive gestational weight gain.

Odds of GDM were higher in mothers who continued to smoke the same number of cigarettes or more cigarettes, or less cigarettes during pregnancy (adjusted odds ratio [OR] 1.46, 95% CI 1.25-1.71 and OR 1.22, 95% CI 1.08-1.38, respectively). Smoking the same or more cigarettes was associated with higher odds of GDM in all subgroups of prepregnancy BMI or gestational weight gain.

The authors noted that in their study, inadequate gestational weight gain was associated with higher odds of GDM. One possible explanation, they said, “might be that women who have been diagnosed with GDM are placed on a strict diet with or without insulin and increased physical activity regime that may contribute to reduced weight gain during the last weeks of pregnancy, thereby limiting the overall gestational weight gain.”

Conclusions
Based the study’s findings, the researchers said that “a large prospective cohort study, collecting data on all known risk factors and possible confounders of GDM, with biochemical validation of GDM and smoking status, is needed to provide a more definitive answer on whether there is a casual association between prenatal smoking and GDM and the interplay with gestational weight gain.”