Smoking in pregnancy was linked to abnormal bone development in offspring, a population-based study from Sweden found.
Mothers who smoked during pregnancy had children with a higher risk of fracture during their first year of life, compared with children who were not exposed to smoking (HR 1.27, 95% CI 1.12-1.45), reported Judith Brand, PhD, of Sweden’s Örebo University School of Medical Sciences, and colleagues.
The association between maternal smoking and bone fracture in children followed a dose-dependent pattern. Researchers found hazard ratios of 1.20 (95% CI 1.03-1.39) for offspring of women smoking 1-9 cigarettes a day and 1.41 (95% CI 1.18-1.69) when maternal smoking reached 10 cigarettes a day or more, Brand’s group indicated in The BMJ.
Previous research has tied smoking during pregnancy to fetal growth restrictions and low birthweight among offspring, the researchers wrote. While there is evidence that exposure to maternal smoking may have an effect on offspring bone health, studies investigating this association have not found conclusive evidence.
“Maternal smoking during pregnancy is a well-known risk factor for intrauterine fetal growth restriction, including reduced fetal skeletal growth,” Brand told MedPage Today in an email. “But its long-term effect on offspring bone health is less understood.”
Stephen Contag, MD, professor of obstetrics and gynecology at the University of Minnesota Medical School in Minneapolis, who was not involved in this research, said that “the notion of smoking during pregnancy affecting fracture rates is unique.” While smoking is known to be a risk factor for outcomes such as preterm delivery, compromised placental function, and fetal development issues, he said that this research provides further evidence for women to cease from smoking during pregnancy.
Brand and colleagues merged data from multiple Swedish registries to analyze more than 1.6 million singleton infants, born from 1983 to 2000. Data on smoking in early pregnancy was gathered by midwives at the first prenatal appointment. Smoking intensity was stratified into three groups: none, 1-9 cigarettes per day, or 10 or more per day.
In addition to an analysis of the whole population, researchers used a sibling comparison to control for unmeasured familial environmental and genetic factors. “This is relevant when studying an exposure like smoking, a trait that is strongly correlated with socioeconomic and lifestyle factors that pass from generation to generation,” Brand said. This analysis also showed increased infant fracture risk with maternal smoking.
Stephen Chasen, MD, maternal-fetal medicine specialist and professor of obstetrics at Weill Cornell Medicine in New York City, said that a unique aspect of this study was examining siblings who had different exposures to maternal smoking in utero.
Chasen, who was not involved in this research, said that the medical community has long known that smoking affects fetal growth and development, as well as calcium metabolism, which supports bone health. But, “this unique study design provided convincing evidence that smoking during pregnancy is an independent risk fracture for fractures, particularly in the first year of life,” Chasen told MedPage Today in an email.
Brand and colleagues followed offspring from birth until a maximum age of 32 years, tracking almost 377,970 fractures. Mothers who smoked were younger, of shorter stature, more likely to be multiparous, and more likely to be over or underweight compared to mothers who did not smoke. Infants of mothers who smoked were more likely to have a lower gestational age and birthweight.
The standardized fracture rate during the first year of life for offspring exposed to maternal smoking was 1.59 per 1,000, compared to 1.28 per 1,000 in offspring unexposed to smoking. Children exposed to maternal smoking in utero had a slightly increased risk of fracture between age 5 and 32; However, this association was slight, did not follow a dose-dependent pattern, and was not consistent with the sibling analysis. No relationship between maternal smoking and offspring’s fracture risk was seen during ages 1-5.
The information on maternal smoking in this study is based on questions administered by midwives, which might mean maternal smoking was underestimated. In addition, this research is limited by the assessment of maternal smoking only in the first trimester.
This study was supported in part by the Örebo University Hospital Research Foundation Nyckelfonden and British government grants.