A large, prospective, U.K. multicenter study reported that breastfeeding was associated with younger age at onset of JIA, as well as with lower disease activity, pain and functional disability scores at diagnosis.38
The larger Swedish registry case-control study based on medical records found that prolonged gestation (longer than 42 weeks) was associated with increased risk of JIA,21 whereas a U.S. hospital case-control questionnaire study found premature birth to be associated with increased JIA risk.22
Low birth weight was protective against the development of oligoarticular JIA in the German case-control questionnaire study,25 but no effect of birth weight on JIA risk was observed in the Swedish registries or in two Washington state case-control studies, one based on birth certificate records and one a clinic-based questionnaire study (see Table 2, Birth weight).21,22 No effect of mode of delivery was seen in these studies.
In a small Brazilian case-control study based on questionnaire data, maternal occupational exposure to chalk dust or gasoline vapor was associated with JDM.39
Smoking
Because smoking is less frequent in children than adults, studies examined parental smoking during pregnancy and passive smoke exposure in the home. Studies of maternal smoking during pregnancy suggest mixed effects (see Table 2, Smoking).
The Finnish Medical Birth Registry study showed that maternal smoking was associated with a higher risk of JIA in girls, with greater risk among girls whose mothers were heavy smokers (more than 10 cigarettes per day).40 In contrast, no effect of maternal smoking on risk of JIA was seen in the Swedish inpatient registry case-control study based on maternal birth registry records.21
JIA was associated with lower levels of maternal prenatal smoking compared with controls, and this effect was more pronounced among patients in the oligoarticular/extended oligoarticular JIA subgroups in a Washington state case-control study based on case identification through ICD-9 coding and examination of birth certificates. Although the analysis was adjusted for confounders, residual differences in socioeconomic status or differential reporting may have contributed to those effects.24
Maternal smoking during pregnancy was also found to be an independent risk factor for JDM in a small Brazilian case-control questionnaire study.39
The effects of exposure to passive smoking in childhood may also affect JIA. One disease-comparison study showed a higher prevalence of JIA compared with other chronic diseases in children exposed to daily passive tobacco smoke.41 By contrast, the Australian case-control study found a protective effect of paternal indoor smoking, which they speculated could be due to cessation of smoking with onset of disease symptoms (see Table 2, Smoking).36
Air Pollution
Several studies examined the effects of air pollution on the development of pediatric systemic autoimmune diseases. In a study in Utah, higher concentrations of fine air particulate matter (42
