Table 3

Associations between occupational factors in 2017 and risk of preterm birth among 934 firefighters and 1356 pregnancies*†

Preterm births
N (%)
RR (95% CI)
Model 1‡
RR (95% CI)
Model 2§
Firefighter subgroups
Employment, stratified by wildland firefighter status
 Structural firefighterNoYes
  Career713 (87%)103 (13%)1.00 (Ref.)1.00 (Ref.)
  Volunteer104 (81%)25 (19%)1.54 (0.98 to 2.41)1.47 (0.92 to 2.33)
 Wildland/combination firefighterNoYes
  Career352 (94%)22 (6%)1.00 (Ref.)1.00 (Ref.)
  Volunteer24 (71%)10 (29%)3.86 (1.56 to 9.52)2.82 (1.19 to 6.67)
 Interaction p-value¶<0.010.09
Work practices
Shift schedule of career firefightersNoYes
 Less than 24 hours on shift229 (87%)34 (13%)1.00 (Ref.)1.00 (Ref.)
 24 or more hr on shift835 (90%)92 (10%)0.78 (0.52 to 1.15)0.88 (0.58 to 1.32)
  • Combination= does wildland firefighting in addition to working for a career or volunteer department.

  • *Generalised estimating equations with Poisson distribution and sandwich variance estimators were used to estimate relative risks and 95% CIs.

  • †Three pregnancies had missing information for shift schedule and were not included in those models.

  • ‡Model 1 is adjusted for age-at-pregnancy, modelled as age-at-pregnancy and age-at-pregnancy.2

  • §Model 2 is additionally adjusted for highest education completed (some college/at least college degree), gravidity (yes/no), BMI (<30 kg/m2/≥30 kg/m2) and smoking status (current or former/never). Highest education completed, BMI and smoking status were measured in 2017 at time of survey. Gravidity was assessed for each pregnancy.

  • ¶P value for interaction term between employment and wildland firefighter status.

  • BMI, body mass index ; RR, relative risk.