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Table 3 Associations between occupational factors and risk of miscarriage among 1074 firefighters and 1864 pregnanciesa,b

From: Occupational factors and miscarriages in the US fire service: a cross-sectional analysis of women firefighters

  Miscarriages
N (%)
RR (95% CI)
Model 1c
RR (95% CI)
Model 2d
Firefighter subgroups
 Employment, stratified by wildland firefighter status
  Structural No Yes   
   Career 891 (76%) 281 (24%) 1.00 (Ref.) 1.00 (Ref.)
   Volunteer 140 (69%) 64 (31%) 1.41 (1.11–1.79) 1.42 (1.11–1.80)
  Wildland/WUI No Yes   
   Career 388 (90%) 41 (10%) 1.00 (Ref.) 1.00 (Ref.)
   Volunteer 38 (68%) 18 (32%) 3.09 (1.76–5.42) 2.53 (1.35–4.78)
  Interaction p-value e    < 0.01 < 0.01
Work practices
 Shift schedule of career firefighters, stratified by age at pregnancy
   < 35 years old No Yes   
   Less than 24 h on shift 232 (77%) 69 (23%) 1.00 (Ref.) 1.00 (Ref.)
   24 or more hr. on shift 772 (84%) 144 (16%) 0.67 (0.50–0.90) 0.74 (0.55–0.99)
  35+ years old No Yes   
   Less than 24 h on shift 64 (78%) 18 (22%) 1.00 (Ref.) 1.00 (Ref.)
   24 or more hr. on shift 211 (70%) 89 (30%) 1.29 (0.84–2.00) 1.28 (0.82–1.99)
  Interaction p-value f    0.01 0.02
  Worked fire/rescue calls at pregnancy start No Yes   
   No 138 (70%) 58 (30%) 1.00 (Ref.) 1.00 (Ref.)
   Yes 1250 (79%) 340 (21%) 0.81 (0.60–1.09) 0.82 (0.61–1.11)
  1. RR Relative risk, CI confidence interval, BMI Body mass index (kg/m2), WUI wildland urban interface
  2. a Generalized estimating equations models with Poisson distribution and sandwich variance estimators were used to estimate risk ratios and 95% CIs
  3. b 86 pregnancies had missing information for wildland status (3), shift schedule (5) and fire/rescue calls (78) and were not included in those models
  4. c Model 1 is adjusted for age at pregnancy, modeled as age at pregnancy and (age at pregnancy)2
  5. d 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
  6. e p-value for interaction between employment and wildland firefighter status
  7. f p-value for interaction between shift schedule and age at pregnancy, among career firefighters