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Table 2 Frequency and factors associated with incident and recurrent preterm birth in the second pregnancy, 2001–2015 (n = 1,764,025)

From: Differences in risk factors for incident and recurrent preterm birth: a population-based linkage of 3.5 million births from the CIDACS birth cohort

Second pregnancy variables Incident preterm birth Recurrent preterm birth Pdifference b
n (%) ORa 95% CI n (%) ORa 95% CI
Urban/rural area of residence
 Urban 86,747 (7.20) Ref Ref 18,003 (18.26) Ref Ref 0.96
 Rural 24,279 (6.70) 0.90 0.89–0.92 4395 (17.24) 0.90 0.87–0.94  
Household overcrowding
 No 63,415 (6.70) Ref Ref 12,155 (17.57) Ref Ref <0.00
 Yes 43,080 (7.62) 1.09 1.07–1.10 9469 (18.65) 1.01 0.98–1.04  
Maternal race/ethnicity
 White 33,781 (6.73) Ref Ref 7237 (18.13) Ref Ref <0.00
 Black/Mixed-race 71,741 (7.25) 1.04 1.03–1.06 14,003 (17.93) 0.95 0.92–0.98  
 Indigenous 981 (10.26) 1.34 1.24–1.44 215 (21.76) 1.14 0.96–1.35  
Maternal education
 ≥ 12 years of study 5659 (5.94) Ref Ref 1,060 (15.95) Ref Ref 0.27
 8 to 11 years of study 66,183 (7.18) 1.21 1.17–1.25 13,523 (18.16) 1.13 1.05–1.22  
 <8 years of study 41,944 (7.08)
41,944 (7.08
1.12 1.09–1.16 8430 (18.04) 1.06 0.98–1.14  
Marital status
 Married, civil union 53,922 (7.34) Ref Ref 10,132 (18.26) Ref Ref <0.00
 Single, divorced, widow 60,422 (6.86) 0.85 0.84–0.86 12,948 (17.80) 0.90 0.87–0.93  
Number of prenatal visits
 ≥ 4 visits 90,941 (6.25) Ref Ref 17,608 (15.91) Ref Ref <0.00
 1 to 3 visits 19,606 (14.05) 2.44 2.40–2.49 4531 (30.65) 2.24 2.14–2.33  
 None 3967 (14.58) 2.56 2.47–2.66 902 (29.62) 2.16 1.98–2.36  
Interbirth interval
 ≥24 months 91,934 (6.80) Ref Ref 15,703 (16.46) Ref Ref <0.00
 12 to 23 months 21,499 (8.06) 1.04 1.02–1.06 6366 (20.64) 1.22 1.17–1.26  
 <12 months 2344 (14.17) 1.89 1.80–1.98 1293 (36.46) 2.58 2.38–2.79  
Maternal age at birth
 20 to 34 years 86,081 (6.69) Ref Ref 15,415 (16.97) Ref Ref <0.00
 14 to 19 years 24,548 (8.46) 1.16 1.14–1.18 7080 (20.24) 1.02 0.98–1.06  
 35 to 49 years 5148 (8.93) 1.42 1.38–1.47 867 (21.95) 1.45 1.33–1.58  
Type of delivery
 Vaginal 64,128 (7.05) Ref Ref 14,656 (18.67) Ref Ref <0.00
 Cesarean 51,494 (7.13) 1.09 1.08–1.11 8679 (16.97) 0.96 0.92–0.99  
Newborn sex
 Female 54,808 (6.88) Ref Ref 11,078 (17.39) Ref Ref 0.25
 Male 60,956 (7.27) 1.06 1.05–1.07 12,283 (18.59) 1.08 1.05–1.12  
  1. aAnalysis adjusted for all model variables
  2. bPdifference represents the interaction between incident and recurrent preterm birth for the respective characteristic. The equation for this model is as follows: Logit [ Pr (PTB2=1/Risk factors)] = β0 + β1 (PTB1) + β2 (risk factor X2) + β3 (PTB1) × (risk factor X2), where PTB1 and PTB2 are indicators of a preterm birth in the first and second pregnancy, respectively. Risk factor X2 is a risk factor in the second pregnancy. The model shows a single risk factor, but additional risk factors were added by including additional terms for each risk factor and its interaction with the first pregnancies’ preterm delivery status, PTB1