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Table 3 - Factors associated with breastfeeding duration: results obtained by Weibull regression

From: Factors associated with duration of breastfeeding in Spain: a cohort study

Factor Time ratio
(95% CI)
p Adjusted time ratio (95% CI)a p
Maternal age (per year) 1.03 (1.01, 1.04) 0.002 1.02 (1.00, 1.04) 0.03
Maternal educational level
 Primary studies (ref.) 1 1
 Secondary studies 1.26 (0.93, 1.70) 0.14 1.28 (0.95, 1.74) 0.11
 Foundation degree 1.05 (0.83, 1.34) 0.68 0.99 (0.77, 1.26) 0.92
 University studies 1.73 (1.37, 2.17) <  0.001 1.53 (1.21, 1.95) <  0.001
Maternal occupation
 Working (ref.) 1   
 Unemployed 0.99 (0.79, 1.24) 0.93   
 Inactive 0.84 (0.64, 1.09) 0.19   
 Student 1.20 (0.51, 2.79) 0.68   
Smoking in pregnancy
 No (ref.) 1 1
 Yes 0.51 (0.40, 0.66) <  0.001 0.59 (0.46, 0.76) <  0.001
Duration of pregnancy
  ≥ 37 weeks (ref.) 1 1
 34–36 weeks 0.75 (0.49, 1.13) 0.17 0.97 (0.62, 1.53) 0.91
  < 34 weeks 0.55 (0.30, 0.98) 0.04 0.91 (0.46, 1.79) 0.78
Type of delivery
 Vaginal (non-instrumental)(ref.) 1   
 Instrumental 0.93 (0.68–1.27) 0.66   
 Cesarean rate 0.93 (0.76–1.13) 0.46   
Newborn gender
 Male (ref.) 1   
 Female 0.92 (0.78, 1.10) 0.37   
Twin pregnancy
 No (ref.) 1 1
 Yes 0.56 (0.37, 0.85) 0.006 0.57 (0.37, 0.89) 0.01
Newborn weight
  < 2500 0.62 (0.46, 0.83) 0.001 0.77 (0.53, 1.11) 0.16
 2500–4000 g 1 1
  > 4000 g 1.11 (0.81, 1.53) 0.52 1.08 (0.79, 1.48) 0.63
  1. aTime ratio adjusted for the remaining variables in the table. Occupation, type of delivery and gender were excluded from the multivariate analysis because of their lack of relationship with breastfeeding duration in the crude analysis
  2. Note: that results are not express as hazard ratios but as time ratios. Therefore, values higher than 1 indicate longer breastfeeding, while values lower than 1 indicate the opposite. For instance, time ratio = 0.55 in participants with pregnancy duration lower than 34 weeks means that breastfeeding duration in those participants about halved that of participants with pregnancy duration > 37 weeks