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Table 2 Adjusted hazard ratios of reproducing (HR) in relation to twinning, parental socio-demographic characteristics and birth characteristics by gender

From: Reproductive patterns among twins - a Swedish register study of men and women born 1973-1983

  Females Males
  HRa 95% CI HRb 95% CI HRa 95% CI HRb 95% CI
Twinbirth 0.89 0.86-0.93 0.91 0.88-0.95 0.92 (0.87-0.97) 0.96 0.91-1.01
Mother’s educational level      
 9-10 years Reference level Reference level
 11-13 years 0.84 0.83-0.85 0.95 0.94-0.97 0.89 0.88-0-91 0.84 0.83-0.85
 ≥14 years 0.67 0.66-0.68 0.80 0.79-0.82 0.74 0.73-0.76 0.66 0.66-0.68
 Missing* 1.05 1.03-1.07 0.64 0.62-0.65 1.02 1.00-1.05 1.05 1.03-1.07
Father’s educational level      
 9-10 years Reference level Reference level
 11-13 years 0.89 0.88-0.90 0.89 0.88-0.90 1.00 0.98-1.03 0.91 0.89-0.92
 ≥14 years 0.88 0.68-0.69 0.68 0.67-0.69 0.91 0.88-0.93 0.72 0.71-0.74
 Missing* 1.01 0.99-1.02 1.01 0.99-1.02 0.72 0.70-0.74 1.00 0.98-1.02
Mother’s civil status      
 Divorced/widowed Reference level Reference level
 Married 0.81 0.79-0.83 0.81 0.79-0.83 0.88 0.85-0.91 0.88 0.85-0.91
 Unmarried 0.84 0.82-0.86 0.84 0.82-0.86 0.87 0.84-0.91 0.87 0.84-0.90
Parental country of origin      
 One or both non-Nordic Reference level Reference level
 Both Nordic 1.34 1.32-1.37 1.34 1.32-1.37 1.20 1.17-1.23 1.20 1.17-1.23
Mother’s age at birth      
 ≥34 years Reference level Reference level
 13-19 years 2.25 2.19-2.31 2.25 2.19-2.30 2.08 2.02-2.20 2.08 2.02-2.16
 20-26 years 1.59 1.56-1.62 1.58 1.55-1.61 1.56 1.52-1.60 1.55 1.52-1.59
 27-33 years 1.14 1.12-1.16 1.14 1.12-1.16 1.17 1.14-1.20 1.17 1.14-1.20
Mother’s parity         
 Previous children Reference level Reference level
 No previous children 0.82 0.81-0.83 0.82 0.81-0.83 0.83 0.82-0.84 0.83 0.82-0.84
Year of birth $ 1.03 1.03-1.03 1.03 1.03-1.03 0.94 0.94-0.94 0.94 0.94-0.94
Size for gestational age        
 Appropriate for gestational age Reference level Reference level
 Small for gestational age    1.02 1.00-1.05    0.97 0.94-1.01
 Large for gestational age    1.02 0.99-1.05    1.03 0.99-1.07
Birthweight         
 Normal birthweight (> = 2500 g) Reference level Reference level
 Low birthweight (1500–2499 g)    0.97 0.94-1.00    0.95 0.91-1.00
 Very low birthweight (<1500 g)    0.87 0.75-1.00    1.03 0.85-1.24
Gestational age         
 Term (37–42 weeks) Reference level Reference level
 Preterm (32–36 weeks)    0.96 0.94-0.99    0.97 0.94-1.00
 Very preterm (<32 weeks)    0.88 0.78-0.99    0.78 0.68-0.90
 Post term (>42 weeks)    1.01 0.98-1.04    0.93 0.97-1.04
  1. *Parents with missing information on education were mainly immigrants and therefore included as a separate category in the analysis.
  2. $Included in the model as a continuous variable.
  3. aAdjusted for socio demographic variables presented in Table 1.
  4. bAdjusted for socio demographic and birth characteristics presented in Table 1.