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Table 3 Adjusted hazard ratios of reproducing in twins vs. singletons (HR) in males and females and in sub-groups defined by age and birth characteristics

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

  Female Male
  HRa 95% CI HRb 95% CI HRa 95% CI HRb 95% CI
Age         
 <= 25 years 0.86 (0.81-0.91) 0.92 (0.86-0.97) 0.91 (0.84-0.98) 0.90 (0.84-0.98)
 >25 years 0.91 (0.87-0.96) 0.95 (0.90-1.00) 0.93 (0.87-0.99) 1.01 (0.94-1.08)
Year of birth         
 1973-1975 0.93 (0.88-0.98) 0.96 (0.91-1.02) 0.96 (0.89-1.03) 1.01 (0.94-1.09)
 1976-1979 0.87 (0.82-0.92) 0.88 (0.83-0.94) 0.89 (0.82-0.96) 0.92 (0.84-1.00)
 1980-1983 0.83 (0.75-0.91) 0.82 (0.74-0.91) 0.84 (0.72-0.99) 0.84 (0.72-0.98)
Gestational age         
 Term (37–42 weeks) 0.92 (0.88-0.96) 0.92 (0.88-0.96) 0.94 (0.89-1.00) 0.97 (0.91-1.03)
 Post term (>42 weeks) 0.72 (0.46-1.15) 0.70 (0.44-1.12) 0.98 (0.55-1.79) 1.03 (0.57-1.88)
 Preterm (32–36 weeks) 0.85 (0.78-0.91) 0.86 (0.79-0.92) 0.89 (0.80-0.98) 0.90 (0.81-0.99)
 Very preterm (<32 weeks) 1.28 (1.02-1.62) 1.29 (1.02-1.63) 1.34 (1.01-1.78) 1.33 (1.00 -1.77)
Birthweight         
 Normal birthweight (≥2500 g) 0.90 (0.85-0.94) 0.90 (0.86-0.94) 0.92 (0.86-0.98) 0.92 (0.87-0.98)
 Low birthweight (1500–2499 g) 0.91 (0.86-0.97) 0.92 (0.86-0.98) 1.05 (0.96-1.16) 1.04 (0.95-1.15)
 Very low birthweight (<1500 g) 1.11 (0.84-1.46) 1.10 (0.83-1.46) 0.81 (0.52-1.25) 0.79 (0.51-1.24)
Size for gestational age         
 Appropriate for gestational age 0.87 (0.84-0.91) 0.89 (0.85-0.93) 0.91 (0.86-0.96) 0.93 (0.88-0.99)
 Small for gestational age 0.93 (0.86-1.00) 0.96 (0.88-1.03) 1.04 (0.92-1.16) 1.09 (0.97-1.23)
 Large for gestational age 1.15 (0.70-1.88) 1.22 (0.74-2.02) 0.72 (0.34-1.50) 0.78 (0.36-1.69)
Optimality at birth         
 Optimal$ 0.90 (0.86-0.93) 0.90 (0.86-0.93) 0.92 (0.87-0.97) 0.92 (0.87-0.97)
 Non-optimal$$ 0.88 (0.86-0.95) 0.91 (0.86-0.96) 0.95 (0.88-1.02) 0.99 (0.92-1.07)
  1. aAdjusted for socio demographic variables presented in Table 1.
  2. bAdjustedfor socio demographic and birth characteristics presented in Table 1.
  3. $Optimal birth characteristics include persons who were not SGA, LGA or preterm at birth.
  4. $$Non-optimal birth characteristics include those who were born SGA, LGA and preterm.