Table 3

Risk of breast cancer related to pregnancy outcomes among parous women ≤40 years of age (470 case/control pairs)

Pregnancy characteristicCases/controlsUnivariate OR (95% CI)Multivariatea OR (95% CI)Trend PHomogeneity test P
Full-term pregnancy only255/2521.001.00
Any history of induced abortion118/1091.07 (0.79–1.46)1.05 (0.75–1.48)
Number of induced abortions
 178/731.09 (0.77–1.54)1.03 (0.70–1.50)
 ≥240/361.12 (0.70–1.81)1.19 (0.69–2.04)0.59
Any history of miscarriage117/1180.98 (0.72–1.33)1.00 (0.71–1.41)
Number of miscarriages
 191/801.13 (0.81–1.58)1.13 (0.78–1.65)
 ≥226/380.69 (0.42–1.15)0.76 (0.44–1.34)0.69
Any other pregnancy outcome
 Current pregnancy15/111.34 (0.61–2.93)1.47 (0.60–3.61)
 Ectopic pregnancy12/42.93 (0.94–9.11)2.69 (0.79–9.18)
 Molar pregnancy1/1
Outcome of first pregnancy
 Induced abortion54/481.11 (0.71–1.72)0.98 (0.60–1.61)
 Miscarriage55/481.13 (0.73–1.75)1.17 (0.72–1.89)
 Other pregnancy outcomeb5/14.69 (0.55–40.31)2.34 (0.25–22.15)
 Full-term, but later incomplete101/1210.82 (0.60–1.13)0.87 (0.61–1.25)
Induced abortion as the outcome of first pregnancy
 Gestation length
  ≤8 weeks38/311.21 (0.72–2.02)1.01 (0.56–1.82)
  >8 weeks16/170.94 (0.47–1.89)0.93 (0.42–2.02)0.48
 Calendar era of induced abortion
  Before 197330/231.30 (0.73–2.31)1.19 (0.62–2.27)
  1973 or later24/250.96 (0.52–1.76)0.78 (0.39–1.55)0.35
 Age
  <20 years22/201.11 (0.57–2.15)0.99 (0.47–2.06)
  ≥20 years32/281.14 (0.66–1.97)0.97 (0.52–1.79)1.00
  • a For each type of pregnancy outcome, multivariate models include adjustment for first degree family history of breast cancer, age at menarche, months of oral contraceptive use, average hours per week of exercise activity since menarche, use of hCG among women with low and high body mass index, age at first full-term pregnancy, number of full-term pregnancies, months of breast feeding, and other pregnancy outcomes.

  • b Other pregnancy outcomes include ectopic (5 cases) and molar (1 control) pregnancies.