Table 3.
Associations of parity and lactation history with incident hip fractures*
| Predictor | Unadjusted model† | Multivariable-adjusted model‡ | ||
|---|---|---|---|---|
| HR (95% CI)§ | p | HR (95% CI) | p | |
| Parity characteristics | ||||
| # pregnancies lasting ≥6 months | 0.005 | 0.15 | ||
| Never pregnant | Reference** (1.00) | Reference (1.00) | ||
| Never had pregnancy lasting ≥6 months | 0.74 (0.50–1.11) | 0.79 (0.46–1.34) | ||
| 1 | 0.96 (0.75–1.22) | 1.14 (0.85–1.52) | ||
| 2 | 0.75 (0.61–0.92) | 0.87 (0.68–1.11) | ||
| 3 | 0.75 (0.61–0.92) | 0.80 (0.62–1.02) | ||
| 4 | 0.79 (0.63–0.99) | 0.85 (0.65–1.11) | ||
| 5+ | 0.98 (0.78–1.22) | 0.90 (0.68–1.18) | ||
| How many times pregnant | 0.03 | 0.34 | ||
| None | Reference (1.00) | Reference (1.00) | ||
| 1 | 0.93 (0.71–1.20) | 1.10 (0.81–1.50) | ||
| 2 | 0.84 (0.68–1.03) | 0.94 (0.73–1.20) | ||
| 3 | 0.72 (0.59–0.89) | 0.81 (0.63–1.04) | ||
| 4 | 0.77 (0.62–0.96) | 0.88 (0.68–1.14) | ||
| 5 | 0.79 (0.62–1.01) | 0.85 (0.63–1.14) | ||
| 6 | 0.97 (0.74–1.28) | 0.85 (0.60–1.21) | ||
| 7 | 0.77 (0.53–1.13) | 0.63 (0.39–1.03) | ||
| 8+ | 1.07 (0.78–1.46) | 0.91 (0.61–1.35) | ||
| Age at first birth (years) | <.0001 | 0.21 | ||
| Never pregnant/No pregnancy lasting ≥6 months | Reference (1.00) | Reference (1.00) | ||
| <20 | 0.56 (0.43–0.73) | 0.86 (0.62–1.20) | ||
| 20 – 29 | 0.85 (0.72–1.01) | 0.87 (0.71–1.07) | ||
| 30+ | 1.14 (0.90–1.44) | 1.08 (0.82–1.42) | ||
| Lactation pattern among participants who had been pregnant (n=83,563) | ||||
| Breastfed for at least 1 month | 0.27 | 0.03 | ||
| No | Reference (1.00) | Reference (1.00) | ||
| Yes | 0.93 (0.83–1.06) | 0.84 (0.73,0.98) | ||
| How many children breastfed? | 0.30 | 0.14 | ||
| None | Reference (1.00) | Reference (1.00) | ||
| 1–2 | 0.89(0.77–1.02) | 0.83(0.70–0.99) | ||
| 3–5 | 1.00(0.84–1.18) | 0.85(0.69–1.04) | ||
| 6+ | 1.14(0.75–1.74) | 1.00(0.60–1.65) | ||
| How old when first breastfed? (years) | 0.09 | 0.39 | ||
| Never breastfed | Reference (1.00) | Reference (1.00) | ||
| <20 | 0.66(0.49–0.88) | 0.83(0.57–1.20) | ||
| 20 – 24 | 0.91(0.78–1.06) | 0.81(0.67–0.99) | ||
| 25 – 29 | 1.03(0.87–1.22) | 0.83(0.67–1.02) | ||
| 30 – 34 | 1.12(0.86–1.46) | 1.00(0.74–1.36) | ||
| 35 – 39 | 0.94(0.57–1.55) | 0.97(0.57–1.66) | ||
| 40 – 44 | 0.34(0.05–2.44) | 0.40(0.06–2.87) | ||
| 45+ | NA | NA | ||
| How old when last breastfed? (years) | 0.001 | 0.39 | ||
| Never breastfed | Reference (1.00) | Reference (1.00) | ||
| <20 | 0.69(0.43–1.12) | 0.93(0.51–1.71) | ||
| 20 – 24 | 0.79(0.63–0.98) | 0.78(0.59–1.03) | ||
| 25 – 29 | 0.77(0.64–0.93) | 0.79(0.63–0.99) | ||
| 30 – 34 | 1.12(0.95–1.33) | 0.92(0.75–1.13) | ||
| 35 – 39 | 1.03(0.82–1.30) | 0.81(0.61–1.06) | ||
| 40 – 44 | 1.42(0.99–2.04) | 0.99(0.65–1.50) | ||
| 45+ | 1.79(0.57–5.56) | 0.93(0.23–3.76) | ||
| Number of months breastfed | 0.68 | 0.19 | ||
| Never breastfed | Reference (1.00) | Reference (1.00) | ||
| 1–6 | 0.95(0.82–1.10) | 0.85(0.71–1.01) | ||
| 7–12 | 0.88(0.71–1.07) | 0.77(0.60–0.99) | ||
| 13–23 | 0.99(0.79–1.23) | 0.88(0.68–1.15) | ||
| 24+ | 0.88(0.66–1.16) | 0.92(0.67–1.27) | ||
Cox proportional hazards regression models
Includes predictor only without any covariates
Includes the following covariates: age, race/ethnicity, education, smoking status, alcohol consumption, hysterectomy, bilateral oophorectomy, years since menopause, previous fracture, parental history of fracture, number of falls in the past 12 months, estrogen use, estrogen route of delivery, recency of estrogen use, use of prescription osteoporosis medication, use of aromatase inhibitors, tamoxifen use, antidepressant use, corticosteroid use, proton pump inhibitor use, anti-epileptic use, anti-neoplastic use, thiazolidinedione use, body mass index, total energy expenditure per week from physical activities (log-transformed), total calcium intake (log -transformed), total vitamin D intake (log-transformed) at baseline, and weight change since age 35.
HR denotes hazard ratio; CI denotes confidence interval.
Ref denotes reference group.