Table 1.
Relative risk of endometrial cancer after a pregnancy compared with having one pregnancy less by pregnancy number and type
| Pregnancy No and type | Relative risk (95% CI) | |
|---|---|---|
| Adjusted for age and period* | Adjusted for age, period, and socioeconomic factors*† | |
| First pregnancy | ||
| Induced abortion | 0.52 (0.44 to 0.62) | 0.53 (0.45 to 0.64) |
| Childbirth | 0.66 (0.61 to 0.71) | 0.66 (0.61 to 0.72) |
| Any subsequent | ||
| Induced abortion | 0.80 (0.76 to 0.85) | 0.81 (0.77 to 0.86) |
| Childbirth | 0.88 (0.85 to 0.90) | 0.86 (0.84 to 0.89) |
| Second pregnancy: | ||
| Induced abortion | 0.77 (0.66 to 0.90) | 0.79 (0.67 to 0.92) |
| Childbirth | 0.85 (0.79 to 0.91) | 0.83 (0.77 to 0.89) |
| Third pregnancy: | ||
| Induced abortion | 0.78 (0.70 to 0.87) | 0.80 (0.71 to 0.89) |
| Childbirth | 0.86 (0.80 to 0.92) | 0.84 (0.78 to 0.90) |
| Fourth pregnancy: | ||
| Induced abortion | 0.81 (0.70 to 0.94) | 0.82 (0.71 to 0.95) |
| Childbirth | 0.92 (0.81 to 1.04) | 0.91 (0.80 to 1.03) |
| Fifth pregnancy: | ||
| Induced abortion | 0.81 (0.63 to 1.03) | 0.81 (0.63 to 1.04) |
| Childbirth | 0.92 (0.72 to 1.16) | 0.91 (0.72 to 1.16) |
Adjusted for age, period, pregnancy history, and interaction between age and period.
Additionally adjusted for educational attainment, marital status, urbanicity, and interaction between period and educational attainment, marital status, and urbanicity, respectively. P value for difference between induced abortion and childbirth for risk of endometrial cancer, adjusted for age, period, and socioeconomic factors, are 0.07 (relative risk 0.53 v 0.66) and 0.25 (0.81 v 0.86), respectively, for first and any subsequent pregnancy. Test for difference between model with different effect of each pregnancy compared with model with same effect for all pregnancies after the first pregnancy gave P=0.99. Sensitivity analysis with start of follow-up from 20 years of age presented in supplementary table S9 gave identical results.