br We assessed total number
We assessed total number of breastfeeding episodes, breastfeeding some or all offspring, average duration of breastfeeding per breastfeeding episode (defined as total breastfeeding duration divided by total number of offspring breastfed), and the duration of breastfeeding for the first and last breastfeeding episodes.
Case-control differences in demographic and other factors were ini-tially assessed using X2 tests. Multivariable unconditional logistic re-gression was used to calculate odds ratios (ORs) and 95% confidence intervals (CIs) for the association between each breastfeeding factor and EOC (separate models for each factor). Age at reference date, total OC duration, parity, race, education, tubal ligation status, hysterectomy status, and family history of breast or ovarian cancer were selected a priori as potential confounders. A variable was retained if its removal re-sulted in at least a 10% change in the effect estimate. Family history of breast and ovarian cancer, tubal ligation, and hysterectomy did not af-fect the relationship between breast feeding and ovarian cancer risk and were not included in the final models. Sensitivity analyses showed estimates were not changed when including factors that may influence a woman's decision to breastfeed in our models (body mass index (BMI), smoking history, ever use of alcohol, and ever use of aspirin or
NSAIDs); therefore, they were not included in the final models. Sensitiv-ity analyses showed estimates were also unchanged by adjusting for age at first live birth, age at last live birth, decade of subject's birth, and year of birth as being pre vs post 1950 and therefore we not included in the final models. Thus, the final models included age at reference date, total OC duration, and parity as continuous variables, and Erastin and education as categorical variables. Tests for trend were performed by coding the ex-posure of interest as a grouped linear variable. Analyses were repeated examining only women age 50 or over and restricting cases to women with invasive disease only. Further analyses examined the associations limiting to high grade serous EOC (the most common histotype); limited cases of other histotypes precluded meaningful analyses. All p-values were two-sided and considered statistically significant at p b 0.05. Analy-ses were conducted using Stata version 9.1 (StataCorp).
Cases were more likely to be non-white and less educated, as well as to have an increased family history of breast or ovarian cancer (Table 1). They were less likely to have used OCs, borne children, or had a tubal ligation.
Longer total breastfeeding duration across all breastfeeding episodes was also associated with increased protection (OR = 0.75 and 0.62 for less than and greater than 1-year total duration, respectively; Table 3). An average duration of 3 months per breastfeeding episode was associ-ated with statistically significant risk reduction (OR = 0.73, 95%CI = 0.58–0.93). Longer average duration per breastfeeding episode ap-peared more protective although the difference was not statistically sig-nificant (OR = 0.73 and OR = 0.67 for 1–3 and 4 or more months average per episode, respectively). Longer duration for both the first (OR = 0.75 and OR = 0.66 for 1–3 and 4 or more months, respectively) and last (OR = 0.75 and OR = 0.65) breastfeeding episodes was associ-ated with statistically significant reduced risk.
More recent breastfeeding was associated with a statistically signif-icant 44% reduction in EOC risk (OR = 0.56, 95%CI = 0.32–0.95 for time since last breastfeeding within the last 10 years, Table 4). Although the association with reduced risk decreased over time, the effect persisted for more than 30 years after the last breastfeeding episode (OR = 0.69, 95%CI = 0.53–0.88).