Our results suggest that amongst parous women, ever breast feeding and longer durations of breast feeding per breast-fed child are associated with decreased risks of ovarian cancer independent of numbers of births. We had expected that exclusive breast feeding would confer greater risk reduction than breast feeding after supplementary food/milk was introduced but our results did not support that hypothesis. We did however find some evidence that the effects of breast feeding might vary according to ever-use of hormonal contraceptives and histological subtype of ovarian cancer.
Strengths of this study include the large number of cases and controls, its population-based design, the relatively high response proportions amongst both cases and controls and the detailed information available about women’s breast feeding practices and potential confounding factors. A weakness is that all the information was recalled, often from many years in the past, so it is likely that there is some error in the estimates. However, highly structured interviews and lifetime calendars were used to aid recall and, as the association between breast feeding and ovarian cancer is not well established or widely known, it is likely that any error would be non-differential.
Although the World Cancer Research Fund report from 2007[14
] concluded that ‘there is limited evidence suggesting that lactation protects against ovarian cancer’, a meta-analysis of nine case-control studies reported a 30% reduction in risk for women who had ever breast fed.[4
] Also, as reviewed by Danforth and colleagues, [7
] most other more recently published population-based studies have found inverse associations with breast feeding [5
] although not all results have been statistically significant or shown trends of decreasing risk with increasing duration of lactation. Since the Danforth paper [7
] a further three studies have investigated this association. A Danish study of borderline ovarian tumors found an overall reduction in risk of around 2% per month of breast feeding.[6
] While we found some suggestion of risk reduction for this tumor group, our results were not statistically significant possibly reflecting the much higher proportions of never-breast feeders in our study (26% amongst our controls versus 6% in the Danish study). An Australian study reported a similar risk reduction for ever breast feeding and somewhat stronger inverse associations with total duration and average duration (per birth) of breast feeding (1.4% and 3.5% per month respectively) compared to the current study.[8
] Again the proportion of never breast feeders was lower in that study (18%). That study also considered duration of individual episodes of breast feeding and these analyses suggested that breast feeding a child beyond 12 months conferred no additional risk reduction. In contrast, our results for the longest episode of breast feeding, for breast feeding amongst women with only one child and for average duration per child breast fed suggest that risk reduction continues for episodes of 18 months of lactation and beyond. Finally a recent US-based case-control study found a decreased risk of ovarian cancer for ever breast feeders but little evidence of further risk reduction with increasing duration of lactation (of note, the proportion of never breast feeders amongst controls in that study was 46%).[9
] These authors also reported that the risk reducing effect of breast feeding was restricted to women who breast fed their last child - a finding that we did not replicate. They did however find, as we did, that the risk reduction associated with breast feeding was greatest for the combined group of endometrioid and clear cell cancers. A specific benefit for endometrioid or clear cell cancers has now been reported by five [16
] of nine studies (including the present study), that have investigated the subtype effects. A pooled analysis with the power to investigate the subtypes individually is required to definitively determine whether breast feeding has particular risk-reducing effects for endometrioid and clear cell cancers but such an association is biologically plausible. Endometriosis is a likely precursor lesion for both subtypes [18
] and endometriosis is effectively treated with drugs which induce a profound hypo-estrogenic state; breast feeding can have a similar effect. Indeed, breast feeding has been found to reduce the risk of endometriosis amongst parous women. [19
An overall effect of breast feeding is also biologically plausible. Breast feeding causes gonadatrophin suppression leading to low estrogen levels and anovulation with a resulting period of lactational amenorrhea (LAM) and should thus protect the ovary from most factors postulated to have a strong causative role in ovarian cancer development. [20
] We did not directly ask about the duration of LAM, although we had information about factors such as supplementary feeding and maternal body mass index around the time of child-bearing, which can influence its duration[3
]. In our study the timing of introduction of supplementary feeds did not appear to influence ovarian cancer risk, however if extra food merely supplements breast milk intake without reducing it then gonadatrophin suppression and anovulation may persist [21
]. The final indicator of duration of LAM that we have is duration of breast feeding itself. Duration of episodes of breast feeding is likely highly correlated with duration of LAM and our results do indicate that longer episodes of breast feeding result in lower risks of ovarian cancer. Although recall difficulties may be problematic, direct assessment of the duration of lactational amenorrhea and other practices associated with its duration (e.g. night feeding) may help us understand how the benefits of breast feeding for ovarian cancer reduction might best be achieved.
In conclusion we have found that, amongst women who have had the opportunity to breast feed, ever breast feeding and increasing durations of episodes of breast feeding are associated with a decrease in the risk of ovarian cancer. In line with the general advice from the US National Institute of Child Health and Human Development [23
] our results indicate maternal benefits for breast feeding to 12 months and beyond.