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Breastfeeding for any duration was linked with a reduced risk of borderline and invasive ovarian tumors, a retrospective study indicated.

In a pooled analysis of nearly 24,000 women, breastfeeding at any point was significantly associated with risk reductions of 28% for borderline tumors and 24% for invasive ovarian cancers (including high-grade serous, endometrioid, and clear cell tumors), reported Naoko Sasamoto, MD, MPH, of Brigham and Women's Hospital in Boston, and colleagues.

"The World Health Organization recommends exclusive breastfeeding for at least 6 months and continued breastfeeding with complementary foods for 2 or more years," the authors wrote in . "Our results support these recommendations, while noting that breastfeeding fewer than 3 months per child is still associated with significant ovarian cancer risk reduction."

But longer average durations of breastfeeding per child were indeed tied to a greater reduction in the risk for invasive disease:

• Up to 3 months: (OR 0.82, 95% CI 0.76-0.88)

• 3 to 6 months: (OR 0.75, 95% CI 0.69-0.82)

• 6 to 9 months: (OR 0.69, 95% CI 0.62-0.76)

• 9 to 12 months: (OR 0.66, 95% CI 0.58-0.76)

• 12 months or more: (OR 0.66, 95% CI 0.58-0.75)

For women with one birth, the reduced risk of invasive cancers was only statistically significant for those who breastfed for 6 months or more, while any breastfeeding in women with two or more children was associated with a reduction in cancer risk.

Sasamoto and co-authors explained that the "biological mechanisms" for how nursing might lead to a reduction in ovarian cancer risk are not well established.

"To date, the leading hypothesis has been that ovulation suppression during breastfeeding inhibits epithelial cell division and proliferation, thereby reducing the opportunity to initiate or promote carcinogenesis," they wrote. "This may especially be pertinent in the first few months post partum when immune function and tumor surveillance mechanisms remain suppressed."

But they added that this cannot entirely explain the association, as longer duration of breastfeeding -- during which time ovulation would have typically resumed -- appeared to confer an even greater benefit.

"Several lines of evidence suggest that breastfeeding may also be associated with long-term modulation of inflammatory, immune, or metabolic pathways, which could influence ovarian cancer risk," they added.

For their study, Sasamoto's group examined 9,973 ovarian cancer patients that had given birth and 13,843 women from 13 case-control trials. Mean ages between the two groups were similar (57.4 vs 56.4 years, respectively). History of either endometriosis or a family history of ovarian cancer were both more common in the ovarian cancer cohort. In women who breastfed, the average duration per child ranged from 3.4 to 8.7 months.

By invasive cancer histology, breastfeeding was associated with a 25% reduction in high-grade serous cancers (OR 0.75, 95% CI 0.70-0.81), a 27% reduction in endometrioid tumors (OR 0.73, 95% CI 0.64-0.84), and a 22% reduction in clear cell tumors (OR 0.78, 95% CI 0.64-0.96).

For borderline tumors, breastfeeding was significantly linked to a reduction in both serous (OR 0.77, 95% CI 0.66-0.89) and mucinous tumors (OR 0.68, 95% CI 0.57-0.82).

Among the study limitations, the authors noted, were a mostly white group of women, the possibility of selection bias in certain included studies, as well as the "potential for differential self-reporting of breastfeeding information by disease status."

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