Breastfeeding May be Associated with Reduced Risk of Ovarian Cancer

In a study published in JAMA Oncology, breastfeeding was found to be associated with a significant decrease in risk of ovarian cancer overall, as well as for the high-grade serous subtype.

These findings suggest the breastfeeding may be a potentially modifiable factor that could lower risk of ovarian cancer independent of pregnancy alone.

“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. “Our results support these recommendations, while noting that breastfeeding fewer than 3 months per child is still associated with significant ovarian cancer risk reduction.”

Researchers performed a pooled analysis of parous women with ovarian cancer and controls from 13 case-controlled studies participating in the Ovarian Cancer Association Consortium. All data were collected from individual sites from November 1989 to December 2009, and the analysis took place from September 2017 to July 2019. Overall, 9,973 women with ovarian cancer and 13,843 controls were included in the study.

Breastfeeding was found to be associated with a 24% lower risk of invasive ovarian cancer (OR, 0.76; 95% CI, 0.71-0.80). Moreover, independent of parity, ever having breastfed was associated with a reduction in risk of all invasive ovarian cancer, particularly high-grade serous and endometrioid cancers. 

For a single breastfeeding event, the mean breastfeeding duration of 1 to 3 months was associated with 18% lower risk (OR, 0.82; 95% CI, 0.76-0.88), and breastfeeding for 12 or more months was associated with a 34% lower risk (OR, 0.66; 95% CI, 0.58-0.75). Further, more recent breastfeeding was associated with a reduction in risk (OR, 0.56; 95% CI, 0.47-0.66 for <10 years) that continued for decades (OR, 0.83; 95% CI, 0.77-0.90 for ≥30 years; P for trend = 0.02). 

“While we observed similar associations with breastfeeding patterns and endometrioid ovarian cancers, fewer cases of other invasive histotypes limited our ability to evaluate them with the same level of detail,” the authors wrote. 

Notably, the possible differential self-reporting of breastfeeding information by disease status could have distorted the magnitude of associations within the study. The results may also have been influenced by selection bias, as the researchers indicated that controls who participate in these studies may differ from women with ovarian cancer by factors influencing breastfeeding or ovarian cancer risk, including unknown factors that could not be accounted for in the analysis.

“Biological mechanisms through which breastfeeding could reduce ovarian cancer risk are not well understood. 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,” the authors wrote. “However, we observed a stronger inverse association with longer breastfeeding duration, suggesting anovulation cannot entirely explain the association because ovulation typically returns once solids are introduced.”

Additionally, it has also been suggested that breastfeeding may be associated with long-term modulation of inflammatory, immune, or metabolic pathways, which could impact ovarian cancer risk.

Reference:

Babic A, Sasamoto N, Rosner BA, et al. Association Between Breastfeeding and Ovarian Cancer Risk. JAMA Oncology. doi:10.1001/jamaoncol.2020.0421.