A study published online in August, 2013 in the Journal of Clinical Nursing found that breastfeeding for six months or longer could delay diagnosis of breast cancer by about 10 years. But the protective benefits were cancelled out if mom was a smoker.
The study, conducted at the University of Granada in Spain, examined medical records of more than 500 women aged 19 to 91, who were diagnosed and treated for breast cancer between 2004 and 2009 at a university hospital in the province of Granada.
Non-smokers who did not have children or who breastfed for less than three months were diagnosed with breast cancer at an average age of 57; non-smokers who breastfed between 3 and 6 months were diagnosed at a similar age of 56. Non-smokers who breastfed longer than six months, on the other hand, were diagnosed at an average age of 68 — more than 10 years later. However, women who breastfed longer than six months but were smokers were diagnosed at an average age of 47, or 21 years earlier. These results held steady, even when family history of breast cancer was factored in.