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Starting a family later in life could save women's lives, as delaying childbirth reduces the risk of developing an aggressive form of breast cancer, according to scientists.
Scientists at the Fred Hutchinson Cancer Research Centre found that women who wait at least 15 years after having their first period to have a child reduce their risk of triple-negative breast cancer by up to 60 percent.
Triple-negative breast cancer accounts for 15 percent of breast cancer cases in the UK and its survival rates are far worse than other forms of the disease.
The disease gets its name because it does not have receptors for oestrogen, progesterone or the protein Her2, as most forms of breast cancer do. As a result, it does not respond to hormone-blocking drugs.
A US study of 50,000 women with breast cancer found that just 77 percent of triple-negative breast cancer sufferers survived for five years or more, compared with 93 percent with other forms of the disease.
Christopher Li, from the research centre, said: "We found that the interval between menarche and age at first live birth is inversely associated with the risk of triple-negative breast cancer."
Previous studies have shown that breast feeding helps to reduce the risk of triple-negative breast cancer, so the authors say there are implications for African-American women, who experience abnormally high rates of the disease.
"Our observations that delayed childbearing and breast-feeding are protective against triple-negative breast cancer suggest that variations in reproductive histories by race may to some extent explain the higher rates of triple-negative disease in African-American women," Li said.
However, Li also warned that this was an observational study and the first to focus on premenopausal breast cancer, so results should be "interpreted with some caution".
The reason breast feeding and delaying childbirth reduces risk of triple-negative breast cancer is unknown, he added.
The study concluded: "A strong inverse association between breastfeeding and risk of triple-negative breast cancer has now been consistently observed across numerous studies, and at present it is the most well-established protective factor for this aggressive and lethal form of breast cancer.
"Further studies clarifying the biological mechanisms underlying this relationship and confirming our results with respect to age at first birth and the interval between age at menarche and age at first birth are needed."