"We had looked at giving birth early, giving birth late, as well as breast-feeding, " says Ursin, who's on the faculty at the University of Southern California.
Ursin's group found that breast-feeding cancels out the effect of delaying childbirth past the age of 25 the average age of first birth in the United States these days.
"The good news is that breast-feeding protects against breast caner no matter when you give birth whether you give birth before age 25 or after age 25, " Ursin says.