As someone affected by this and someone who has met countless women in the same boat, this line of research seems important.
Long ago, I was looking at article after article in the New England Journal of Medicine online that described exactly my experience--having a child at an older age (almost 35) and then discovering breast cancer within three years.
The researchers of this latest study are calling breast cancer associated with pregnancy any bc found within five years of giving birth.
When I was reading through the journal articles some years ago, it seemed this is similar to the increased risk women who use Hormone Replacement Therapy face. It seems a concentrated burst of hormones for any reason can trigger bc, especially as a woman gets older than 30.
Anyway, very interesting research. I can't wait to read more about what they discover.