> About 13% of women develop breast cancer during their lifetime. 25% increase is then very significant, this is about additional 4% chance of getting cancer.

The study didn’t measure lifetime risk, so your assertion is not correct. It measured risk of acquiring breast cancer before age 49. This is not the age when most women get breast cancer. Average age of onset for breast cancer in the US is 62.

While you are partly correct, the study just "didn't measure". It means the study does not say anything about getting cancer after 49. What it does not say is that there isn't any increase after the age women typically stop using contraceptives.

In particular, since we don't know the mechanism in which contraceptives cause cancer, we can't say whether trend ends when women stop taking them or will increase their chance of getting cancer for the rest of their lives.

Yes, however, we have over a half century of data on hormonal contraception at this point, more than enough to measure the effect on lifetime risk.

This very large meta-study showed the increased risk is no longer significant about ten years after stopping use: https://pubmed.ncbi.nlm.nih.gov/8656904/

(btw, got that link from this article about the study in the OP, which points out potential confounders: https://www.statnews.com/2023/03/21/small-rise-breast-cancer...)

Thank you for the context. The net increase in risk is what I was looking for.