A. When a woman is pregnant, her breasts are much larger and her estrogen and progesterone levels are huge. Progesterone is sending out signals that provide a skeleton to build the breasts.

Q. Was it a surprise to learn that estrogen and progestins can cause breast cancer?

A. We’ve known there is a cause and effect with hormones and breast cancer since 1896. If a woman is premenopausal and she has breast cancer and you take out her ovaries, the tumors decrease in size. Not all the tumors — if you took 100 women who were premenopausal and took their ovaries out, 35 percent would have a response. And you could get a dramatic response. A tumor that was the size of a walnut could shrink in six months to the size of a pinhead. It turned out that the tumors that responded contained estrogen receptors. This became cause and effect — the estrogen receptor was the mechanism that estrogen used to stimulate tumors to grow. If there was no estrogen receptor, taking away estrogen didn’t do anything at all.

Image Research Leader Dr. V. Craig Jordan studies the effects of estrogen-blocking drugs on breast cancer. Credit... Jessica Roberts for The New York Times

Q. Did taking away estrogen ever make a breast cancer go away completely?

A. This is the basic difficulty. We were dealing with advanced breast cancer, and what we saw was that we could get complete remissions in 4 or 5 percent of the women. In the majority of women, the remission would last for one to two years. Taking away estrogen slowed things down, it reversed the process, but it did not cure.

Q. Do you agree with the latest analysis indicating that breast cancer is declining because so many women stopped taking Prempro and other menopausal hormones?

A. Throughout the 1990s, physicians were recommending that menopausal women take hormone replacement therapy. What happens is that you increased the rate of breast cancer in the whole country. And it shifted the epidemiology. We have seen an increase in the percentage of estrogen-receptor-positive tumors in the 1990s and in the beginning of the 2000s, so that now 70 percent of tumors are estrogen-receptor positive.

This was, if you like, consistent. Everything was ticking in. The Women’s Health Initiative and the Million Women Study in Britain really said: “Here’s a controlled series of studies comparing taking nothing with taking hormone replacement therapy. How many cancers were there at the end of the day?”