"Breast cancer is clearly related to estrogen exposure," he said.

In general, increased estrogen exposure may increase the risk of blood clots.

While the causes of breast cancer are only partly understood, it is known that estrogen exposure is a key factor in the development of this disease.

Long-term unopposed estrogen exposure can be caused by:

An increased incidence of endometrial cancer has been found in association with prolonged, unopposed estrogen exposure.

But experts say estrogen exposure earlier in life may be more potent in stimulating breast cancer than exposure that occurs later.

Mutations that can lead to breast cancer have been experimentally linked to estrogen exposure.

The greatest estrogen exposure by far that a woman ever gets is during the first and second trimesters of pregnancy.

Other factors associated with increased risk, such as obesity and polycystic ovary syndrome, may also be related to increased estrogen exposure.