Most of these recent users were young women. If DMPA initiated breast cancer, women exposed to the largest amounts of DMPA that is, women who used DMPA the longest would have the highest risk of breast cancer. Among all users, however, duration of use makes little difference to risk. Better detection of breast cancer among DMPA users may explain part of the apparently increased risk. DMPA users and users of combined OCs may face similar risks of breast cancer.
Pregnancy has a similar effect on the risk of breast cancer.
This pattern, too, suggests that higher than usual levels of reproductive hormones accelerate the development of existing tumors. If DMPA does increase the risk of breast cancer, the additional cases attributable to DMPA would be slight.