Final answer:
Progesterone breakthrough bleeding is most commonly seen in women who use progesterone-only contraception, due to the hormonal changes and the way these pills function.
Step-by-step explanation:
Progesterone breakthrough bleeding is sometimes seen in women who use progesterone-only contraception. This is due to the way progesterone-only pills (POPs) work; they thicken the cervical mucus and inhibit ovulation, but may also cause irregular menstrual periods. Women on POPs can experience breakthrough bleeding as their body adjusts to the hormonal changes. Breakthrough bleeding can also occur when there's a decline in progesterone levels, such as during menopausal transition or cessation of progesterone therapy.
It is less likely to be associated with obesity or conditions like polycystic ovary syndrome (PCOS), unless those conditions are influencing the effectiveness or metabolism of the contraceptive. Issues such as hormonal imbalances or irregular periods tied to PCOS might indirectly influence the occurrence of breakthrough bleeding as well.