"Estrogen breakthrough bleeding:
Occurs when ovulation does not happen
resulting in a state of high estrogen and low progesterone.
This allows the uterus to be continually exposed to estrogen until the levels are so high that feedback from the brain (in the form of decreased follicle stimulating hormone) causes estrogen levels to decline somewhat.
Progesterone breakthrough bleeding:
This occurs when an excessive ratio of progesterone to estrogen exerts its influence on the endometrium.
Typically this situation occurs in women taking only progesterone-containing birth control pills"