In the early follicular phase, after menstrual flow has ended, the lining of the uterus is at its most thin. Levels of estrogen and progesterone are at their lowest. Later in the follicular phase, proliferation (or thickening) of the uterine lining occurs. This thickened lining is preparing for a possible pregnancy. The follicular phase typically lasts about 14 days. The luteal phase begins when the follicular phase ends.