Updated December 10, 2014.

Definition: Follicular Phase: The follicular phase begins on Day 1 of your menstrual cycle. Day 1 is by definition the day your period starts. The follicular phase ends when a hormone called luteinizing hormone peaks and ovulation occurs.

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.

Also Known As: proliferative phase

Examples: The female hormones estrogen and progesterone are at their lowest levels during the early follicular phase of the menstrual cycle.