Understanding the Menstrual Cycle and its Phases
The menstrual cycle spans 21-35 days and is regulated by the complex interaction of the hypothalamus, anterior pituitary gland, ovaries, and uterus.1 The ovarian and endometrial cycles occur within the overall menstruation process.
Day 1 to ovulation: Follicular and proliferative phase
The follicular phase begins on day 1 (onset of menstruation) and continues through to ovulation, which can take from 7-21 days.
Follicle-stimulating hormone (FSH) stimulates a group of primordial follicles to mature. The dominant follicle produces rising estradiol, which suppresses non-dominant follicles. At the same time, estrogen triggers endometrial proliferation.
Mid-cycle: Ovulation
A surge in luteinizing hormone (LH) triggers ovulation, briefly followed by a drop in estrogen. This marks the transition between the two primary phases (follicular and luteal).
Days ~14: Luteal and secretory phase
After ovulation, granulosa cells transform into luteal cells, which create the corpus luteum, a structure that secretes progesterone. Progesterone prepares the uterus for possible pregnancy by stimulating endometrial glandular development, glycogen accumulation, and the expansion of spiral arteries.
If pregnancy does not occur, the lining of the corpus luteum breaks down. Progesterone and estrogen decline and menstruation follows.
