follicular phase

The follicular phase is the first half of the menstrual cycle, the build-up phase during which the ovary grows a follicle and the body prepares its lining. It runs from the first day of menstruation until ovulation. Think of it as the casting and rehearsal stage: many candidates start, one star emerges, and estrogen rises throughout in anticipation of the main event.

Mechanistically, a small rise in FSH at the start of the cycle recruits a cohort of follicles to grow. As they enlarge, their granulosa cells pour out estradiol. The most responsive follicle becomes dominant and keeps growing even as rising estrogen suppresses FSH, starving its weaker rivals, which undergo atresia. Inside the uterus, this estrogen rebuilds the lining shed during the last period — the proliferative endometrium.

The follicular phase ends when estrogen climbs high enough, for long enough, to flip from suppressing to stimulating the pituitary, producing the LH surge that triggers ovulation. A practical caveat: this phase is the variable part of the cycle, so women with irregular periods usually have follicular phases of differing length, while the luteal phase that follows stays relatively constant.