When does the follicular phase start?

The follicular phase lasts until ovulation and begins on the first day of menstruation. During the follicular phase, the uterine lining thickens, and an ovarian follicle begins to mature. This follicle houses an egg, which is released during ovulation. To regulate follicle growth and maturation, hormones such as follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are produced.

The matured follicle will rupture at the end of the follicular phase, releasing the egg into the fallopian tube, where it may be fertilized by sperm. If the egg is not fertilized, the levels of oestrogen and progesterone in the body fall, causing the luteal phase to begin and, eventually, menstruation.