The proliferative phase is a period of rebuilding within the female reproductive cycle, preparing the body for potential pregnancy. It begins immediately after menstrual flow ceases and concludes with ovulation, the release of an egg from the ovary. This phase is named for the rapid multiplication and growth of cells occurring in the uterine lining. While the follicular phase describes parallel events in the ovaries, the proliferative phase specifically refers to the structural changes in the uterus. Its duration typically ranges from 10 to 16 days, often ending around day 14 of a standard cycle.
Hormonal Signals Driving Growth
The proliferative phase is orchestrated by hormonal signals involving the brain and the ovaries. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
Early in the phase, the gradual rise of FSH acts on the ovaries, prompting several ovarian follicles to begin maturation. These growing follicles produce estradiol, a potent form of estrogen. This steadily increasing level of estrogen acts as the main chemical messenger, driving the cellular growth and preparation that characterizes the proliferative phase.
Rebuilding the Uterine Lining
The rebuilding of the uterine lining, or endometrium, is the main physical event of this phase. Estrogen, secreted by the maturing ovarian follicles, acts directly on the basal layer of the endometrium that survived the previous shedding. This hormonal signal stimulates rapid regrowth, forming a new, thick functional layer.
The lining thickens from a minimal post-menstrual state to about 8 to 12 millimeters by the end of the phase. This growth includes the elongation of endometrial glands and the development of new blood vessels, known as spiral arteries. These changes increase the blood supply, preparing the uterus to nourish a fertilized egg.
Follicle Maturation and Selection
While the uterus rebuilds, egg maturation occurs simultaneously in the ovaries. Under the influence of FSH, a cohort of between 3 and 30 ovarian follicles begins to develop; these are tiny sacs containing an immature egg and support cells.
As they grow, the follicles compete for continued development. The one most responsive to FSH establishes dominance by producing the highest amount of estrogen. This dominant follicle continues to mature, while the less successful follicles regress through a process called atresia, becoming the primary source of the high estrogen levels that dominate the latter half of the phase.
The Shift to the Next Phase
The proliferative phase is brought to a close by the high levels of estrogen it produces. As the dominant follicle reaches its maximum size, it secretes a high concentration of estrogen into the bloodstream. This sustained high level switches the hormonal control system from a negative feedback loop to a positive feedback loop on the pituitary gland.
The pituitary gland responds to this surge of estrogen by releasing a large amount of luteinizing hormone, known as the LH surge. The LH surge is the immediate trigger for ovulation, causing the wall of the mature dominant follicle to rupture and release the egg. This event marks the end of the proliferative phase and the beginning of the ovulatory phase.