Proliferative Phase

The proliferative phase begins after menstruation and lasts approximately 6 to 13 days in a typical 28-day cycle. This phase follows the menses phase and precedes ovulation. During this time, the uterine endometrium undergoes reconstruction under the influence of estrogen released by growing ovarian follicles.

The stratum basale, the basal layer of the endometrium, survives menstruation intact and serves as the source for endometrial regeneration. Stem cells within this layer undergo rapid mitosis, creating a new stratum functionalis layer. This regenerative capacity allows the endometrium to rebuild completely during the proliferative phase.

During the proliferative phase, the endometrium thickens from approximately 2 mm to 4–10 mm, roughly doubling in thickness. Endometrial glands develop and become more numerous, while arterioles coil and lengthen to penetrate the newly formed stratum functionalis. These vascular changes enhance blood supply, giving the endometrium a velvety, thick, and well-vascularized appearance.

The proliferative phase concludes with ovulation, which is triggered by a surge in luteinizing hormone released by the anterior pituitary gland. This hormonal surge causes the dominant follicle to mature into a Graafian follicle and rupture, releasing the oocyte. The luteinizing hormone surge marks the transition from the proliferative phase to the secretory phase.

Estrogen, released by growing follicles in the ovaries, drives endometrial reconstruction during the proliferative phase. It stimulates stem cells in the stratum basale to divide rapidly and promotes the development of endometrial glands and blood vessels. Without adequate estrogen, the endometrium cannot rebuild properly to prepare for potential implantation.

Follicle maturation in the ovaries occurs simultaneously with endometrial reconstruction during the proliferative phase. Growing follicles produce estrogen, which stimulates endometrial thickening and gland development. The phase culminates when the dominant follicle matures into a Graafian follicle, triggering ovulation and ending the proliferative phase.

Arterioles coil and lengthen during the proliferative phase, penetrating the newly formed stratum functionalis to enhance blood supply to the endometrium. This increased vascularization delivers oxygen and nutrients necessary for gland development and tissue thickening. The well-vascularized endometrium becomes prepared to support potential embryo implantation.