Follikelfase

Follikelfase, or follicular phase, is the first phase of the female menstrual cycle. It begins on day 1 with menstrual bleeding and lasts until ovulation. The phase is characterized by recruitment and growth of ovarian follicles under the influence of follicle-stimulating hormone (FSH).

FSH stimulates several primordial follicles to grow into primary and secondary follicles. The granulosa cells of these follicles produce estrogen, mainly estradiol, which stimulates the proliferative growth of the endometrium. Rising estrogen levels regulate the hypothalamic-pituitary axis, initially providing negative feedback to FSH and eventually contributing to a positive feedback that leads to a surge in luteinizing hormone (LH) late in the follicular phase, triggering ovulation.

During the late follicular phase, one follicle typically becomes dominant and reaches maturity, while others regress. The dominant follicle releases a mature oocyte during ovulation. After ovulation, the ovary enters the luteal phase with the formation of the corpus luteum.

Typical duration is about 10 to 14 days in a 28-day cycle, but the length can vary widely between individuals and cycles. In anovulatory cycles, ovulation does not occur and the follicular phase may not lead to corpus luteum formation.

Clinically, the follicular phase is important for fertility, as ovulation defines the fertile window. Hormone measurements and ovulation tests reflect follicular maturation, and deviations in this phase can indicate reproductive disorders.