In the build-up phase, also follicle (maturation) phase, preovulation phase or proliferation phase, the egg cell matures.
The hormones FSH (follicle stimulating hormone) and LH (luteinising hormone), which are produced in the pituitary gland (hypophysis), cause the growth and development of the liquid-filled follicle in the ovary in the first half of the cycle and the maturation of the egg cell in it.
Physiological variations in menstrual cycle length are mainly due to variations in the length of the follicular phase. Although the 14th day for ovulation is persistent, the time of ovulation shifts depending on the length of the pre-ovulation phase. In 50% of cycles, ovulation occurs after the 14th day of the cycle, which is due to a longer follicular phase. In 5% of the cycles, this phase is so much shorter that ovulation takes place before the 12th day of the cycle.