WHAT: The follicular phase begins with the cycle day 1 and ends at ovulation. During the follicular phase, estrogen causes the endometrium (the lining of the uterus) to grow so it can receive a fertilized egg should you become pregnant. Another hormone, follicle-stimulating hormone (FSH), in turn stimulates the growth of ovarian follicles. Each follicle contains an egg. By late in the follicular phase, one egg will reach maturity and be ready to release. The variability in a woman’s cycle length is due to changes in the follicular phase length. In other words - when your period is late, the follicular phase is most likely to blame.