The longest phase of the menstrual cycle is known as the follicular phase, and it can last between 14 to 21 days. During this phase, your ovaries support the growth and development of an egg that will be released later during ovulation. As the follicular phase draws to a close, a woman is at her most fertile and is more likely to become pregnant during sex.

A woman is born with about a million eggs, but not all of these eggs will reach maturity. During the follicular phase of the menstrual cycle, between 11 to 20 eggs start developing in the ovaries. The development of these eggs is prompted by the release of follicle-stimulating hormone (FSH) from the pituitary gland in the brain. This hormone signals the ovaries to begin producing follicles, which are fluid-filled sacs where the eggs can mature.

At one point during the follicular phase, a dominant follicle emerges. This follicle develops faster and stronger than the rest of the follicles, and the egg inside of it will be the egg that is eventually released during ovulation.

At the same time, more estrogen is released in the body to prepare for potential pregnancy. This increase in estrogen causes the lining of the uterus to thicken to be able to support a fertilized egg. If the egg becomes fertilized at this point in the menstrual cycle, it can then implant itself in the thickened lining of the uterus to continue to develop. If the egg does not become fertilized, the lining of the uterus will be shed during menstruation.

As a woman’s estrogen levels rise, her FSH levels drop. This drop in FSH effectively stops the growth of the remaining follicles (those that were not the dominant follicle) and causes them to be reabsorbed into the body.

Lastly, at the end of the follicular phase, the elevated estrogen levels in the body trigger the release of luteinizing hormone (LH). LH is responsible for releasing the mature egg from the dominant follicle and the ovary so that it can migrate to the fallopian tubes where it can be fertilized by a sperm.

The length of the follicular phase, like other phases of the menstrual cycle, varies from woman to woman. In particular, the length of this phase depends on the amount of time it takes a woman’s body to develop a fully mature egg.

References:

• Cleveland Clinic. (2022, August 8). Follicular Phase. https://my.clevelandclinic.org/health/body/23953-follicular-phase
  
  
• Monis, C. N., & Tetrokalashvili, M. (2019). Menstrual cycle proliferative and follicular phase. https://www.ncbi.nlm.nih.gov/books/NBK542229/