The Menstrual Cycle Explained: What Happens Each Month

The female reproductive system is different from the reproductive system of males in that it follows a monthly rhythm, called the menstrual cycle. One of the most visible signs of this cycle is menstruation, the shedding of the uterine lining, driven by a complex interplay of hormones. The first menstrual period, known as menarche, usually occurs around age 12, while cycles naturally stop at menopause, which most often happens around age 51. These recurring changes are a central aspect of female reproductive health and development.

Each month, the female body goes through a natural cycle to get ready for the possibility of pregnancy. The brain sends signals that stimulate the ovaries, helping eggs mature and eventually triggering their release. At the same time, ovarian hormones adjust the cervix to help sperm reach the uterus and thicken the uterine lining to support a fertilized egg. If no pregnancy happens, the body starts over, preparing for the next cycle.

There are two cycles in the menstrual cycle: the Ovarian (follicular phase, ovulation, and luteal phase) and Endometrial (proliferative phase, secretory phase, and menstrual phase) cycles. See more about the phases below.

Preparing for Ovulation

The first part of the menstrual cycle helps the body get ready for ovulation and a possible pregnancy. This includes the follicular phase in the ovaries and the proliferative phase in the uterus.

1. Follicular Phase: This phase starts on the first day of your period and ends when an egg is released. Follicle-stimulating hormone (FSH) helps eggs in the ovaries mature. A dominant egg continues to mature during this stage. 

2. Proliferative Phase: While the egg matures, estrogen from the growing follicles helps the uterus rebuild its lining after menstruation. The lining thickens and becomes full of blood vessels to prepare for a possible pregnancy. Estrogen also changes cervical mucus, making it easier for sperm to travel.

Ovulation

Ovulation happens about two weeks before your next period. When estrogen reaches a high enough level, it triggers a surge of luteinizing hormone (LH), which causes the dominant egg to be released from the ovary. After ovulation, the cervical mucus stays watery to help sperm reach the egg, while estrogen levels drop.

After Ovulation

After the egg is released, the second part of the menstrual cycle begins. This phase includes the luteal phase in the ovaries and the secretory phase in the uterus. It usually lasts about 14 days and ends when your period starts.

1. Luteal Phase: After ovulation, the empty follicle becomes the corpus luteum, which produces progesterone. This hormone prepares the uterus for a fertilized egg by thickening the lining, building glands, storing energy, and reducing cervical mucus elasticity to make it harder for sperm to pass. Progesterone also slightly raises body temperature.

2. Secretory Phase: If an egg is fertilized, it releases Human Chorionic Gonadotropin (hCG), which keeps the corpus luteum active and maintains progesterone production. If there’s no fertilized egg, progesterone and estrogen levels drop. This signals the body to start the next cycle.

Menstruation

When hormone levels fall, the uterus can no longer maintain its lining, and it sheds as menstrual bleeding. The first day of bleeding is day 1 of the next cycle. Menstrual flow usually lasts up to 8 days and contains blood, uterine lining cells, and vaginal fluids.

Photo Credits: https://kindbody.com/the-menstrual-cycle/

Citation

Thiyagarajan DK, Basit H, Jeanmonod R. Physiology, Menstrual Cycle. [Updated 2024 Sep 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK500020/