Introduction to the Menstrual Cycle

The menstrual cycle is a complex series of events that occurs in the female reproductive system, typically lasting around twenty-eight days.

Throughout the menstrual cycle, hormonal fluctuations play a crucial role in regulating the various phases. The main hormones involved include:

• Follicle-stimulating hormone (FSH): Stimulates the development of follicles in the ovaries.
• Luteinizing hormone (LH): Triggers ovulation and promotes the formation of the corpus luteum.
• Estrogen: Produced by the growing follicles, it promotes the thickening of the uterine lining.
• Progesterone: Secreted by the corpus luteum, it helps maintain the endometrium and supports a potential pregnancy.

Understanding the menstrual cycle is essential for assessing reproductive health, identifying fertility windows, managing contraception, and diagnosing menstrual disorders. It also plays a crucial role in understanding various conditions related to the female reproductive system, such as polycystic ovary syndrome (PCOS), endometriosis, and menstrual irregularities.

Endometrium

The layer of tissues lining the uterus (which houses the fetus during pregnancy). This uterus lining consists of two layers:

• The functional layer, which sheds during monthly menstrual cycles
• The basal layer, which aids in feeding the top functional layer.

The Ovarian Cycle

Ovulation is a phase in the menstrual cycle when the ovary releases an egg. Once the egg leaves the ovary, it travels down the fallopian tube, waiting to be fertilized. The average time is day 14 on a 28-day cycle. After this fertilization, the egg implants in the uterus.

Memory Trick

 O – Ovulation

 O – Ovaries release an

 O – Oval shaped egg

 O – Over to the uterus

The Follicular Phase

Before an egg is released (before ovulation), several hormones pop off like a party popper. The hypothalamus releases Gonadotropin releasing hormone (GnRH), which dominos into the release of Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH) from the anterior pituitary. This stimulates the follicles within the ovaries, causing ovulation.

The Luteal Phase

Ovulation begins, and the egg floats away from the ovaries toward the uterus. The empty follicle within the ovary (corpus luteum) releases both estrogen and progesterone (steroid sex hormones) to help to thicken the endometrium and to turn off excess FSH and LH hormones to help a fertilized egg implant and develop into a baby. 

But if the egg is not fertilized within fourteen days, then the egg dies, and the thick endometrium dissolves, causing menstrual bleeding.

The Uterine Cycle

Think of the “catching phase.” The endometrium becomes thick enough to house a fertilized egg, but if the egg is not fertilized within fourteen days, it will shed each month, causing menstrual bleeding.

Pregnancy

Now if the egg is fertilized with sperm, then the menstrual cycle will stop, as the egg attaches to the uterus wall. This embryo releases HCG (human chorionic gonadotropin) hormone, which is the hormone tested during a pregnancy test. 

A positive test means positive for HCG hormone. This HCG helps to keep estrogen and progesterone levels HIGH by keeping the corpus luteum open inside the ovaries. 

Birth Control

Oral contraceptives (or the ring), works by keeping estrogen and progesterone high in order to control ovulation and prevent fertilization.

Plan B Pill (Morning After Pill)

This emergency contraception works by shedding the uterine lining (the endometrium) so that a fertilized egg can not attach and become a fetus.

Menopause

Menopause is a natural biological process that marks the end of a woman’s reproductive years. It is typically diagnosed when a woman has gone twelve consecutive months without a menstrual period. Menopause occurs due to a decline in the production of reproductive hormones, particularly estrogen, and progesterone, by the ovaries.

Menopause marks the end of the menstrual cycle. During menopause, a woman’s ovaries gradually stop releasing eggs, resulting in the cessation of menstrual periods. This is because the decline in hormone production leads to the loss of ovarian follicles, which are responsible for releasing eggs.