The Physiology of Ovulation, Women’s Fertility, and the Menstrual Cycle

By Lindsay Meisel | Published Nov 18, 2015 | Last updated Nov 23, 2020 ✓ Fact checked

Essential Takeaways

Female fertility is determined by a repeating fluctuation of hormones called the menstrual cycle
The primary hormones involved in the menstrual cycle are estrogen and progesterone
Estrogen and progesterone play an important role in egg maturation and uterine lining proliferation

The female menstrual cycle and women’s fertility is complex. In order to best appreciate these complexities, we must first understand the physiology of the body to understand how and why the cycles work as they do. A woman’s fertility repeats itself every 28 days on average, in a cycle that includes preparing for ovulation and then either preparing to sustain a pregnancy or preparing to repeat the cycle. Ovulation, the release of the egg, usually happens at roughly the mid-point of the monthly cycle. A significant part of women’s health centers on hormones, fertility, and the physiological preparation for pregnancy each month.

Menstrual Cycle

A woman’s menstrual cycle begins on the first day of her period and ends the day before her next period begins. Between these two days, hormonal fluctuations help prepare the body for a potential pregnancy. During the first half of the menstrual cycle, called the follicular phase, estrogen levels increase leading to the thickening of the uterine lining. Ovulation, when an egg is released from one of the ovaries, marks the end of the follicular phase and the transition to the second half of the cycle—called the luteal phase. During the luteal phase, progesterone levels increase, which help further prepare the uterine lining for the possible implantation of an embryo.

If the egg is fertilized by a sperm, conception will have occurred. Next, the fertilized egg must begin its journey through the fallopian tubes toward the uterus, where it needs to implant in the uterine lining. If the egg is not fertilized, or if a fertilized egg does not successfully implant in the uterine lining, pregnancy does not occur. Progesterone levels plummet, triggering menstruation. The next cycle begins with this shedding of the uterine lining.

Fertility

At birth, females are born with between one and two million eggs. However, only between 300 and 400 eggs will be released during ovulation over a lifetime. If ovulation coincides with the presence of sperm in the fallopian tubes, fertilization may occur. Fertilization must occur within 24 hours of the release of the egg, and sperm may live up to five days in the fallopian tubes, so the window for conception can last up to six days. The optimal time to have intercourse when actively trying to conceive is the one to three days immediately prior to ovulation; this gives the sperm time to go through the vital process of capacitation before the egg is released.

Ovarian Cycle

The ovarian cycle is the process of ovulation that occurs each month. It includes the follicular phase and the luteal phase. During the follicular phase, the ovaries begin preparing an egg to be released. GnRH stimulates the pituitary gland to produce follicle stimulating hormone (FSH) within the ovaries, which are sacs filled with fluid and eggs. Eventually, one follicle becomes dominant—this is the follicle that will release an egg during ovulation.

After ovulation, the luteal phase begins, which lasts until the next period starts. During this time, the follicle that released an egg transforms into the corpus luteum and begins producing progesterone. If the egg wasn’t fertilized, the corpus luteum will break down after about two weeks. As progesterone levels peak and decline, many women experience premenstrual symptoms: bloating, acne, breast pain, and changes in mood are common. If the egg was fertilized, the corpus luteum continues progesterone production to support early pregnancy.

Uterine Cycle

The uterine cycle includes three phases: menstruation, the proliferative phase, and the secretory phase. Menstruation usually lasts for five or six days. Next comes the proliferative phase, when the uterus begins re-building lining that was shed during menstruation. The uterus needs a thick lining for a fertilized egg to implant and grow if conception occurs. The final phase is the secretory phase, which involves the secretion of chemicals that will help with implantation of a fertilized egg or will help the lining begin to break down so it can be shed again.

Menstrual Disorders

Menstrual disorders can include abnormal uterine bleeding, such as heavy periods, absent periods, called amenorrhea, irregular periods, called oligomenorrhea, or irregular bleeding between cycles. Another menstrual disorder is called dysmenorrhea, and it involves unusually painful periods. Some women also have fibroids, which are noncancerous tumors in the uterus, which can cause heavy or prolonged periods. Anyone who experiences irregular or abnormal menstrual bleeding, excessive pain, headaches, emotional distress, and bloating may need to be evaluated by a physician.

Menopause

Every woman will eventually stop having monthly periods, and when this happens, she has reached menopause and fertility has ended. The average age for going through menopause is 51, and it usually takes several years for a woman to proceed through the stages of perimenopause, menopause, and postmenopause. A woman is considered to have reached menopause when she hasn’t had a period for 12 consecutive months. Common symptoms experienced during menopause include sleep disturbances, hot flashes, and changes in the vaginal and urinary tract as estrogen levels decrease.

By Lindsay Meisel | Nov 18, 2015

Lindsay Meisel

Lindsay Meisel is the Head of Content at Ava. She has over a decade of experience writing about science, technology, and health, with a focus on women's health and the menstrual cycle. Her work has been featured on The Fertility Hour, The Birth Hour, The Breakthrough Journal, and The Rumpus.