What is a follicle on the ovary?

Medically reviewed on August 17, 2022 by Amy Harris, MS, RN, CNM. To give you technically accurate, evidence-based information, content published on the Everlywell blog is reviewed by credentialed professionals with expertise in medical and bioscience fields.

Table of contents

• Ovarian follicles and your menstrual cycle
• Why are follicles important?
• What is the difference between a follicle and a cyst?
• Do follicles on your ovary impact fertility?
• Everlywell: Your source for reproductive health answers

Ovarian follicles are the sacs within your ovary that house your undeveloped eggs until it is time for you to ovulate. When it comes to female fertility, your ovarian follicles are definitely one term to learn more about. If you are wondering whether you might need help getting pregnant through infertility treatments, it can help to learn some basic reproductive biology as a first step. Healthcare providers sometimes check ovarian follicles to help diagnose and treat infertility.

Keep reading to learn more about ovarian follicles and what role they play in your menstrual cycle and fertility.

Ovarian follicles and your menstrual cycle

At birth, female fetuses are born with approximately one million eggs. Surrounding these immature eggs are tiny sacs filled with fluid - your ovarian follicles. At puberty, females typically have between 300,000 to 400,000 follicles. [1]

Your menstrual cycle is the series of hormone-driven events that prepares your body to get pregnant and carry a baby. At the start of each menstrual cycle, several eggs start to mature and develop into an egg that can be fertilized by a sperm. However, only one egg reaches full maturity per cycle. During the first 14 days of your menstrual cycle (called the follicular phase of the menstrual cycle), this dominant, bigger follicle forms. All the other follicles shrink and eventually disappear. Once this egg is ready, the dominant follicle cracks open (a bit like an egg itself). Ovulation happens with this follicle rupture and release of the egg from the ovary. It then travels down the fallopian tubes into the uterus for fertilization.

After the egg has left the follicle, your corpus luteum (a normal cyst that forms each month after ovulation at the surface of your ovary) starts to form from the materials that made up that follicle. This second part of your cycle is called the luteal phase. Every woman’s menstrual cycle is unique. The length of each cycle and its phases can vary based on your age and other factors. Changes in the length of your follicular and luteal phases of your menstrual cycle can sometimes make getting pregnant more complicated.

Why are follicles important?

Follicles on your ovaries play a key role in reproductive health. They help by: [2]

• Protecting your immature eggs
• Nourishing the eggs until they are ready for ovulation
• Secreting the hormones that regulate the menstrual cycle
• Provide information about your status when considering treatment for infertility

It can take almost a year for your ovarian follicles (and the eggs they hold) to develop from their immature state until they are ready to journey to the uterus for fertilization. Whether they do so correctly and in the right sequence can have a big impact on your fertility. Two conditions related to female infertility are directly related to problems with ovarian follicle development.

Premature ovarian insufficiency is when a person’s ovaries stop working normally before the age of 40. The cause of premature ovarian insufficiency is unknown in the majority (90%) of cases. Irregular periods, trouble getting pregnant, and early signs of menopause are the most common symptoms of premature ovarian insufficiency.

In the second condition, polycystic ovarian syndrome or PCOS, people have irregular menstrual cycles, hormonal imbalances, and decreased fertility. The root cause of these changes is abnormal follicular development and an excess number of ovarian cysts.

What is the difference between a follicle and a cyst?

While normal follicle development is a key part of female fertility, ovarian cysts are not. Ovarian cysts are also fluid-filled sacs in your ovaries—they don't serve a real purpose but, in most cases, aren't harmful.

The two most common types of ovarian cysts found inside ovaries are: [3]

• Follicular cysts – A follicle cyst can form when the follicle doesn't break open and release an egg during ovulation. The follicular cells continue to grow, fill with fluid, and become a cyst. You typically won't feel when you have a follicular cyst—most are painless and harmless. The ovary reabsorbs most follicular cysts within a few months. Follicular cysts are not cancerous. Occasionally they can grow large enough to cause pain, feelings of fullness, and even rupture. Seek medical treatment immediately if you feel sharp or sudden pain in your lower abdomen, especially if it’s accompanied by nausea or fever.
• Corpus luteum cyst – Corpus luteum cysts form after the dominant follicle breaks open and releases the egg. Making progesterone is the corpus luteum’s most important job. Progesterone changes the uterus into a healthy environment for a fertilized egg to develop and grow into a fetus. Sometimes the corpus luteum sac doesn't shrink like it should and instead reseals itself, trapping fluid inside and forming a cyst. Like follicular cysts, corpus luteum cysts can also grow big enough to cause pain and eventually burst. Corpus luteum cysts typically go away more quickly than follicular cysts, usually after a few weeks.

So, as you can see, follicles are different from ovarian cysts. Follicles can become cysts. If you are experiencing an ovarian cyst and want to begin practices to get rid of it, discover tips on how to shrink ovarian cysts naturally.

Do follicles on your ovary impact your fertility?

Yes. The most common overall cause of female infertility is the failure to ovulate, which occurs in 40% of people with infertility issues. That is why many fertility evaluations include blood tests to measure hormone levels associated with ovarian follicles. The American College of Obstetricians and Gynecologists (ACOG and the American Society for Reproductive Medicine (ASRM) recommend starting an evaluation for infertility if a woman has been trying to get pregnant for at least one year without success. Because fertility declines with age, women who are older than 35 should see a healthcare provider for a fertility evaluation after six months of trying to get pregnant without success. Age impacts ovarian follicles and female fertility by:

• Making it harder for ovaries to release eggs
• Decreasing the number of eggs to be released
• Making the eggs less healthy (harder to fertilize and develop into a normal pregnancy)

In addition to blood tests, your healthcare provider may order an ultrasound to check your ovarian follicles for: [4]

• Size
• Quantity
• Quality

If your follicles aren't the correct size or contain poor-quality eggs (called low ovarian reserve), you may have a harder time getting pregnant without some help from a fertility specialist (usually a reproductive endocrinologist) or an obstetrician-gynecologist (OBGYN). Infertility problems related to problems with your ovaries, follicles, or menstrual cycles not working correctly can include: [5]

• Hormonal imbalances preventing ovulation
• Failure of your eggs to properly mature in the follicle
• Infections that damage the existing eggs in your ovaries
• Blockages of the tubes that the egg must travel down from your ovaries to your womb (uterus)
• Inability of fertilized eggs to implant in your uterus

Outside of egg quality, other factors leading to infertility can include autoimmune disorders (like lupus), polycystic ovarian syndrome (PCOS), uterine fibroids, and structural issues in the reproductive system such as blockages in the fallopian tubes. In one out of every three infertile couples, male infertility is the cause. If you are trying to get pregnant, it is important to look at both your health and fertility and your partner’s health and fertility. For both men and women, overall health and wellness affect the ability to conceive. Being overweight or underweight, smoking, excessive alcohol use, and exposure to pesticides can all be lifestyle-related factors impacting fertility.

Everlywell: Your source for reproductive health answers

Follicles are normal, fluid-filled sacs that form on your ovaries once you start ovulating (releasing eggs monthly from your ovaries). Your ovaries usually release one (except in the case of twins) follicle with a mature egg each month. Follicles are important for female fertility because the hormones they secrete help drive egg maturation, menstrual cycles, and support an early pregnancy in the uterus if the egg gets fertilized. Problems with either your menstrual cycles, eggs, follicles, or ovarian cysts can make getting pregnant more complicated.

If you've had difficulty conceiving and want to know what's causing your problems, we might be able to help you get started on your hunt for some answers. The Everlywell Female Hormone Test measures the levels of certain fertility-associated hormones in your body. The five hormones measured by the Everywell test are estradiol, luteinizing hormone (LH), follicle stimulating hormone (FSH), testosterone, and thyroid stimulating hormone (TSH). Getting a sense of these hormone levels can be the first step towards better understanding your body and fertility.

You can also browse the suite of fertility products offered by Natalist, including a Women's Fertility Test bundle and prenatal daily packets.

Related content

What ovarian reserve testing can tell you about fertility

What is follicle-stimulating hormone (FSH)? FSH explained

What is a diminished ovarian reserve?

References

1. Cancer.gov. Ovarian Follicle. URL. Accessed August 17, 2022.
2. The Journal of Clinical Investigation. The Ovary: Basic Biology. URL. Accessed August 17, 2022.
3. Womenshealth.gov. Ovarian Cysts. URL. Accessed August 17, 2022.
4. Fertility Research and Practice. Fertility with Early Reduction of Ovarian Reserve. URL. Accessed August 17, 2022.
5. NIH. Possible Causes of Female Infertility. URL. Accessed August 17, 2022.