What exactly happens when you ovulate? 

Ovulation is the moment when your ovary releases a fully matured egg into the fallopian tube. The egg, which is about 1mm in size (the largest human cell, by the way), jumps out of the 20-25mm follicle in which it has matured over a few months.

Your fertile window – the days during which you can get pregnant – starts five days before ovulation and ends 12-24 hours after the egg is released. Sperm can survive for a maximum of five days in your body, although most sperm perish within three days. Immediately after sexual intercourse, sperm can reach the mature egg in 1-1.5 hours.

How to Determine Your Fertile Window >>

When does ovulation take place?

Ovulation takes place approximately in the middle of your cycle. Your cycle is divided into two phases: the follicular phase and the luteal phase. During the follicular phase, the hormone FSH causes several eggs in your ovaries to mature. The hormone estrogen ensures that the uterine mucosa grows and is thus prepared for the possible implantation of a fertilized egg.

ovulation menstrual cycle

Fig. 1: Ovarian cycle.

Usually, only one egg reaches the last stage of maturation. The oocyte is now called the dominant follicle. It is released from the ovary at the end of the follicular phase. Sometimes, two eggs reach the final development stage, making it possible to conceive twins.

The length of the follicular phase varies from woman to woman. A recent study with data from a fertility app showed that ovulation in many women does not occur on the 13th-15th day of the cycle as previously assumed:

  • In women with a shorter cycle length of 21 to 24 days, it took 12.4 days to reach ovulation.
  • In women with a typical cycle length of 25 to 30 days, it took 15.2 days to reach ovulation.
  • In women with a longer cycle length of 31 to 35 days, it took 19.5 days to reach ovulation.

How do I know an ovulation has occured in my cycle?

Unfortunately, there is only one way of knowing for sure that you have ovulatted: watching the ovulation on an ultrasound. However, it is impossible to predict your ovulation to the second, which is why it is also impossible to safely confirm that it took place.

Ovulation tests are a fairly safe parameter for whether and when ovulation takes place. The increase in the luteinizing hormone (LH) measured by ovulation tests is quite reliable but cannot confirm or rule out an ovulation with 100 percent certainty.

Some women feel pain in their lower abdomen and thereby know they’re ovulating. This pain can be a sudden twinge lasting for a few minutes, or a dull cramp lasting for a day or two. It’s usually on either the left- or right-hand side, depending on which ovary is releasing the egg.

A reliable sign that ovulation is about to take place is fertile discharge. The discharge is thin, clear, and slippery, much like an egg white. It’s a sign not only that ovulation is approaching but also that you are in your fertile window. Fertile cervical fluid helps sperm move up the cervix to fertilize an egg. It also keeps the sperm healthy during the journey.

Last but not least, the question remains whether the oocyte – assuming ovulation has taken place – is actually mature and, thus, has the potential to develop into an embryo. Unfortunately, this question cannot be answered. What is certain is that as you get older, ovulation becomes less frequent, and the quality of those eggs that are released deteriorates. Studies show that in women aged 40-42 years, only half of all eggs are capable of development, and there is no ovulation at all in 30-40 percent of all “normal” cycles.

How long does it take for an egg to mature?

Unlike boys, which constantly develop sperm in the testicle, girls are born with all the egg cells ever available to them. This amount is known as the ovarian reserve. At the time of a girl’s first period, about 400,000 eggs are left in her ovaries. Most of the follicles in the ovaries are in the stage  of primordial follicles, the earliest stage of development. Primordial follicles, in contrast to mature follicles, are only 0.03 mm in size. The egg cells contained therein are too immature to be fertilized.

At any given moment, cohorts of follicles are selected from the ovarian reserve to develop further. They go through the following stages of development over a period of about three months:

  • Primary follicles
  • Secondary follicles
  • Antral follicles
  • Mature follicles

In the first cycle phase, the follicular phase, a dominant follicle is selected from several antral follicles that are 6-8 mm in size. The dominant follicle continues to mature until ovulation. How exactly the selection of the dominant follicle works is still subject to research.

The corpus luteum forms from the empty follicle left behind after ovulation. The corpus luteum is responsible for producing the hormone progesterone, which stimulates the uterus to thicken even more in preparation for the implantation of a fertilized egg.

Fig. 2: Follicle maturation in the ovary. Over the course of about three months, follicles grow from 0.3 mm in size to 8 mm in size. Only one follicle makes it to 25 mm, releasing the egg at ovulation.


All the follicles that remain in an earlier stage of development undergo a degenerative process called follicular atresia. It is assumed that several hundred eggs perish in each cycle. In some women, it can be as many as several thousand. Of the hundreds of thousands of follicles originally present at your birth, only about 1,000 remain until menopause. Ninety-nine percent of all follicles you are born with do not make it to ovulation.

If you never use hormonal contraception and never get pregnant, you will ovulate an estimated 400 times. This average does not include women with polycystic ovary syndrome (PCOS), which typically comes with irregular cycles and infrequent ovulation.

What happens to ovulation and the ovarian reserve during pregnancy?

Once you are pregnant, follicle selection and maturation is reduced to a minimum. Because you don’t ovulate, fewer eggs continue to grow. The ovarian reserve is used up more slowly than normal. The same is true when you breastfeed your baby. Some studies have even shown that women who give birth to several children and breastfeed for at least half a year after birth have a significantly later onset of menopause.

Menopause and Fertility >>

What happens to ovulation if you use hormonal contraception?

If you take the pill, use a hormonal IUD, or get hormone injections, there is no ovulation. Nevertheless, the eggs in your ovaries keep on maturing. Unfortunately, you cannot save eggs by using hormonal contraception.

Hormonal contraception interferes with the hormonal interaction during your cycle and, thus, not only prevents ovulation but also influences the structure of the uterine lining and vaginal discharge, preventing pregnancy when used properly.

The anti-Mullerian hormone (AMH) is reduced by about a third during the use of hormonal contraceptives. If you’re getting your ovarian reserve tested, this situation should be taken into consideration by your doctor. If you need help with your hormone results, just e-mail me at silvia@fertinesse.com!

Does a menstrual period prove that I have ovulated?

A regular cycle with regular periods is an indication that your ovaries are still maturing follicles, but it does not prove that an ovulation is actually taking place. Menstruation also takes place without previous ovulation, as long as you have not entered menopause.

If you’re not sure whether you ovulate or not, drop me a note at silvia@fertinesse.com.I’m happy to help!


Sources:

Prior JC et al: Ovulation Prevalence in Women with Spontaneous Normal-Length Menstrual Cycles – A Population-Based Cohort from HUNT3, Norway. PLoS One. 2015; 10(8): e0134473.

Baerwald AR, Gree PA, Pierson RA: Ovarian antral folliculogenesis during the human menstrual cycle: a review. Human Reproduction Update. 2012 Jan-Feb;18(1):73-91.

Kaczmarek M: The timing of natural menopause in Poland and associated factors. Maturitas. 2007 Jun 20;57(2):139-53. Epub 2007 Feb 20.

Bull JR et al: Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digit. Med. 2, 83 (2019).

Shadyab AH: Maternal Age at Childbirth and Parity as Predictors of Longevity Among Women in the United States: The Women’s Health Initiative.
At J Public Health. 2017 January; 107(1): 113–119.