When am I most fertile?

Here you will find information about the menstrual cycle and the period when you are most fertile.

Women are fertile for only a few days a month. This is not true for men, also for them there are a number of other factors that can affect the chances of pregnancy.

The fertile period depends on when ovulation takes place within the menstrual cycle. To know when you are fertile, it is useful to gain insight into your menstrual cycle.

Research shows that the chances of pregnancy are highest if you have sexual relations in the three days before ovulation.

If you have sex three days before ovulation you have a 24% chance of getting pregnant, the day after (2 days before ovulation) it is 26% and one day before ovulation you have a 21% chance of getting pregnant. If you have sex on the day of ovulation itself, you only have a 10% chance of getting pregnant. This is because sperm cells can live for up to 72 hours (3 days) and eggs for only 12 hours. So the chances of getting pregnant are - on average - higher if you had sex in the days before ovulation.  

There are tools to calculate ovulation, but these tools are usually based on the assumption that ovulation always occurs 14 days before the next period. Meanwhile, research shows that this is not always the case. If you are normally fertile, it is enough to have sexual intercourse several times a week to get pregnant within a reasonable time.

The menstrual cycle

Most women have their periods about every 4 weeks from puberty to menopause. There are wide variations in the length of the menstrual cycle, the number of days of bleeding, the severity of bleeding and the symptoms experienced during periods. 

Length and duration of menstrual cycle

The menstrual cycle starts on the first day of the period and ends on the day before the next period. A normal menstrual cycle lasts between 21 and 35 days and can be regular or irregular. With a very irregular cycle, pregnancy may take longer. If this is the case, it is best to consult a doctor. Even if your cycle is shorter than 21 days or longer than 35 days, you should make an appointment with your GP or gynaecologist.  

During the menstrual cycle, the uterus prepares itself (under the influence of hormones) for the possible implantation of a fertilised egg. An extra layer of tissue then grows in the uterus. If fertilisation has not taken place, this extra layer of tissue (again under the influence of hormones) starts to crumble and is secreted (menstruation). Periods normally last from 3 to 7 days.

The different phases of the menstrual cycle

In a 28-day menstrual cycle, the first 14 days consist of the follicular phase. During this phase, several vesicles containing eggs grow in the ovaries (follicles), but normally only one follicle will fully mature per cycle. The follicle is fully matured around day 14 and ovulation takes place during this period.

Ovulation: it was long assumed that ovulation always took place 14 days before the next period, but we now know that there is more variation. Depending on cycle length, ovulation can take place between day 10 and day 26. Women with a shorter cycle (21 - 27 days) get their ovulation earlier (from day 10), women with a longer cycle (29 - 35 days) get their ovulation later. The mature egg released at ovulation is collected by the fallopian tubes. If intercourse takes place during this period, and thus sperm are available, fertilisation can take place in the fallopian tubes. The fertilised egg then takes a few days to reach the uterus and implantation in the uterus occurs about a week after ovulation.

The two weeks after ovulation are called the luteal phase. During this period, the uterus will get ready to possibly receive a fertilised egg (embryo). If an embryo implanted, then you will have no periods.

Ovulation is the moment when an egg is released from the ovaries. If you want to calculate the moment of your ovulation, you can keep track of when your periods start for several months. The first day of your period is the first day of your menstrual cycle. If your cycle always consists of the same number of days, then you have a regular cycle and it is easier to calculate the day of your ovulation. Ovulation usually occurs 2 weeks or about 14 days before your next period will start.

If, for example, you notice that your cycle (counting from the first day of your period) always lasts 28 days, then you can expect your ovulation around the 14th day after the start of your period and you will be most fertile from the 11th day after the start of your period until the 13th day after the start of your period.


If your cycle lasts 30 days each time, you should expect your ovulation around the 16th day after the start of your period and are therefore most fertile from the 13th day after the start of your period to the 15th day after the start of your period.

If you do not have a regular menstrual cycle, it is more difficult to calculate when to expect your ovulation. Since a normal cycle lasts between 21 and 35 days, there is a good chance that your fertile days fall between day 9 and day 16 of your cycle. If you have several sexual relations during this period, you are more likely to get pregnant.

Contact a doctor if your cycle is shorter than 25 days or longer than 35 days or if there is more than a 3-day difference in your cycle each time. For women who menstruate regularly, it is recommended not to focus too hard on the fertile period, but to have sexual intercourse several times a week. On the other hand, you can also recognise your fertile period by the nature of vaginal loss. Around the time of ovulation, you have a bit more vaginal loss and this mucus is also a bit clearer, smoother and more stretchy. Incidentally, this vaginal loss is also more accessible to sperm and some women are also more are to have sex during their fertile period.

You can also use an ovulation test to learn about your fertile period.  With an ovulation test, you can measure when certain hormones rise based on a urine sample. Most tests indicate when luteinising hormone (LH) starts rising. Since this hormone starts rising only one to two days before ovulation, you can miss fertile days with these tests. There are now new tests that also measure oestrogen levels, which means fewer fertile days are missed. In general, all ovulation tests are quite expensive and if you start measuring too early, you often use several tests needlessly.

Since the temperature rises slightly after ovulation, it is sometimes recommended to track the temperature daily to recognise the fertile period. However, this procedure is difficult and will only indicate when the fertile period is over. Moreover, your body temperature can also be affected by other factors. 

The window of fertility

During the menstrual cycle, most women are fertile for several days. You have a higher chance of pregnancy if you have sexual intercourse during this period. After ovulation, the egg survives for another 24h. Therefore, the fertile period of a cycle ends 24h after ovulation, given that the egg is no longer usable after this period. The sperm, on the other hand, can survive in the female body for up to 5 days. Generally, if one has sexual relations in the three days before ovulation, the chances of pregnancy are higher.

There are a lot of individual differences and even with a regular cycle, it is difficult to predict the exact fertile period.

If you are normally fertile, you have about a 90% chance of becoming pregnant within the year if you have regular sexual intercourse around the middle of the cycle. Even in perfectly fertile couples, it can take slightly longer than a year to get pregnant. 


The words "woman" and "man" are increasingly ambiguous concepts and should be avoided where possible. For ease of communication on this platform, we use the terms "woman" and "man" when providing information about biological processes. In this context, the term "woman" refers to a person with "female" sex characteristics (e.g., vulva, uterus, ovaries, etc.), and the term "man" refers to a person with "male" sex characteristics (e.g., penis, testes), although we are aware that:

1) There are many intersex individuals or individuals with variations in sex characteristics;

2) There are many people for whom gender identity does not fully correspond to or align with sex characteristics.

When providing information about relational aspects, however, the term "woman" does not refer to biological characteristics, but rather to gender. This also includes trans women (individuals who were legally registered as male at birth based on sex characteristics but have a female gender identity) and intersex individuals (born with variations in sex characteristics) who identify as women. Similarly, the term "man" refers to gender in this context, including trans men and intersex individuals who identify as men.