Did you know that there are actually only a few days in the menstrual cycle when it’s possible to get pregnant? This is called the “fertile window”. In this article, we cover everything you need to know about when you are most fertile, and proven ways to calculate your fertile window so you can boost your chances of conceiving.
What is the fertile window?
A normal menstrual cycle can last between 21-35 days. The first day of your period represents the first day of a new cycle. There are only 6 days within your cycle when it’s possible to get pregnant, which is known as the fertile window.
Sperm can live in your reproductive tract for up to 5 days. Once an egg has been ovulated, there’s a period of fewer than 24 hours where it can be fertilized, otherwise, it gets dissolved back into the body, and your period begins. So, the 5 days leading up to ovulation and the day of ovulation itself make up your fertile window.
When is a woman most fertile?
Fertility peaks right around ovulation. Your most fertile days are the two days before ovulation and the day that ovulation happens. In the graph below, you can see the probability of getting pregnant within the fertile window.
How to calculate ovulation
The day ovulation occurs varies from woman to woman and can also change from month to month. So can the length of the cycle. That’s why it can be a bit hard to predict when exactly ovulation will take place.
Ovulation typically happens between days 11-21 of your cycle. In a 28-day cycle, the most fertile days are days 11-14. In a shorter cycle of 24 days, the most fertile days are around days 7-10, while in a longer cycle of 35 days, they’re about days 18-21.
There are several methods to calculate your fertile window and predict the day of ovulation:
- Ovulation calendar
- Ovulation tests that measure the amount of luteinizing hormone (LH) in your urine
- Cervical fluid
- Basal body temperature
- Wearables
- Ultrasound and blood tests
Method 1: Ovulation calendar
Many women use a calendar to keep track of their cycles. You can do this on a paper or digital calendar or with a cycle tracking app.
Here’s how to use this method to calculate your fertile window:
- Write down the day you start your period (this is cycle day 1).
- Track how many days you bleed and how long your cycle is.
- Do this for eight to 12 months to find an average.
- Subtract 18 from the total number of days of your shortest cycle to predict your first fertile day.
- To estimate your last fertile day, subtract 11 from the total number of days in your longest cycle.
To increase accuracy, use this method in combination with some of the following methods. Some apps have a spot to record your basal body temperature, changes in cervical fluid, and ovulation test results.
Method 2: Ovulation tests
Another way to know when you’re ovulating is with an ovulation test. This works like a pregnancy test: you pee on a stick to check your hormone levels. Before ovulation, you’ll have a surge in luteinizing hormone (LH), which triggers your ovary to release an egg. You can expect to ovulate around one to two days after your LH levels rise.
Here are some tips to use urine tests to calculate ovulation:
- Do a test once per day at the same time for 5-7 days.
- When you should start the test depends on how long your cycle lasts.
- Generally, you can count down 18 days from your usual cycle length to start taking the test.
- When the test strip has the same dark color as the comparison strip, it means that LH has surged and ovulation is about to happen.
One thing to note though: An LH increase isn’t a completely fool-proof way to find when you are most fertile because sometimes this hormone can rise even without ovulation.
Method 3: Cervical fluid
Another way to calculate ovulation is by checking your cervical fluid, which goes through changes when you’re about to ovulate. You can feel your cervical fluid inside the lower part of your vagina.
Here’s what happens to your cervical fluid throughout your cycle:
- After you stop bleeding, the vagina is dry and there’s no cervical fluid.
- Then the fluid becomes sticky and rubbery.
- When it gets very wet, creamy, and white, you have started your fertile window
- Next it’ll become slippery, stretchy, and clear (like an egg white). Grab the fluid between your thumb and pointer finger, and if it stretches when you spread your fingers apart, it could mean that you’re about to ovulate, and you’re in your most fertile days.
- Once ovulation has occurred, there’s no cervical fluid or it’s quite thick (like bubble gum).
Note that there are certain things make this method of calculating ovulation less reliable:
- Breastfeeding
- Hormonal birth control
- Vaginitis or sexually transmitted infections
- Surgery on the cervix
