Fertile window represented with a circular illustration of the menstrual cycle, highlighting the pregnancy window
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What is the fertile window?

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Written by Lauren McKay

Lauren McKay

Lauren McKay is a writer and journalist with more than ten years of experience writing across a variety of topics. She is a passionate advocate for driving women’s health knowledge and is a trained yoga teacher. She earned a Masters in Creative Writing from the University of Glasgow and currently lives in Scotland.

Medically reviewed by Jack Pearson, Medical Affairs Manager at Natural Cycles

Jack Pearson

Dr. Jack Pearson is a previously HCPC registered Embryologist with a PhD in reproductive medicine. Prior to joining Natural Cycles leading Medical Affairs, he worked for more than 10 years in a clinical setting working at some of the busiest fertility clinics in the UK. Today he spends most of his time working with experts at the world’s leading institutions to carry out important research with the vision to further the field of female health. He earned his PhD from the University of Sheffield specializing in Sperm Metabolism and currently lives in London.

Contrary to popular belief, you can’t get pregnant every day of your cycle. There is actually a small number of days when it’s possible to conceive. So, what is the fertile window? Read on to find out when you are most likely to conceive, when’s the best time to get pregnant, and how understanding the fertile window can also prevent pregnancy...

This article is also available in Spanish

What does fertility mean?

Fertility relates to our ability to be able to reproduce. In short, being fertile means having the ability to conceive, or have children. We are able to conceive over a number of fertile days in the menstrual cycle, known as the fertile window, or pregnancy window. Male fertility doesn’t change in the same way as female fertility, but it does decrease over a man’s lifetime and is closely connected to sperm count.

What is the fertile window?

A good understanding of menstrual cycles is crucial when it comes to getting to grips with the fertile window. Your cycle starts on the first day of your period and ends when you start your next period. 

Although the textbook average menstrual cycle was always thought to be 28 days long, our research looked at 600,000 cycles and found that was only the case for 13%. A 29-day cycle was more common, but in reality, cycle length varies from person to person, and anything between 21 and 35 days is within the regular range.

The female fertile window is around six days long. It begins up to five days before you ovulate, and includes the day of ovulation. Then, once an egg cell is released, it is viable for a maximum of 24 hours before it deteriorates. 

That means you could potentially get pregnant at any time during your fertile window, thanks to the rate of sperm survival, as sperm can live in the female reproductive system for up to five days before an egg cell is even released!

The best time to get pregnant in the fertile window

Unlike resilient sperm cells, the egg cell starts to deteriorate quite quickly after ovulation happens. Within 24 hours the egg cell usually dies, and even before this, as each hour passes, the cell loses quality and conceiving becomes less likely. For this reason, it’s best to have sex as close to ovulation as possible.. 

Can you get pregnant outside of your fertile window?

It’s not possible to get pregnant outside the fertile window, although depending on the length and regularity of your cycle, there’s a small chance you can get pregnant on your period. This is highly unlikely, though not impossible. Because cycle lengths vary from woman to woman, it’s difficult to predict ovulation without knowing your unique cycle.

For people with a 28-day menstrual cycle, it’s highly unlikely that they’ll get pregnant right before their period. However, for those with a short cycle, it’s a possibility, depending on when ovulation happens.

It’s also unlikely that you’ll get pregnant directly after your period has finished but again, it could be possible depending on what your cycle looks like. If you have a short cycle and a long period, then it’s possible that you may ovulate straight after your period, meaning you could get pregnant if you have sex then.

Preventing pregnancy in the fertile window

While finding the fertile window can help identify your best time to get pregnant, this knowledge can also be used to do exactly the reverse: preventing you from getting pregnant. A short fertile window means there are only actually six days in a month when a woman can get pregnant.

This means, in theory, that identifying ovulation can be used as a form of contraception if a woman uses protection or abstains from sex during the time when there is a risk of getting pregnant. However, ovulation day can vary from cycle to cycle and sometimes we don’t know until after it has happened. 

That’s why timing intercourse to take place only outside your period of fertility isn’t always reliable unless you’re sure of when you’re ovulating. That means it’s best to use contraception, such as condoms, every time you have sex if you want to reduce the risk of pregnancy. 

Alternatively, you can use various different methods of tracking your cycle to determine when your monthly fertility window is. That means you’ll know when you’re ovulating and you can plan to have sex then if you’re trying to conceive (or abstain if you’re not trying to get pregnant!).

How to predict ovulation

There are a number of ways to predict ovulation and to plan the best time to get pregnant during the fertile window. Indicators such as a rise in basal body temperature, positive ovulation tests and consistency of cervical mucus can help identify and narrow down the days you have the highest fertility and help plan a pregnancy.

The calendar method

The calendar method involves tracking your cycle on a calendar or in an app by marking when your period begins every month. Your cycle length, then, is the number of days between that date and your next period. Over time (it’s recommended that you monitor for six cycles before relying on this method), you’ll start to build up a picture of your average cycle length.

That can help you to understand how long your fertile window lasts. Most people ovulate around 12 to 14 days before the start of a new menstrual cycle, and your window occurs in the days leading up to ovulation, plus the day of ovulation, and 1 or 2 days after it.

If you have irregular periods and your cycle varies from month to month, then you should be cautious of relying on this method, as it’s harder to predict your fertile window with any accuracy.

Ovulation calculators

Similarly, you can use online ovulation calculators to work out when you’re ovulating. You add information about your most recent period, such as the date it started and how long it lasted for, and the calculator will work out when you’re most likely to conceive.

These calculators shouldn’t be relied upon if they only take data from your last period, as it may not give the full picture of your menstrual cycle. It’s also worth noting that, again, your fertile window may change from month to month if you have irregular periods.

Checking your cervical mucus

In the days leading up to ovulation, you might find that your vaginal mucus changes to become stringier. When your vaginal mucus becomes slippier, it makes it easier for sperm to travel through the cervix and uterus to meet the egg in the fallopian tubes.

You can track the look and feel of the mucus every day throughout your menstrual cycle, noting down the results on a chart, to help determine when you’re ovulating.

It can take three to four months of daily checks to recognize patterns in your mucus, but it’s important to remember that the consistency of your mucus can be affected by a range of different factors, including medication, sexual intercourse, breastfeeding, lubrication, and feminine hygiene products. That means that, on its own, it’s not always a reliable indicator of whether you’re ovulating.

Ovulation predictor kits

You can buy tools to use at home to help predict when you’re ovulating. They work by testing your urine for levels of luteinizing hormone (LH), which plays an important role in your menstrual cycle.

LH is always present in your body, but as your estrogen rises, LH spokes, which kickstarts ovulation.

Ovulation predictor kits, then, measure how strong your hormone levels are. If they’re high, it could mean that you’ll soon be ovulating, as an increase in LH usually happens 24-48 hours before ovulation. An ovulation test isn’t a guarantee that ovulation will occur – rather, it’s an indication that it is going to happen. It’s also important to note that these tests may not be reliable if you don’t have an average cycle, or have a condition like polycystic ovarian syndrome (PCOS).

Basal body temperature measurement

During your ovulation window, your basal body temperature (BBT) will also increase. This is your body’s temperature when at rest, and it usually rises by around 0.5°F or 0.3°C just after ovulation.

Tracking your basal body temperature can help you to understand where you are in your cycle, and help to predict ovulation. Other methods, like checking your cervical mucus, counting your cycle days, and taking tests can help to predict when you might be ovulating, but measuring your BBT confirms when ovulation has happened. That means you can definitively pinpoint when your most fertile time is.

BBT is measured using a basal thermometer, a special kind of thermometer that has two decimal points. You should take your temperature first thing in the morning before you get out of bed so that you measure your body’s lowest resting temperature. 

It’s best to monitor your basal body temperature every day without any breaks, for several cycles, to get a more accurate picture of your cycle.

How does Natural Cycles calculate the fertile window?

The Natural Cycles app is powered by an algorithm that analyzes temperature data (amongst other factors) to predict and identify ovulation. This technology means the app can learn your unique cycle and removes an element of human error involved in traditional fertility awareness-based methods. It’s also completely hormone-free.

The app works in two ways: NC° Birth Control and NC° Plan Pregnancy so you can tailor it to your reproductive health needs. Natural Cycles provides a daily fertility status and, as birth control, tells you if you need to use protection to prevent pregnancy. The app does not protect against sexually transmitted infections.

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