Fertile window: Ovulation & when to have sex

What is the fertile window?

The fertile window refers to the short span of days in each cycle when unprotected sex can result in pregnancy. Let’s recall that pregnancy happens when an egg cell and a sperm cell meet in the female reproductive system. With that in mind, there are a few aspects that play into exactly how long this fertile window lasts:

• Ovulation: This is when a female egg cell is released from an ovary. Ovulation happens once per cycle, roughly around the middle of the cycle. Once the egg is released, it lives for up to 24 hours in the reproductive system [2].
• Sperm survival: When sperm enter the female reproductive system during unprotected intercourse, they can live for up to five days in the body, and can fertilize the egg cell during this time [3].

Since sperm can live up to five days inside the female reproductive system, and the egg survives about 24 hours, the fertile window lasts only six days per cycle. Although sperm can only fertilize the egg cell once it’s released at ovulation, we include the five days prior because if you have unprotected sexual intercourse during those five days, you can get pregnant during ovulation that month [1]. Learn more about how sperm cells reach the egg cell here.

Fertile days to get pregnant in the menstrual cycle

Considering that ovulation usually happens at about the middle of your menstrual cycle, the fertile window usually falls around the middle of the cycle as well — about five days before and the day of ovulation as mentioned above. 

Ovulation typically happens about 12 to 16 days before your next period starts [4]. This reflects the length of the luteal phase, which is the more stable part of the cycle. In fact, a large-scale study of 600,000 cycles logged on Natural Cycles found that ovulation happens on average 12 days before the start of the next period [5]. However, the timing of ovulation can still vary between individuals (and even cycle to cycle) as there are many factors that can affect when you ovulate [1].

How to predict ovulation

So naturally, the next question here is how to predict when you’re going to ovulate? There are a number of ways to predict ovulation and use that knowledge to serve your fertility goals. 

Traditional fertility awareness methods like the calendar or rhythm methods can give a rough estimation of when your fertile window will fall, but these are based on generalized data and aren’t tailored to your unique cycle. Natural Cycles works a bit differently, taking a comprehensive view of your cycle and biometric data to calculate the fertile window. Additionally, you can also use ovulation tests to help predict the day of ovulation. Physical indicators such as a rise in basal body temperature or changes to the consistency of cervical mucus can also identify the days when you have the highest fertility during each cycle. 

Let’s go into more detail about each of these together:

How does Natural Cycles calculate the fertile window?

Natural Cycles is an FDA-cleared app that uses body temperature patterns to calculate when you’re most fertile. Right after ovulation, basal body temperature (BBT) rises slightly. By collecting this data from a basal thermometer, Oura Ring, or your Apple Watch, the app can identify when ovulation has occurred and predict fertile days in future cycles. In fact, research shows that couples using Natural Cycles to plan pregnancy take three cycles to conceive on average [6]. 

Calendar methods

Calendar tracking methods involve logging your cycle on a calendar or in an app by marking when your period begins each month. You can then calculate your cycle length — the number of days between one period and the start of the next period. Over time, you’ll start to build up an idea of your average cycle length [7]. Since we know that people tend to ovulate about 12 days before the start of their next period, you can gather an approximate idea about when it’s likely that you’ll ovulate based on your cycle length [5].

One major drawback here is that it’s recommended to log at least six cycles before relying on this method to determine your average cycle length and likely ovulation date. Furthermore, calendar methods are not recommended for those with irregular cycles due to a shifting fertile window. And even for those of us with more regular cycles, it’s important to note that these methods cannot pinpoint ovulation beyond estimates based on static cycle data, meaning it’ll be harder to predict ovulation with accuracy [7].

Ovulation calculators

You can also use a newer tool called an ovulation calculator to try to figure out where you are in your cycle and when you’re likely to ovulate. Simply enter information about your most recent period, such as the date it started and how long it lasted, and the calculator will give you an estimation of n you’re most likely to ovulate next cycle [1]. 

Keep in mind these calculators only offer a basic estimation, calculated using a snapshot of data, and they cannot give you the full picture of your menstrual cycle without access to other important fertility indicators. And again, your fertile window may change from month to month, particularly if you have irregular periods [1].

Ovulation tests

Ovulation test kits can be used at home to help predict when you’re ovulating. They work by measuring levels of luteinizing hormone (LH) in your urine. This hormone plays an important role in your menstrual cycle. LH is always present in your body, but when you approach ovulation, estrogen rises and LH spikes, which can trigger ovulation. This LH surge usually indicates that ovulation will occur within the next 12 to 36 hours, but it’s important to note that an LH spike doesn’t always guarantee that ovulation will happen [1]. 

Natural Cycles works seamlessly with ovulation tests, allowing you to log test results in the app in addition to your temperature. Keep in mind that ovulation tests only predict ovulation, and cannot guarantee that ovulation has happened.

Checking your cervical mucus

Cervical mucus (also known as vaginal discharge) changes in color and consistency throughout each menstrual cycle, so if you know what you’re looking for, it can give you a lot of information about your fertility. In the days leading up to ovulation, you might find that your cervical mucus becomes similar in consistency and color to egg whites — it’ll be slippery, clear, and a bit stretchy. After ovulation, your cervical mucus may appear cloudy and feel more sticky. At the beginning of your cycle (after your period), you may notice very little or no discharge for a few days. Tracking these qualitative changes throughout your menstrual cycle can help determine when you’re ovulating each month [1, 8]. 

However, it can take a few months of daily evaluation to start to recognize patterns in your cervical mucus. It’s also important to note that these patterns may be affected by factors other than your cycle, including medication, sexual intercourse, breastfeeding, lubrication, and feminine hygiene products [7]. Ultimately, the cervical mucus method can teach you a lot about your body and help you understand your fertile window, but it is subject to interpretation and not as objective as some fertility indicators, such as ovulation tests or basal body temperature.

Measuring basal body temperature 

As we mentioned earlier, your BBT increases slightly (by about 0.5°F or 0.2°C) just after ovulation. Tracking this metric can help you understand where you are in your cycle. While other methods can help to predict when you might be ovulating, measuring your BBT confirms that it has happened. 

To track your BBT, take your temperature first thing in the morning before you get out of bed with a basal thermometer. Over time, this metric can help you predict ovulation and the fertile window each month, but you’ll need to do the calculations on your own, unless you download the Natural Cycles app which does this for you! [1]. Natural Cycles can also measure skin temperature to detect ovulation by integrating with your Apple Watch or Oura Ring. Learn more about ways to measure with NC°.

The best time in the fertile window to get pregnant 

So if you’re trying to conceive, figuring out your ovulation date, and therefore your fertile window, is essential. Once you’ve identified your fertile window with one (or multiple) of the methods outlined above, having unprotected intercourse often during this window gives you the best chances at pregnancy [4].

Should I have sex before, during, or after ovulation?

If you’re having sex during your fertile window, you’re already off to a great start, and for many people, this will be enough to conceive. You and your partner can have unprotected intercourse every two to three days, or even daily, during this window to improve your chances of conception. However, if you want to get even more specific with your timing, it helps to have sex as close to ovulation as possible — ideally just before — so that sperm will be ready to fertilize the egg quickly after it’s released [3, 4]. The Natural Cycles app shows your fertility status as a scale, so you’ll know the days when you’re predicted to have peak fertility vs days when conception is less likely.

Can you get pregnant outside of your fertile window?

Pregnancy is only possible within the fertile window because ovulation needs to happen in order for sperm to meet the egg, so outside of that window, chances of conception are extremely low. That said, ovulation isn’t perfectly predictable, and it can happen earlier or later in the cycle than expected, so your fertile days may shift as a consequence [2]. For those who struggle to find their fertile window due to irregular or infrequent ovulation, fertility treatments like in vitro fertilization (IVF) can help with conception. 

What if I have irregular periods?

Irregular cycles are usually defined as lasting less than 21 days or more than 35 days [4]. Significant variations in cycle length can correlate with certain lifestyle changes, like losing weight, perimenopause, or conditions like an underactive thyroid or PCOS. Getting pregnant with irregular cycles can be more challenging, as ovulation may be less predictable from cycle to cycle [4]. However, many people with irregular cycles still manage to conceive, although it may take a little longer. 

If you’re trying to navigate getting pregnant with irregular periods, ovulation tests can help by providing some insight, but it’s important to note that LH surges don’t always result in ovulation [1]. In this context, tracking your BBT can be particularly helpful to identify if ovulation has occurred. Natural Cycles can be an invaluable tool for those with irregular cycles, helping monitor body temperature changes and using this data to pinpoint ovulation. In addition to temperature data, the Natural Cycles algorithm uses period data and optional ovulation tests to learn about the pattern of your cycle. It can even flag irregularities like recurring anovulation or important changes in cycle phase length. All of these changes are documented in the handy Cycle Report feature, which can be downloaded and shared with your healthcare provider. This report can help you have informed discussions about what next steps to take regarding fertility.

How else can I prepare for pregnancy?

If you’re looking for the best chances of conception, it’s helpful if you and your partner limit smoking and alcohol consumption while preparing for pregnancy. It’s also recommended to eat a healthy diet and start taking folic acid, a prenatal vitamin, each day as you're trying to get pregnant [4].

Using Natural Cycles to calculate the fertile window

The Natural Cycles app is powered by an algorithm that analyzes temperature data, and other indicators like ovulation tests, to help you identify and predict fertile days more accurately than manual tracking methods. You’ll also get handy notifications of your fertility status each day, taking the guesswork and calculations off your plate. With multiple ways to use the app, including NC° Plan Pregnancy and NC° Follow Pregnancy, you can tailor it to your current reproductive health needs and switch seamlessly between modest. See how Natural Cycles can support your fertility goals today.