Best time to have sex to avoid pregnancy: What are "safe" days?

Before we dive in, it's important to note that no method of birth control is 100% effective. Discuss with your healthcare provider to determine the best option for you, taking into account your medical history and lifestyle.

When am I most likely to become pregnant?

Each cycle, you have a select number of days during which unprotected sex can lead to pregnancy. This part of your cycle is named the fertile window. The fertile window varies from person to person and even from cycle to cycle, but for most individuals it usually falls around the middle of their cycle.

During ovulation, an egg cell is released and has up to 24 hours to be fertilized. But the fertile window includes the five days before ovulation, plus the 24-hour ovulation period. This is because sperm can survive for up to five days in the female reproductive system [1, 2].

So, if you can predict your ovulation, you can also calculate the timing of this six-day fertile window when you are most likely to become pregnant following intercourse.

Best times to have sex without getting pregnant

The best time to have sex without getting pregnant is any time outside the six-day fertile window. In other words, you want to avoid having sex on ovulation day, or the five days before this. In theory, this is straightforward, providing we know which day of the cycle we ovulate, and that stays the same every cycle. For many people, ovulation day will happen around the mid-point of the cycle. However, in practice, it’s not that simple. Ovulation is hard to pinpoint, and even those of us with regular cycles can experience changes to our periods and cycles from time to time. We don’t recommend simply guessing where you are in your cycle as a means to find fertile days for this reason. 

However, there are steps you can take to find your fertile days with more certainty. Timing sex based on your fertile window is actually a form of birth control, usually referred to as natural family planning, or fertility awareness. There are a few different methodologies, each with its own practices and effectiveness rates, and caveats. We’ll go into these methods in more detail later. 

Can you get pregnant if you have unprotected sex during your period?

The first day of your period, also known as menstruation, is day one of your menstrual cycle. When your period starts, you are at the beginning of your cycle and counting down to ovulation, which usually occurs 14 days before your next period (and next cycle) [5]. It’s a common assumption that most cycles follow a 28-day pattern, but research by Natural Cycles found that cycle length among its users varies widely, with an average of 29 days [17].

Thinking about the six-day fertile window again, the likelihood of your fertile window occurring at the same time as your period is low. However, some individuals may be at risk of getting pregnant when having unprotected intercourse during their period. If you experience short cycles (less than 21 days), your ovulation may be earlier in your cycle than most, which can push the fertile window to overlap with your period. 

If you experience irregular cycles, for example, related to a hormonal condition such as PCOS, ovulation can be unpredictable or particularly early in your cycle, and this can put you at risk of pregnancy when having unprotected intercourse during your period. When it comes to something like preventing pregnancy, it’s best not to leave this up to guesswork.

Can I get pregnant if my periods are irregular?

An irregular period can refer to the menstrual bleed itself or the characteristics of the overall menstrual cycle. Regular menstrual periods usually last for up to 7 days. Your periods may be irregular if you [4]:

• Experience bleeding or spotting between periods
• Have a menstrual cycle that is shorter than 21 days or longer than 35 days
• Have a period that lasts for more than 7 days
• Don’t have a period for 3 months
• Have a period that bleeds much more or less than usual
• Have a cycle length that varies by more than 7 to 9 days

Even with irregular periods, chances are that you’re still ovulating and can therefore get pregnant. However, it’s harder to pinpoint the fertile window if your ovulation day tends to move. Your fertile window may also shift significantly from month to month, making it harder to predict. If you have an irregular cycle or irregular periods, speak to your healthcare provider about any concerns, including avoiding pregnancy. If you’re using Natural Cycles, it’s helpful to know that the app is just as effective for those with irregular cycles, but you might get assigned more Red (fertile) Days to allow for a shifting ovulation day.

Ways to better predict and confirm when ovulation occurs

To predict the best time to have sex without getting pregnant, you must understand the rhythm of your cycle and how to use this to predict ovulation. Fertility awareness methods refer to several types of cycle tracking methodologies. Some are based on population-wide estimations of when ovulation will occur, while others can help predict and confirm an individual’s ovulation. 

Remember that whichever fertility awareness method you choose, you must use additional contraceptive protection on fertile days or abstain from sex. 

Calendar method

The calendar method refers to several calendar-based approaches to tracking your menstrual cycle, such as the rhythm method and the standard days method. Both techniques involve counting the days of your cycle, from day one when your period starts to the last day before your next period begins. Tracking these days in a calendar gives you an understanding of the average length of your menstrual cycle [18]. 

Once you have tracked your cycle length for at least six cycles, you can calculate when your fertile window is likely to occur. The rhythm method bases the calculations on your shortest cycle to estimate your fertile window. The standard day method is a more straightforward approach that assumes a fertile window from day 8 to 19 for everyone.

In terms of reliability, studies are limited, but the standard days method has been shown to be 88% effective with typical use and 95% effective with perfect use [6]. To use calendar methods effectively, you must track and count the days of your cycle for at least six cycles before relying on the data. Calendar-based methods carry a risk of human error if you calculate the dates yourself and are not suitable for cycles shorter than 26 days or longer than 32 days.

Basal body temperature

Basal body temperature (BBT) refers to the lowest, resting temperature of your body [3]. Each menstrual cycle, your basal body temperature follows a clear pattern, fuelled by hormonal changes [7]. After ovulation, the hormone progesterone causes your BBT to rise by a small but detectable amount [8]. Your BBT will remain elevated throughout the luteal phase until it drops when your period arrives and your next cycle begins [7]. By observing these temperature shifts, you get a clear indication of what phase of your cycle you are in. 

To use the BBT method correctly, there are specific rules to follow. Consistency is key. To observe the temperature rise associated with ovulation, you must take reliable temperature measurements throughout your cycle. Temperature must be taken first thing in the morning, after at least a few hours of uninterrupted sleep, and before you undertake any activity. Once you sit up and get out of bed, your temperature begins to fluctuate, and any thermometer readings will not be an accurate indication of your BBT. 

The temperature shift after ovulation is also very subtle, rising by around 0.5-1°F [7]. To record this shift accurately, you must use a dedicated basal thermometer that measures to two decimal places. It is also important to consider external factors that can affect BBT, such as illness, alcohol, lack of sleep, certain medical conditions, and some medications.  

Natural Cycles

The first FDA-cleared birth control app, Natural Cycles, shares some similarities with the BBT method. Like traditional temperature tracking methods, it works by finding fertile days based on the small rise in temperature that happens after ovulation. Unlike traditional charting methods, Natural Cycles is powered by a more sophisticated algorithm that learns the pattern of your cycle over time and does the hard work for you. 

The algorithm assigns Red Days and Green Days, depending on where you are in your cycle to indicate whether or not you could be fertile that day. On Red Days, you should use protection to prevent pregnancy, while on Green Days, you’re not fertile and can have sex without risk of pregnancy. Natural Cycles is 93% effective with typical use and 98% effective with perfect use. It’s just as effective with irregular cycles, and unlike traditional family planning, can be relied on from day one. 

Cervical mucus method

Have you noticed your vaginal discharge change throughout your cycle? Cervical mucus is a type of vaginal discharge secreted from the cervix. The amount, color, and consistency of cervical mucus change throughout your cycle, influenced by fluctuations in your estrogen levels [9].

The beginning of your cycle is marked by the start of your period. After your period, your body produces very little cervical mucus. You may feel “drier”, or if you do have any discharge, it may feel sticky. As you enter your fertile window and your ovulation day is nearing, cervical mucus may increase in quantity and may feel and look creamy. During ovulation, your mucus may become more wet or slippery, and will have a consistency similar to egg white. After ovulation, cervical mucus begins to decrease again and may appear thicker. 

You can use these changes in cervical mucus to track your cycle stages, fertile window, and safe days to have sex if you are avoiding pregnancy. To do this, you must analyze samples of your cervical mucus and track changes throughout your cycle. You can analyze mucus on toilet paper when you wipe, or insert a clean finger into your vagina, as close to the cervix as possible, to collect a sample [9].

While the cervical mucus method can indicate which stage of your cycle you are in, it has its limitations. Identifying and distinguishing the different consistencies of cervical mucus may need dedicated practice and is open to human error [10]. The cervical mucus method is also not suitable for those taking hormonal medication, breastfeeding, or experiencing any type of vaginal or sexually transmitted infection, as these can influence the consistency and presentation of cervical mucus and discharge [11, 12].

At-home ovulation tests

While ovulation tests themselves are not a fertility-awareness method, they are a useful tool you can use in conjunction with other cycle tracking methods. Luteinizing hormone (LH) is a natural chemical in our bodies that plays a vital role in the menstrual cycle. Working with other hormones such as estrogen, it regulates and coordinates your cycle [13]. LH levels usually surge around 1-2 days before ovulation [14], increasing from an average of 1 – 12 IU/L in the follicular phase to 16 – 104 IU/L around ovulation [15]. 

At-home ovulation tests will only tell you what is happening on the day you take the test. Therefore, you may need to take multiple tests throughout your cycle to detect the LH surge. The NC° app can analyze ovulation test data on your behalf to more accurately predict your ovulation and fertility window.

Track ovulation and more

The best way to know when it is safe to have sex without getting pregnant, whichever method you choose to use, is through efficient and consistent tracking. The NC° app is the only FDA-cleared contraceptive app, using period dates, temperature data, and optional LH testing to accurately predict your fertile window and safe days to have sex.

By tracking and recording your basal body temperature each day, Natural Cycles can predict your ovulation and fertile window, and clearly advise whether you are on a non-fertile day or potentially fertile and need to use protection, such as condoms. You can also keep track of other symptoms throughout your cycle, such as cervical mucus, and log any results from ovulation tests to help the app predict your fertile window. Natural Cycles is effective from day one of use and, through peer-reviewed research, has been proven to be more accurate at finding the fertile window than calendar-based methods [16]. Click “get started” and find out if Natural Cycles could be right for you, today!