Natural birth control and Fertility Awareness Methods: How effective are they?

By Maegan Boutot, Science Writer for Clue

Fertility Awareness-Based Methods (FAMs) are tools that people use both to become pregnant and prevent pregnancy (1–4). There are a few different methods, but the goal of all FAMs is to predict when a person will ovulate (1–4).

These methods are based on the facts that (A) a person can only become pregnant if they have unprotected heterosexual sex during the six days leading up to and including ovulation (this is called the fertile window) and (B) people ovulate once per cycle (5). The length of the fertile window is a combination of how long sperm can live in the uterus (about 5 days) and the lifespan of an egg (12–24 hours).

(In Clue, we give an estimate of your fertile window, but this estimate is not necessarily your actual fertile window. You have to determine when you’re ovulating to know your actual fertile window, and the timing of ovulation can change cycle-to-cycle.)

Having sex too far before or too late after ovulation cannot result in a pregnancy, even if that sex is unprotected, because it is outside of the fertile window. Therefore, if a person wants to prevent pregnancy, they need to abstain from unprotected sex during the fertile window.

There are a few different trackable indicators used in FAMs. These include:

  • Recording basal body temperature (BBT) — a person’s body temperature rises about 0.5–1 degree Fahrenheit/0.3–0.6 degrees Celsius at the beginning of luteal phase (i.e. the second half of their cycle, after ovulation). Recording BBT is not a reliable FAM on its own, as a rise in BBT only tells a person that they have already ovulated, not when they are going to ovulate. However, tracking the day BBT rises from month to month can help a person estimate when they might ovulate in the next cycle.
  • Monitoring cervical mucus — cervical mucus changes throughout the menstrual cycle in response to estrogen and progesterone, and usually increases in amount when a person is approaching ovulation or is ovulating. An egg-white-like cervical mucus usually suggests that ovulation will occur soon or has occurred within the last day, whereas thicker, clumpy cervical mucus usually suggests a person has already ovulated.
  • Tracking cycles using a calendar or app
  • Using luteinizing hormone (LH) urine tests — LH spikes within 24 hours before ovulation and this hormone can be detected with at-home urine tests (1–4).

Trackable-indicators themselves aren’t usually considered FAMs, but are tools implemented by FAMs. Some well-known FAMs include:

  • The rhythm method — this is the oldest FAM and is calendar-based. A person should track their menstrual cycles for at least six months before using this method. After having tracked multiple cycles, a person should use their longest cycle and shortest cycle to determine the time during which they are most likely to be fertile and should avoid sex or use a second form of contraception. If your cycles aren’t regular and between 26 and 32 days, this is probably not a good method for you. There is no current estimate for how well the rhythm method works.
  • Standard Days method — this method is similar to the rhythm method. In short, a person avoids sex or uses a second form of contraception from days 8 to 19 of their cycle (with day 1 being the first day of their period). If your cycles aren’t regular and between 26 and 32 days, this method is not recommended. Five out of 100 people will get pregnant per year if they use this method perfectly, and this method is currently considered a modern contraceptive by the World Health Organization.
  • TwoDay Method— a person who uses this method will check for certain types of cervical mucus every day of their cycle. If yesterday and/or today a person has slippery or egg white cervical mucus, they are potentially fertile and should avoid sex. Four out of 100 people will get pregnant per year if they use this method perfectly.
  • Billings Ovulation Method — Similar to the TwoDay method, the Ovulation method uses cervical mucus to estimate the fertile period. People record descriptions of their cervical mucus onto a chart and follow a set of rules as to when they can have sex. Three out of 100 people will get pregnant per year if they use this method perfectly.
  • Sensiplan (sometimes just called the symptothermal method) — this method uses cervical mucus and BBT readings to determine the fertile window in each individual cycle. Fewer than one out of 100 people will get pregnant per year if they use this method perfectly.
  • Fertility tracking via mobile app — these are relatively new tool that aren’t methods in themselves, but typically use calendar estimates and BBT, and sometimes other symptom inputs, results from luteinizing hormone tests etc. According to recently published research on one such app, one out of 100 people will get pregnant per year if they use this method perfectly; however, these estimates are based off studies with a significant amount of missing data, which may affect this estimate. (6–10)

Although the efficacy rate, or the “perfect-use”, for these methods is high, these tools can be challenging to use properly and consistently, and so the effectiveness rate, or the “typical-use” rate, is probably much lower.

How effective are FAMs as contraceptives?

Most people don’t use FAMs (or most other forms of birth control) perfectly, and so the effectiveness rate (i.e. what we tend to see in practice) for FAMs is estimated to be lower. How much lower, though, is up for debate. Different FAMs probably have different effectiveness rates, but there are few studies looking at each individual FAM type (ex. Standard Days or TwoDay methods) to know how well the results are generalizable.

Furthermore, many factors can affect the effectiveness ratings, and there is variability among FAM effectiveness estimates. These include A) research factors, B) a person’s menstrual cycle and C) the accuracy of measurements made by a FAM user.

Research Factors

Study design can have a strong impact on results. In contraceptive research, it is common to group all types of FAMs together, as far fewer people use FAMs than other forms of contraception or birth control. This is why it is estimated that 1 out of every 4 people who rely on FAMs as a form of contraception will become pregnant in one year (6), as anyone reporting using any FAM is grouped into a single category. This means that someone who doesn’t track their cycles very well and only occasionally abstains from sex is grouped with a person who avoids sex during their entire fertile cycle and is tracking BBT and/or cervical fluid regularly with a well-defined program (like StandardDays or Sensiplan). It’s unlikely that these two people would see the same effectiveness using their respective FAM.

The length of time and the number of people who stay enrolled in a study also have an effect on the efficacy and effectiveness ratings (6). In many FAM studies, half or more of the participants drop-out (i.e. participants quit) or are “lost to follow up” (i.e. the researchers don’t know what happened to the participants) (6–10).

Although participants may have a good reason for wanting to drop out (they don’t like the method, they want to get pregnant, etc.), this can make it hard for researchers to accurately predict if their method is effective for everyone, or if it’s effective just for the people who remain in their study. People who remain in a study can be different from lost participants in many ways. For example, people who remain may be better at using the contraceptive method than those who quit, may have sex less often (and thus be at decreased risk for pregnancy), or may be older or less fertile than people who quit (6).

For example, in a study estimating the efficacy rate for one fertility-focused mobile app,, about 5 out of 10 people quit the study before one year, and about 400 people were lost to follow up (10). Because of the way the study was conducted, researchers don’t know if people who remained were less at risk than those who quit, either due to having sex less or some other factor. Their estimates for pregnancy rates should be given as a range, rather than an absolute number, given that they don’t know what happened to many of their participants. Unfortunately, this is a common problem in FAM research, and can lead to an inaccurate perception of how effective a method is.

Individuals’ menstrual cycles

Some FAMs, like the Standard Days method, are highly influenced by the regularity of a person’s menstrual cycle. Unfortunately, these methods cannot always accurately predict ovulation before a person enters the fertile window. These forms of FAMs either guess at when a person will enter their fertile window based on past cycles, or tell a person they are in or already past the fertile window.

Calendar-based FAMs, such as the Standard Days Method, are used by people whose cycles are quite regular. These methods use the assumption that each cycle is similar to the previous cycle. This might be true on average, but most people have some variability in the length of their cycles and experience unusual cycles occasionally. Stress, jet lag and working night shifts may cause an occasional irregular cycle (11–13). Similarly, people who are approaching menopause or who are in adolescence are more likely to have irregular cycles, and it’s not always obvious when a person has entered or left these phases in their life. Sometimes unusual cycles just happen without an obvious reason.

Measurement accuracy

Combined symptothermal methods have the highest efficacy rates of FAMs for contraception (6, 7). BBT and cervical mucus tracking are cumbersome forms of FAMs, because their use requires daily measurements, usually one or two a day, and highly accurate readings. For example, tracking BBT requires a person to take their temperature immediately upon waking (or after another period of extended rest), and use of the TwoDay method requires checking for cervical mucus at least twice a day (2). If a person does not do this consistently, their recordings and estimates won’t be reliable.

Even if a person regularly checks their fertility signs, the measurements may not be accurate. BBT in particular is affected by sleep and sickness (1, 2), and so recordings taken when a person hasn’t slept well or is sick can’t be trusted to be accurate.

FAMs are made more reliable by widening the abstinence period past the fertile window, which is what many formal guidelines for combination FAMs (such as the Standard Days method) suggest. Although the biological fertile window is about six days, a person using FAMs can add days to their potential fertile window to compensate for variability. This comes at a cost, though; the more days added to the potential fertile window, the fewer days a person can have sex without another form of contraception.

Again, increasing the fertile window in this way only works for people who have a limited amount of variability. For the 1 in 5 people whose cycle length variability is 14 days or more, or for people who are perimenopausal or in adolescence, widening the potential fertile window might not offer enough protection from unintended pregnancy (14).

Who might FAMs be a good fit for?

Using certain FAMs can be a lot of work, but some people will consider the work worth it. FAMs have no side effects, are free or inexpensive to use, and can be stopped or started without the help of a healthcare professional. For people who cannot afford, who do not have access to, or are religiously opposed to, or who simply don’t want to use other forms of contraception, using FAMs is an effective alternative (1–3).

It is important to note, though, that people with irregular menstrual cycles, and/or who are younger than 18 or who are older than 40, tend not to be included in FAM research (6–10). So it’s hard to say how effective FAMs are among these groups of people.

Also, because FAMs don’t protect against sexually transmitted infections (STIs), they’re probably not right for people who are having sex with multiple and/or untested partners.

FAMs and a second method

FAMs used in conjunction with other forms of contraception, such as condoms or spermicide, can decrease the risk of getting pregnant, especially if a person entirely avoids sex during the potential fertile window and uses another method at every other point (1–3). Using condoms in conjunction with FAMs also reduces the risk of STI transmission.

Why record BBT in Clue?

Clue on its own isn’t recommended for use as a contraceptive. BBT and ovulation tests can be tracked to better improve Clue’s estimates of your cycle length, luteal phase length, fertile window, and ovulation, but this is for informational purposes only and to help you understand how your cycle, body and health.

Download Clue today to learn more about your cycle and your body.


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