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Illustration: Emma Günther

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Natural birth control and Fertility Awareness Methods

How effective are they?

Top things to know:

  • Fertility Awareness-Based Methods (FABMs, or FAMs) are tools that people use both to become pregnant and prevent pregnancy

  • There are a few different methods, but the goal of all FABMs is to predict when a person will ovulate

  • FABMs have no side effects and free or inexpensive

These methods are based on the facts that a person can only become pregnant if they have unprotected vaginal sex during the six days leading up to and including ovulation (this is called the fertile window) and 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).

It’s untrue that you can only get pregnant on the day of ovulation, so you should always use some form of birth control if you don’t want to become pregnant.

Download Clue to start tracking your fertile days.

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There are a few different trackable indicators used in FABMs. 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 FABM 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).

Some well-known FABMs include:

  • The rhythm method—this is the oldest FABM 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 methodthis 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 predictable 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 tools that aren’t methods in themselves, but typically use calendar estimates and BBT, and sometimes other symptom inputs, like results from luteinizing hormone tests. Estimates on how many people get pregnant each year vary based specifically on each app and how rigorous each is studied and tested (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 FABMs as contraceptives?

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

Many factors can affect the effectiveness ratings, and there is variability among FABM effectiveness estimates. These include research factors,  a person’s menstrual cycle, and the accuracy of measurements made by a FABM user.

Research factors

Study design can have a strong impact on results. In contraceptive research, it is common to group all types of FABMs together, as far fewer people use FABMs than other forms of contraception or birth control. This is why it is estimated that 1 out of every 4 people who rely on FABMs as a form of contraception will become pregnant in one year (6), as anyone reporting using any FABM 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 window 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 FABM.

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 FABM studies, half or more of the participants drop-out (participants quit) or are “lost to follow up” (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 FABM research, and can lead to an inaccurate perception of how effective a method is.

Individuals’ menstrual cycles

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

Calendar-based FABMs, 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 variable cycle (11–13). Similarly, people who are approaching menopause or who are in adolescence are more likely to have variable 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 FABMs for contraception (6, 7). BBT and cervical mucus tracking are cumbersome forms of FABMs, 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.

FABMs are made more reliable by widening the abstinence period past the fertile window, which is what many formal guidelines for combination FABMs (such as the Standard Days method) suggest. Although the biological fertile window is about six days, a person using FABMs can add days to their potential fertile days 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 FABMs be a good fit for?

Using certain FABMs can be a lot of work, but some people will consider the work worth it. FABMs 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 FABMs is an effective alternative (1–3).

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

Also, because FABMs 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.

FABMs and a second method

FABMs 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 FABMs also reduces the risk of STI transmission.

Why track with Clue?

Tracking better improves Clue’s estimates of your cycle length, luteal phase length, and estimated ovulation. Understanding your body can improve usage of non-hormonal birth control. Download Clue to track your cycle and learn more about birth control options.

Article originally published on December 14, 2017

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