Top things to know:
For now, the Clue app should not be used as birth control
The predicted fertile window in Clue is only an estimate
It’s normal for the length of menstrual cycles to vary, which means that your fertile window can change from cycle to cycle
There is still substantial risk for pregnancy before and after the predicted fertile window shown in Clue
There are a few different methods of “natural birth control”, and apps that use them. But Clue is currently not designed to be a contraceptive.
If avoiding pregnancy is important to you, you shouldn’t rely on Clue for birth control.
Tracking BBT and ovulation tests will help make your fertile window in Clue more accurate. but you’ll need to follow a formal Fertility Awareness Method (FAM) method if you want to incorporate these measurements into a FAM for contraception.
You have to know what you’re doing for FAMs be effective, and be comfortable with each method’s level of risk. If your cycle tends to be quite irregular, you can turn your fertile window off in settings.
You can use Clue to learn a lot more about your body.
Clue was built as a tool to track periods, symptoms, and sensations across menstrual cycles, to learn about the body, and to create an important health record.
We love building Clue, and we’re proud to have created something great: a 2016 study published in the journal Obstetrics & Gynecology rated Clue as the top of 108 free menstrual tracking apps after evaluating for accuracy, features, and functionality.
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It’s not possible to create accurate predictions of the fertile window based only on tracking your period, and Clue’s predictions are shown for informational purposes only.
The “fertile window” that appears in Clue may align with your own actual fertile window often, sometimes, or never.
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Why exactly should I not use Clue as a contraceptive? First: remember the phases of the menstrual cycle
The menstrual cycle is more than just your period. A lot happens in your body before and after your menstruation. Ovulation (the release of an egg from your ovary) divides the cycle into two phases: the follicular phase and the luteal phase. Pregnancy is possible for about six days each cycle, called the fertile window.
1. The follicular phase: from the first day of your period until ovulation
The follicular phase lasts from cycle-day-one until ovulation. It’s the time of the cycle when an egg prepares to be released from the ovary.
This phase is considered the most “plastic” of the cycle, meaning it can change a lot in length from cycle-to-cycle (1,2).
The follicular phase typically lasts 10-22 days, but it can be longer or shorter, and can change over time (1,2,6). A follicle develops over many cycles, but most growth happens in this phase leading up to ovulation.
If you do a 10-day fast or spontaneously decide to bike the Tour de France during this phase, for example, your ovulation may be delayed (and may not happen at all).
One recent study found that a heightened immune response in the follicular phase was more likely to affect the length of that follicular phase than if the immune response had occurred in the previous luteal phase (7).
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2. Ovulation and the fertile window
Ovulation is the release of an egg from the ovary into the fallopian tube. It happens at the end of the follicular phase, and marks the beginning of the luteal phase. The fertile window mainly overlaps with the follicular phase, and includes the five days leading up to ovulation (when estrogen rises sharply) as well as the 24 hours after ovulation occurs (the window of time when exposure to sperm can lead to pregnancy)(8).
3. The luteal phase: from ovulation until the day before your next period
The luteal phase is when the body prepares for a possible pregnancy. It lasts from after ovulation until the day before your period, and includes the days when some people experience premenstrual symptoms.
The average luteal phase is about 14 days long, but yours might be between 9-16 days (1,6). The length of the luteal phase can change over time, and vary somewhat cycle to cycle, but it’s far less flexible in length than the follicular phase (1,9). This is why the timing of ovulation is estimated retroactively when each cycle is complete.
How does Clue estimate my fertile window?
Clue’s algorithm estimates and predicts the timing of ovulation and fertile window based on combined data from your cycle length, tracking history, and global averages. You can read more about exactly how those predictions are made here.
We start with what an average cycle looks like (not necessarily your cycle) based on research, and then the algorithm adjusts as you track. If someone tracks basal body temperature (BBT) or ovulation tests, the information gained from these inputs is incorporated into estimates and predictions. Someone might have a 12 day luteal phase, for example, and predictions and estimations will adjust accordingly.
Why, exactly, you shouldn’t use your fertile window in Clue as birth control
Some apps have been developed with the goal of being used as birth control. They estimate and indicate days where they consider a user’s risk of pregnancy to be low, or high. To do this, they take into account the rules of certain fertility awareness methods, cycle regularity, and sometimes measures including cervical fluid.
In these methods, the block of time indicated as high-risk are often much longer than a ~6-day fertile window—which is very different from Clue. The efficacy of these apps (how well they work when someone uses them perfectly) varies, as does their effectiveness (how well they work given how people tend to use them) (10-12). Their ways of communicating risk levels to users also varies, and might even be confusing. These methods are not one-size fits all.
But here’s why Clue should not be used that way (for now):
1) A “regular” cycle is not the norm (yes, you are a unique a snowflake)
The timing of your ovulation (and thus your fertile window, as well as your cycle length) can vary cycle-to-cycle. And if you’re not tracking ovulation, you shouldn’t rely on a six-day fertile window to be accurate.
Cycles vary a lot for the couple of years after menarche, and usually take about six years to settle into their typical “normal” (13). They also vary a lot after pregnancy, and again as they come to an end during perimenopause. In between these times, some people have a very regular cycle, but most people don’t (1,2).
The follicular phase leading up to ovulation commonly varies in length from cycle-to-cycle. Additionally, the luteal phase used in Clue’s initial estimates might not be the length of your typical luteal phase.
That means the predicted fertile window displayed in Clue won’t always (or maybe ever) represent your true fertile window, even if your period comes at the same time each cycle.
Even for people who report being regular, variability is common. A study looking at ovulation in 213 women (696 menstrual cycles) found that women who considered their cycles as “regular” still had a 1-6% chance of being within the fertile window on the day they would typically expect their period to start. Of all the women in the study, over 7 in 10 were within their fertile window before or after days 10-17 of the cycle. About 2% were considered fertile by day 4 of the cycle, and about 17% were considered fertile by day 7 (2).
Another study found that in people who considered their cycles regular, only about 6 in 10 of their fertile days were actually included in a fertile window estimated by counting backwards from the first day of menstruation (a standard way to estimate ovulation in the absence of other biological tests) (13).
If your cycle varies a lot, you can turn off your predictions in the settings of your Clue app.
2) Accurately predicting ovulation takes more than counting days
To know when your actual fertile window happened in any given cycle, you have to determine when you’ve ovulated. Tracking ovulation at home requires taking your BBT each morning or doing ovulation urine tests (though neither method are perfectly accurate). Because ovulation can change cycle-to-cycle, you have to do this on a continual basis.
But still, these tools only show when ovulation has happened in the past (like with BBT), or when it may be just about to happen (like with ovulation tests). That only covers 1-2 days of your fertile window. They don’t predict when ovulation will happen days in the future, or when you’ve entered into the beginning of your fertile window.
Unfortunately, there are few ways to reliably predict the timing of ovulation (tell your next generation of tech-scientists-to-be that we really need innovation here).
The best at-home method we have for estimating a current fertile window and predicting an upcoming ovulation is by monitoring cervical fluid.
Monitoring fluid consistency is a key part of some FAMs—symptothermal contraceptive methods like Sensiplan or Billings Ovulation method. Other methods use only BBT, with longer windows of time for avoiding unprotected sex. These methods take some training, daily work, and an understanding of what to look for and when. They can also incorporate knowledge of how each day’s data relate to risk assessment and behavior. Clue does not currently incorporate cervical fluid tracking into its predictive algorithm.
Not all FAMs require cervical fluid monitoring. Calendar-based FAMs only require tracking periods, and others require only tracking BBT. But these methods also require abstaining from unprotected sex for far more days than just the 6-day fertile window. The Standard Days Method, for example, requires at least 12 days of abstinence. And they are only recommended for people whose cycles meet certain criteria of regularity, and who are willing to understand and be okay with the degree of risk of any respective method.
Petra Frank Hermann, a researcher and one of the world’s leading FAMs experts, pointed out in a letter to the editor of a journal last year that “the six day window comes from a study that was not designed to explore the outside limits of the fertile window, and that could not exclude a substantial probability for pregnancy from intercourse outside of the six-day window. The fertile window based on ovulation determined by basal body temperature is several days wider.”
3) False confidence can get you into trouble
It’s important to cultivate body literacy and learn about how your reproductive system works. It can be exciting, illuminating, and empowering to learn for the first time that you can’t get pregnant all the time, or that you can feel abdominal sensations around the time of ovulation, or that you experience your mid-cycle high-estrogen differently than your friend.
But it’s not unheard of for people to go off their hormonal birth control, learn about the fertile window with the intention of incorporating body-awareness in their contraceptive choices, and then become unintentionally pregnant. Overconfidence can lead to people falsely assessing their risk. Someone may think they’ve already ovulated, for example, and then have unprotected sex, only to learn that their ovulation was later than usual that cycle. Someone might ovulate earlier than expected and be in their fertile window a day before they would typically stop having unprotected sex. Someone may decide to have intercourse specifically on the days before their fertile window, knowing they’ll have to abstain from unprotected sex for several days, or decide to have intercourse just after those days are over, having just abstained. If their fertile window is a day off of what they think it is, this behavior may increase the chance of unintended pregnancy more than if they had known nothing and timed their intercourse at random. It happens. Don’t let it happen to you.
Why show a “fertile window” in the app at all, then?
We get a lot of emails from new Clue users asking questions about how fertility and the cycle work. It’s common to think that you are fertile all the time, or that nothing much happens between periods. Understanding ovulation and the fertile window is an important part of learning about your body and reproductive system. Many people use it as a general reference when they are beginning to consider becoming pregnant, or actively trying to become pregnant for the first time (but aren’t ready to start tracking things like BBT).
Also, the days leading up to and following ovulation are ones of big hormonal changes. Estrogen is highest around this time, and some people are interested in paying attention to how they experience these changes—which sensations and symptoms happen consistently, such as changes in sex drive, or ovulation pain. Others are interested in paying attention to how they experience the first days of the luteal phase, as progesterone rises.
Mid-cycle changes are relevant and interesting, but if you’re hoping to successfully prevent pregnancy, you’ll need to follow a proven method of contraception, and use it diligently.
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