The Rules of the Sympto-thermal Method of Fertility Awareness

This excerpt from The Cycle was written by Groove’s founder Jennifer Aldoretta

We’ve finally gotten to the good stuff. Before we get started, I must mention that it is imperative that you have a thorough understanding of the female reproductive system, female sex hormones, and fertility signs before attempting to learn or practice the sympto-thermal method of fertility awareness (STM), especially as natural birth control. Although there are other methods that fall into the category of “fertility awareness” — including the Billings Method, which monitors only cervical fluid (cervical mucus) changes — the sympto-thermal method is the best way to get a clear picture of your fertility.

Not only is it the most effective method of fertility awareness, but it can be used by most individuals regardless of cycle length, cycle regularity (or irregularity), or reproductive disorders. When the rules presented here are strictly adhered to, the effectiveness of the sympto-thermal method of fertility awareness rivals the birth control pill.

Practicing the sympto-thermal method can also help you get pregnant or simply keep track of gynecological health. After you’ve read through this section, you can then read about using the sympto-thermal method as natural birth control, using the sympto-thermal method to get pregnant, and using the sympto-thermal method to improve or maintain reproductive health. Just make sure to read about how to check your fertility signs and the effectiveness of the sympto-thermal method as natural birth control before diving into any of the more specialized information.

It is absolutely crucial (and I cannot stress this enough) that you read this section very carefully before attempting to practice the sympto-thermal method. If you don’t quite get it after the first read, then read it again! Just make sure you have a good understanding of the material before attempting to use this method for any purpose, but especially as a form of natural birth control. I also highly recommend that if you do not feel confident in your knowledge of the sympto-thermal method (or if you or your partner are confused by any fertility signs), get in touch with a fertility awareness educator to help out with the transition.

Another thing I absolutely must bring up, even though I’m confident that you’re a smart lassie (or lad…or anything in between).

Practicing a fertility awareness-based method in no way protects you from sexually transmitted infections (STIs) or HIV.

Knowing that you (or your partner) are not in the fertile window isn’t an invitation or excuse to forgo a condom. I’m not going to give you a full parental lecture, but if you’re planning to practice the sympto-thermal method (or any other method of fertility awareness) and you’re concerned about the possible transmission of STIs or HIV, use a condom!

Ladies and gents, without further ado, I present the rules of the sympto-thermal method.

1. First Five Days Rule

You are considered infertile during the first five days of a new cycle, but only if you experienced a spike in basal body temperature 12 to 16 days prior to menstruation. Since some people experience mid-cycle spotting or bleeding, it’s very important to assume infertility only the first five days of an actual menstrual period, which occurs 12 to 16 days after a rise in basal body temperature (BBT). If menstruation lasts for more than five days, you are potentially fertile due to the possibility of menstrual fluid masking fertile cervical fluid, especially for those who tend to ovulate early. Since you must have experienced a temperature shift (indicating ovulation) prior to menstruation, it’s important to wait until at least one full cycle has been charted before relying on this rule.

If you have experienced any cycles that are less than 25 days in length, if you are just starting out with the sympto-thermal method, or if you are perimenopausal, this rule will change to assume infertility only the first three days of a new cycle. This is due to the possibility of early ovulation and a fertile window that begins early enough in the cycle to be masked by menstruation. Anyone starting out with the sympto-thermal method must adhere to this alternative rule until it can be confirmed that your cycles are consistently longer than 25 days.

Although rare, it is possible to get pregnant during menstruation. A previous physician of mine swears she got pregnant on the last day of her period. After she admitted she was using the rhythm method at the time, I knew this was due to one of several things (aside from the fact that she was using the rhythm method). Perhaps she ovulated very early during the cycle in question and didn’t take previously short cycles into consideration; cervical fluid can easily be masked by menstrual fluid. Or maybe she was experiencing heavy mid-cycle spotting that she mistook for menstruation; mid-cycle spotting is often thought to occur on your most fertile day(s). It is also possible that she didn’t ovulate during her previous cycle; if this was the case, she could have had fertile cervical fluid while experiencing withdrawal bleeding (which is not a true menstrual period). She was married at the time, and this surprise pregnancy happily ended in the birth of one of her two lovely daughters. But had she been practicing the sympto-thermal method instead of the outdated, unscientific rhythm method, she could have avoided this unplanned pregnancy!

2. Dry Day Rule

I discussed cervical fluid patterns in another section, and I mentioned that there is typically no fluid on the days following menstruation. However, there are exceptions that I will discuss below. A lack of cervical fluid, or cervical mucus, means sperm cannot survive for more than a few hours in the body (the vagina is too acidic). Because of this, during the time between menstruation and ovulation, you are considered infertile the evening of every day you experience a dry day — or a day with no visible cervical fluid. As soon as you notice cervical mucus — whether it is sticky, creamy, egg white, or watery — sperm are now able to survive in your body and your fertile window is officially open!

This rule specifies evenings as “safe” assuming that you check your cervical fluid at the vaginal opening rather than at the cervix. If you decide to do some spelunking and instead check your fluid at the opening of the cervix, you will have a much more accurate picture of your fluid type for that day. If you just can’t muster the courage to check it at the cervix, you can instead check at the vaginal opening; but you will not be considered infertile until that evening since fluid may take several hours to make the journey from the cervix to the vaginal opening. Since cervical fluid can quickly change from dry to fertile, it’s necessary to check several times throughout the day. I will discuss how to check cervical fluid in an upcoming section.

Some people may find that they have cervical mucus every day. It may be sticky, tacky, pasty, gummy, or creamy day after day with no obvious “dry” days. For example, if you check your cervical fluid for several cycles and establish that you consistently experience sticky fluid on the days you would have expected to be “dry,” your basic infertile pattern (BIP) is sticky.

For these folks who do not experience “dry” days, the dry day rule will be different: if you experience a BIP that is not dry, you are considered infertile the evening of every day you experience no change in your cervical fluid (cervical mucus). Those who experience non-dry BIPs must use incredible care when checking cervical fluid to ensure you are getting an accurate representation. When in doubt, don’t take the risk!

Semen, lubricants, and spermicides can all mask cervical fluid. If any of these are present in the vagina during “dry” days, it’s important for them to be expelled so they don’t mask potentially fertile fluid on the following day. An easy way to expel these substances is to perform kegels — which is the repeated contraction and relaxation of the pelvic floor muscles. A great way to practice is to start and stop your urine stream in the middle of using the bathroom…that’s a kegel! 

If semen, lubricant, or spermicide is not expelled from the vagina after intercourse, then unprotected intercourse is not safe on the following day since each one has the potential to mask fertile cervical fluid (cervical mucus). To be more conservative, some people do not consider themselves safe the day after semen, lubricant, or spermicide presence, even if it is a dry day.

2a. Doering Rule

The “Doering Rule” that I’m about to discuss is a conservative addition to the “Dry Day Rule” for those who may want a little extra protection. I’d like to note that adding the Doering Rule is not necessary to properly practice the sympto-thermal method; it’s all a matter of personal preference. Practicing this rule in addition to the others will make the sympto-thermal method more effective, but it will typically lead to a longer window of abstinence or barrier backup use. If this rule is added, it is used in addition to the Dry Day Rule.

Because the use of the Doering Rule adds a calendar-based rule to the sympto-thermal method, at least 12 cycles of charts are required before it is used. The Doering Rule states that your fertile window begins seven days before the earliest cycle day on which a temperature shift has occurred over the past 12 cycles. However, if cervical fluid is seen before this “Doering day,” the fertile window is open. Presence of cervical mucus before the “Doering day” ALWAYS overrides the Doering Rule.

For example, if the earliest temperature shift that Mary has experienced during the past year occurred on cycle day 16 (the rest were on cycle days 17-20), then she would consider her fertile window open on cycle day nine of each cycle (16 – 7 = 9). However, during Mary’s last cycle, she noticed cervical fluid on cycle day eight, and she, therefore, considered cycle day eight the start of her fertile window instead of cycle day nine.

It’s imperative that the Doering Rule is ONLY practiced if at least twelve cycles of basal body temperature data are available, because it is possible to ovulate on cycle day 20 during one cycle and cycle day 12 the very next cycle. Since the day of ovulation will likely change from cycle to cycle, the “Doering day” must be recalculated each cycle to include the most recent cycle.

3. Peak Day Rule

The last day of slippery, slick, or lubricative cervical fluid (cervical mucus) is considered your peak day. Because your peak day is the last day of slippery fluid before a drying pattern begins, you won’t be able to identify your peak day until the following day. Some individuals may never experience slippery fluid — if you fall into this category, your peak day is the last day of the most fertile type of fluid you observe, even if it is not slippery, watery, or egg white.

Your peak day may occur a few days before ovulation, and it’s generally the most fertile day of your menstrual cycle. If you experience a sudden temperature shift (as described in rule #4 below), you are considered infertile starting in the evening of the third day after your peak day. However, if you experience a slow-rise temperature shift (as described in rule #4 below), you are not considered infertile until the evening of the fourth day after your peak day.

Whether you’re considered infertile three days after your peak day because of a sudden temperature shift or four days after your peak day because of a slow-rise temperature shift, the “Peak Day Rule” MUST be double-checked with the “Temperature Shift Rule”. I will explain this in more detail in the “Double-Check Rule” (rule #5) that I will discuss shortly. We’ll also discuss how to identify your peak day in an upcoming section.

4. Temperature Shift Rule

After you ovulate, there is a surge of progesterone production that causes your basal body temperature to rise or “shift”. This rise in temperature can take two forms: (1) a sudden temperature shift and (2) a slow-rise temperature shift, each of which I will discuss below. To make these high, post-ovulatory temperatures easier to visualize, you can draw a horizontal line — called a coverline — across your chart. I’ll discuss how to draw a coverline after we go through the remaining rules, but I want to mention it because I will reference it in the next few paragraphs.

Sudden temperature shift

A sudden shift in basal body temperature occurs the day that your basal body temperature is at least 0.2 °F (or 0.1 ºC) higher than the highest temperature you recorded during the previous six days. That means that if you look back at your basal body temperature for the past six days (not including today), and the highest temperature that occurred during that time is at least 0.2 °F (or 0.1 ºC) lower than today’s temperature, you can assume that today is the day of your temperature shift.

If you experience a sudden temperature shift, you are considered infertile for the remainder of your current cycle starting in the evening of the third consecutive day that your temperature is above the coverline, as long as the third temperature is at least 0.3 ºF (0.15 ºC) above the coverline. If the third temperature after your temperature shift doesn’t reach 0.3 ºF (0.15 ºC) above the coverline, this rule will change and you must follow the rule associated with a slow-rise temperature shift (explained below).

If your basal body temperature drops below the coverline during any of the first three days after your temperature shift, you must restart the three-day count. Additionally, if any basal body temperature readings during those first three days after your temperature shift are either artificially high or missing, the three-day count must be restarted. Please note that these two approaches are very conservative, and if you choose not to follow these approaches, there will be an extra risk encountered.

For example, if Katherine’s basal body temperature drops below the coverline (or if it is artificially high or she simply forgot to take it) on the second day after her temperature shift, to be conservative she would restart her count. However, if Katherine acknowledges and is okay with the extra risk, she can resume her count the following day.

I will discuss how to pick out an artificially high basal body temperature reading in an upcoming section. The charts that follow display the temperature shift rule as it applies to the two types of temperature shifts: “sudden” and “slow-rise”.

Slow-rise temperature shift

Not everyone will experience a sudden temperature shift after ovulation. Some will experience a slow-rise temperature pattern, where basal body temperature will rise in increments of about 0.1 °F (or 0.05 ºC). For those who experience this type of rise, the temperature shift will occur on the day that basal body temperature is at least 0.1 °F (or 0.05 ºC) higher than the highest basal body temperature that occurred during the previous six days. 

In the case of a slow-rise temperature shift — or if you experience a sudden temperature shift but your third temperature was not at least 0.3 ºF (0.15 ºC) above the coverline — your post-ovulatory infertile period begins in the evening on the fourth consecutive day that your basal body temperature is above the coverline. 

If basal body temperature falls below the coverline, wasn’t taken, or is artificially high for any reason during any of these four days, restart the count to be conservative. The rules for drawing a coverline in this case are different than with a sudden temperature shift, and I’ll discuss this later in this section. A sample chart displaying a slow-rise temperature pattern is shown below.

5. Double-Check Rule

This last rule is somewhat redundant, but it must be followed in order to practice the sympto-thermal method correctly. It is possible that the third day after your peak day and the third day after your temperature shift will occur on the same day. However, this is not always the case. You must wait until both the “Peak Day Rule” and the “Temperature Shift Rule” have been satisfied before you consider yourself infertile for the remainder of your current cycle.

For example, the third day after Sonya’s peak day falls on a Tuesday and the third day after her temperature shift occurs the next day on Wednesday. In this case, Sonya can consider herself infertile on Wednesday evening after both rules are satisfied.

Optional Cervical Position Check

As we’ve learned, the position and hardness of the cervix changes with fluctuating estrogen and progesterone levels. Estrogen secretion before ovulation causes changes in both the cervix (including making it softer and more open) and the cervical fluid (causing it to become progressively clear, stretchy, and slippery). Progesterone secretion after ovulation causes these changes to reverse and produces a sustained rise in basal body temperature.

After ovulation, as sort of a “confirmation” that the infertile period has begun, many people will check the position of the cervix to confirm that it has returned to its infertile (low, firm, and closed) position. It is a great way to provide a little extra peace of mind! I will discuss how to check your cervical position, and what to look for, in an upcoming section.

Sympto-thermal Method Rules Summary

While that bigger, more educated brain of yours processes all of the information I just dumped on you, here is a breakdown of the rules you just learned. Remember, the “Doering Rule” (rule 2a) is optional, but all other rules must be followed to correctly practice the sympto-thermal method and to maintain effectiveness of this method.

1. First Five Days Rule: you are considered infertile during the first five days of a new cycle, but only if you experienced a spike in basal body temperature 12 to 16 days prior to menstruation. However, if you have experienced any cycles that are less than 25 days in length (or if you are perimenopausal or just starting out with the sympto-thermal method) this rule will change to assume infertility only the first three days of a new cycle.

2. Dry Day Rule: you are considered infertile the evening of every day you experience a “dry” day — or a day with no visible cervical fluid. However, if you happen to have a non-dry basic infertile pattern (BIP), you are considered infertile the evening of every day you experience no change in your cervical fluid. Additionally, if semen, lubricant, or spermicide is not expelled from the vagina after intercourse, then unprotected intercourse is not safe on the following day since each one has the potential to mask fertile cervical fluid (cervical mucus).

2a. Doering Rule (a conservative addition to the Dry Day Rule): your fertile window begins seven days before the earliest cycle day on which a temperature shift has occurred over the past 12 cycles. At least 12 cycles of basal body temperature data are necessary to use this rule. Presence of cervical fluid (cervical mucus) will always override this rule.

3. Peak Day Rule: this rule is satisfied in the evening of the third day after your peak day, unless you experience a slow-rise temperature shift, in which case the rule is satisfied in the evening of the fourth day after your peak day. The “Double-Check Rule” (rule #5) must be followed to determine when your post-ovulatory infertile period begins.

4. Temperature Shift Rule: this rule is satisfied after three consecutive days of a basal body temperature that is above the coverline, as long as the third temperature after your temperature shift is at least 0.3 ºF (0.15 ºC) above the coverline. For a slow-rise temperature shift, or in the case that your third temperature was not at least 0.3 ºF (0.15 ºC) above the coverline, this rule is satisfied after four consecutive days of a basal body temperature above the coverline. The “Double-Check Rule” (rule #5) must be followed to determine when your post-ovulatory infertile period begins.

5. Double-Check Rule: you must wait until BOTH the “Peak Day Rule” AND the “Temperature Shift Rule” are satisfied before you consider yourself infertile for the remainder of your current cycle.

The following sample charts show all of the sympto-thermal method rules in action:

Sudden Temperature Shift

Sudden Temperature Shift & Early Peak Day

Sudden Temperature Shift & Late Peak Day

Slow-rise Temperature Shift

Slow-Rise Temperature Shift & Early Peak Day

Slow-Rise Temperature Shift & Late Peak Day

Non-Dry Basic Infertile Pattern (BIP)

More sample charts

Above, you saw a handful of sample charts that show you how to implement the rules of the sympto-thermal method that were discussed earlier in this article.

If you’re interested in visualizing cycles that include things like anovulation, low progesterone, or stopping hormonal contraception, you can do that here.

How to Draw a Coverline

Drawing a coverline isn’t totally necessary, but doing so will make it much easier to visualize the temperature shift. I mentioned that some folks experience a sudden temperature shift, while others have a slow-rise pattern. The rules for drawing the coverline are different for each of these two scenarios.

Sudden Temperature Shift Coverline

For someone who experiences a sudden temperature shift — meaning the temperature shift occurs on the day when basal body temperature is at least 0.2 °F (or 0.1 ºC) higher than the highest basal body temperature that occurred during the previous six days — the coverline is drawn horizontally across the chart 0.1 °F (or 0.05 ºC) higher than the highest basal body temperature that occurred in the six days immediately prior to the temperature shift.

Slow-Rise Temperature Shift Coverline

Someone who experiences a slow-rise temperature shift — meaning a temperature that rises in about 0.1 °F (or 0.05 ºC) increments — will draw the coverline horizontally across the chart through the day that basal body temperature is at least 0.1 °F (or 0.05 ºC) higher than any of the previous six days. If you’re still confused, take a look at the example chart below.

So, that’s that! The sympto-thermal method of fertility awareness may seem a bit overwhelming or daunting at first, but what isn’t? Learning to tie your shoes was probably pretty stressful when you were first figuring it out, too. The benefits of practicing the sympto-thermal method — whether it’s for natural birth control, to get pregnant, to maintain optimum health, or simply to track your cycles — quickly become apparent after starting out. You might even be asking yourself why you ever resorted to using anything else!