This excerpt from The Cycle was written by Jennifer Aldoretta
In order to understand the rules of the sympto-thermal method (STM) and to properly and effectively practice them, it is absolutely essential that you first gain a thorough understanding of the information presented in this section. I previously discussed the hormone changes that produce the menstrual cycle. These hormonal fluctuations have both an unseen and an observable effect on your body, and this section will focus on how those changes manifest in an obvious and useful way. Your body’s fertility signs are plain as day; you simply must know where—and how—to look.
Cervical fluid is definitely the most important fertility sign, as it allows for the detection of the beginning of the fertile window. Cervical fluid is one of the two fertility signs—along with basal body temperature—that absolutely must be observed to practice the sympto-thermal method (STM). As ovarian follicles develop and secrete increasing amounts of estrogen in the first half of the menstrual cycle, the properties of cervical fluid change to allow sperm to survive in your body. This results in a fertile window of about six days [31,32], though the precise length varies from person to person. This fertile window is the only time during the menstrual cycle when a pregnancy is possible. Cervical fluid undergoes several changes during the fertile window as a result of the estrogen rise, each of which I will explain in the paragraphs below. The easiest and most reliable way to check cervical fluid is to gather some with your fingers and then pay attention to the way it feels when you rub it between your fingers; while this might invoke an “ew” or two, checking on a daily basis opens up a whole world of incredible knowledge and insight! I’ll get into the details of how to check cervical fluid later, but that little tidbit is necessary to keep in mind as I go through this section.
I mentioned earlier that the length of the fertile window will be different for everyone, and the same can be said for cervical fluid patterns. A different body means different hormone fluctuations and, therefore, different fertility patterns! For the sake of simplicity, I will use a 28-day cycle to explain what changes occur. However, this model will serve only as a guide (even for those who do have a 28-day cycle), since you will likely find that your individual pattern is different than the one I’m about to discuss.
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So let’s make a completely ridiculous statement and say that every human female has a 28-day menstrual cycle and ovulates exactly on cycle day 14 (which by now we know just isn’t true). Day one of the cycle is the first day of menstruation, which typically lasts about five days. On cycle day six, after menstruation has ended, the person in our example will not notice any cervical fluid. This is because estrogen production by the developing follicles is not yet significant enough to cause observable changes in cervical fluid. A slight dampness will still be present when you check your cervical fluid—just like when you touch the inside of your mouth. The vagina is a mucous membrane, after all! However, this dampness will evaporate from your fingers fairly quickly. This “dry” cervical fluid pattern will continue until about cycle day 9, when the developing follicles begin secreting enough estrogen to cause cervical fluid to develop fertile qualities.
At this point, cervical fluid will become noticeable, unlike the mysteriously-named “dry” fluid that is nowhere to be found. From cycle day 9 until ovulation on cycle day 14, this person’s fluid will progress from “sticky” to “creamy” to “egg white,” each of which I will discuss below. However, this is definitely not the pattern everyone will experience. In most cases, the presence of any cervical fluid means you’re in your fertile phase, since it’s possible for sperm to survive in sticky or creamy fluid long enough to fertilize the egg. However, there are exceptions that I will discuss when I introduce the rules of the sympto-thermal method. In this example, cycle day 9 will see sticky fluid, which can be white or yellow in color and may have a texture that is thick, pasty, crumbly, tacky, gummy, or rubbery. It may stretch slightly or form small peaks between two fingers, but it will not feel slippery or lubricative. As estrogen levels continue to rise, cervical fluid may become creamy. Creamy fluid is noticeably thinner than sticky fluid—due to the higher water content—and may take on a milky or lotiony texture. In our example, this creamy fluid will continue until cycle day 12.
Cycle days 13 and 14 will see a dramatic change in the look and feel of cervical fluid. In the days just before ovulation, the fluid in this example takes on an “egg white” quality, which is appropriately named because it closely resembles raw egg white. The makeup of egg white cervical fluid is similar to that of seminal fluid, allowing for sperm survival and motility in an otherwise hostile vaginal environment. Egg white fluid will feel very slippery and lubricative due to the high water content, and many people find that it stretches up to several inches. Depending on the person, this egg white fluid may be clear or streaked, or it may be brown or pink-tinged due to pre-ovulatory spotting.
Many people find that the most fertile cervical fluid they encounter during their cycle is the slippery, lubricative egg white fluid. However, others may notice that their most fertile fluid becomes so watery, thin, and slippery before ovulation that it leaves them feeling leaky. This fluid is known to leave a lovely circular stain on your underwear and may cause a cold, wet veginal sensation.
After ovulation—which occurs on cycle day 14 in our example—your cervical fluid will begin a drying pattern. It may be thick, sticky, creamy, pasty, or even stretchy, but it will no longer feel slippery or lubricative. This “drying” of fertile cervical fluid coincides with the sudden secretion of progesterone by the corpus luteum (and, as it so happens, with a rise in basal body temperature) after ovulation. As I mentioned earlier, one of the many ways progesterone prevents a pregnancy from occurring after ovulation is to dry up slippery cervical fluid for the remainder of your cycle. Thanks, progesterone! The chart below shows just one of the possible cervical fluid patterns that might occur during a single menstrual cycle. Every fertile window will manifest in a slightly different way.
This person's fertile window starts on cycle day 9 with the presence of sticky cervical fluid. There is no longer fertile-quality fluid on day 15, but cervical fluid alone cannot confirm the end of your fertile window.
You may notice that your cervical fluid doesn’t seem to “fit” into one of the categories I just discussed, but that doesn’t necessarily mean you’re not completely normal and healthy! The most important thing to remember about cervical fluid is that it will become progressively wetter and more slippery leading up to ovulation; so paying close attention to your body’s pattern is vital. I will discuss the different fluid types, along with their many possible variations, in more detail when I discuss how to check cervical fluid (or cervical mucus).
If you’re a visual learner, take a look at these images of cervical fluid changes throughout the menstrual cycle. Though keep in mind that individual patterns will vary.
Basal body temperature (BBT) is the second fertility sign that must be observed in order to practice the sympto-thermal method of fertility awareness. It is defined as the core temperature of the body at rest, and is measured first thing upon waking. I’ll detail how to measure your basal body temperature in another section. BBT follows a very predictable—and scientifically supported—pattern during the menstrual cycle, although the exact pattern will differ from cycle to cycle and from person to person. Measurement of BBT is an important practice when using STM, as it allows you to confirm ovulation. This allows you to detect the post-ovulatory infertile period, or the time in your cycle when unprotected sex will not result in a pregnancy.
BBT is low during the pre-ovulatory stage of the cycle, which includes menstruation, the proliferative phase, and the follicular phase. However, it’s important to mention that BBT may be a bit high for the first few days of a new cycle due to lingering effects of progesterone that was produced during the previous cycle. This high menstrual BBT is perfectly normal and does not indicate pregnancy or fertility.
The low pre-ovulatory BBT can manifest in several ways. For one person, it may mean temperatures ranging from 97.5 ºF to 97.9 ºF (or from 36.38 ºC to 36.61 ºC); for a different person, it may mean temperatures between 96.8 ºF and 97.2 ºF (or between 36.00 ºC and 36.22 ºC). It all depends on the person.
After ovulation occurs, BBT will rise due to the presence of progesterone that is secreted by the corpus luteum (which, remember, is made from the empty follicle that recently ovulated). The post-ovulatory stage of the cycle, which includes both the luteal phase (ovarian cycle) and the secretory phase (uterine cycle), is defined by a consistently high BBT compared to the pre-ovulatory stage. The person who had the pre-ovulatory temperatures between 97.5 ºF and 97.9 ºF (or 36.38 ºC and 36.61 ºC) may have post-ovulatory temperatures ranging from 98.2 ºF to 98.6 ºF (or 36.80 ºC to 37.00 ºC). The person with the lower pre-ovulatory temperatures of 96.8 ºF to 97.2 ºF (or 36.00 ºC to 36.22 ºC) may experience post-ovulatory temperatures between 97.5 ºF and 97.9 ºF (or 36.38 ºC and 36.61 ºC). The important thing to note here is that this sustained rise in BBT will only occur if ovulation has already taken place. This makes detecting the end of the fertile window a breeze! Take a look at the chart on the next page for an example of typical BBT fluctuations seen over the course of a cycle.
On cycle day 17, you can see that this BBT pattern spikes and remains high for the rest of the cycle. This is because the corpus luteum secretes progesterone that has a thermal effect, causing BBT to increase. A luteal phase count is started on the day of the temperature shift to highlight the start of the high post-ovulatory temperatures and to make it easy to determine the luteal phase length. The horizontal line is called the Coverline, which is drawn to make the temperature spike easier to visualize. I'll discuss how to draw a Coverline in another section.
The data in the chart above shows a luteal phase length of 12 days (this is the number of days between the post-ovulatory temperature shift and the start of the next menstrual period). However, not everyone will experience the same pattern. Luteal phase lengths typically fall between 12 and 16 days and will be quite consistent for each individual .
My personal luteal phase tends to be exactly 13 days. On rare occasion, it will be 12 or 14 days, but even this small variation makes it easy to predict my next period. Even though my overall cycle length often varies by seven days or more, I always know when to expect my period. Gone are the days spent worrying why my period is already five days later than my last cycle. Gone are the pregnancy scares and the uncertainty!
Someone trying to avoid a pregnancy will look to their BBT rise to determine when they are no longer at risk of getting pregnant during their current cycle. Conversely, someone trying to achieve a pregnancy will look to their BBT rise to determine when their window of baby-making opportunity has passed until their next cycle. There are rules that must be followed to practice the sympto-thermal method correctly, so make sure to read them carefully.
It’s important to point out that there are certain events—such as a fever, alcohol consumption the previous night, or restless sleep—that can cause an artificially high BBT. There are simple ways to detect an artifically high basal body temperature that I’ll discuss in another section.
While observing cervical position is not necessary to practice the sympto-thermal method, it can provide great insight, as well as a way to double-check fertility signs, for those who feel they need a little something extra. The cervix undergoes physical changes in the presence of fluctuating estrogen and progesterone levels. Since the cervix itself produces cervical fluid, it certainly makes sense that changes in cervical fluid also mean changes in the cervix itself. For most of the menstrual cycle, the cervix sits low in the vagina and is very firm, like the tip of your nose. The opening in the cervix, or the os, is tightly closed at this time, indicating that you’re not yet fertile. As ovulation grows closer, the cervix becomes soft like your lips; its position in the vagina becomes high, and it opens up to allow easier passage of sperm into the uterus. After ovulation, it returns back to its low, firm, and closed position rather quickly and remains in this position until the next cycle. Read about how to check your fertility signs for more information.