A basal body temperature (BBT) is the lowest temperature attained by the body during a twenty-four hour period, which is usually attained during a period of rest. For most women, the rest necessary for a BBT occurs after three or more hours of uninterrupted sleep, which usually coincides with nighttime sleep. In women, basal body temperature is also affected by hormones, most notably estrogen and progesterone. Prior to ovulation when the body is dominated by estrogen, basal body temperatures remain low.
However, after ovulation when progesterone dominates, the BBT rises an average of approximately half a degree. (See “Classic Rise” chart.) For many women, this noticeable temperature spike occurs immediately after ovulation due to the increase in progesterone and decrease in estrogen. However, not all women with ovulatory cycles experience an ideal thermal shift. The following sections describe the most common variations—and perfectly normal—in basal body temperature chart patterns.
Variations in Basal Body Temperature
The first most common variation in the BBT chart pattern is the sloping rise. In a sloping rise pattern, a thermal shift is obvious, but the individual temperatures rise more gently in a curved pattern over a few days rather than abruptly. (See “Sloping Rise” chart.) Eventually, the post-ovulation temperatures plateau at least half of a degree above the pre-ovulation temperatures.
The second variation in the basal body chart pattern is the slow rise, which is extremely similar to the sloping rise. In a slow rise pattern, a thermal shift is not apparent until a few days after ovulation. (See “Slow Rise” chart.) Like the sloping rise, temperatures in a slow rise increase gradually but in even smaller increments and over a longer period of time. The temperatures in a slow rise pattern also eventually reach the highest post-ovulation temperatures after four to six days.
The third common variation in the BBT chart pattern is the fallback rise, which is also sometimes referred to as a fallback thermal shift. In a fallback rise, an obvious thermal shift occurs immediately after ovulation. Following the obvious shift, the temperature drops the next day, sometimes under the coverline. However, on the third day after ovulation, the temperatures return to high post-ovulation temperatures and continue to remain high. (See “Fallback Rise” chart.)
The fourth variation in the basal body temperature chart pattern is the staircase rise. In a staircase rise, a thermal shift is not always obvious. Temperatures generally alternatively increase and decrease over a period of a few days until reaching the highest post-ovulation elevated temperatures. Sometimes, the first decrease falls below the coverline. However, after the period of a few days, the post-ovulation temperatures plateau at approximately half a degree above the pre-ovulation temperatures. (See “Staircase Rise” chart.)
Other Irregular Basal Body Temperatures
In addition to the four variations in basal body temperature, other irregular temperatures sometimes happen that make reading the BBT fertility chart more difficult. The first irregular temperature is a significant temperature drop immediately before the thermal shift. Such a decrease is often referred to as an ovulation dip and is the result of an increase in estrogen immediately before ovulation. Not all women experience an ovulation dip.
The second irregular temperature is a significant temperature drop about a week after ovulation. Such a decrease is often referred to as an implantation dip and is the result of a fertilized egg implanting in the uterine wall. An implantation dip may be accompanied by implantation bleeding. Not all women who have conceived experience an implantation dip, nor does a dip after ovulation always indicate pregnancy.
The third irregular temperature is a temperature that seems out of place either before ovulation or during the luteal phase after ovulation. In general, out-of-place temperatures may be explained by an insufficient period of rest before the taking of the basal body temperature. By following the rule of thumb, a woman may ignore one or two unusual temperatures before or after ovulation when reading her chart.
Although some women experience a clear shift in their basal body temperature immediately following ovulation, not all women can identify such obvious patterns in their BBT fertility charts. Fortunately, some variation and irregularity is perfect normal. Ovulation is possible without an obvious temperature shift as clearly demonstrated by the four variations in basal body temperature.
How to Take Your BBT
Basal body temperature is the lowest temperature of the body during rest, usually taken in the morning upon waking up. To take your BBT accurately, following a few guidelines is important. First, try to take your temperature at the same time every day, ideally after at least three to five hours of uninterrupted sleep. Taking your temperature before getting out of bed or engaging in any physical activity is best because movement can affect your temperature reading. Keep your thermometer next to your bed so that you can take your temperature with minimal movement.
Second, use a basal body thermometer, which is more sensitive and accurate than a regular thermometer such as thermometers for determining fevers. You can purchase a basal body thermometer at most pharmacies or online. I bought my first basal body thermometer at my local drug store. Newer models even come with apps to make tracking even easier. Be sure to read the instructions carefully and familiarize yourself with how to use the thermometer properly. Generally, you should place the thermometer under your tongue and close your mouth and then keep the thermometer in place for several minutes the screen displays a reading.
Finally, record your temperature on a chart or in a tracking app, which will help you monitor any patterns or changes in your temperature over time. There are several charting methods available including graph paper, spreadsheets, online tools, and apps. Choose the method that works best for you and be consistent in recording your temperature every day. By following these guidelines, you can ensure that your BBT readings are accurate and reliable, which can help you monitor your reproductive health and identify ovulation or other changes in your cycle.
Other Factors That Affect BBT
While basal body temperature is a useful tool for tracking ovulation and fertility, also keep in mind that there are several factors that can affect BBT readings. For instance, medications can interfere with BBT readings. Certain medications such as thyroid hormone replacement therapy or nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen can affect your temperature and make interpreting BBT charts accurately more difficult. Talk to your healthcare provider about any medications you are taking and the ways in which they may affect your BBT readings.
Other factors that can affect BBT readings include travel, illness, stress, and sleep disturbances. Travel across different time zones can disrupt your sleep pattern, which in turn can affect your BBT readings. Illness, especially illness with a fever, can also cause a temporary increase in body temperature, which can make detecting ovulation trickier. Stress can affect your sleep quality and cause fluctuations in your BBT readings. Sleep disturbances such as insomnia or sleep apnea can also interfere with accurate BBT readings. Talk with your healthcare provider about any factors that you think might be affecting your basal body temperature. Alcohol consumption can also affect BBT readings. Alcohol is a depressant that can lower your body temperature temporarily, which can cause a false decrease in your BBT readings. Limiting alcohol consumption or avoiding drinking altogether if you are trying to track your BBT accurately is important. By keeping these factors in mind and taking steps to minimize any potential impact, you can ensure that your BBT readings are as accurate as possible, which can help you track your reproductive health effectively.
When to Seek Medical Advice
While basal body temperature charting can be a useful tool for tracking ovulation and fertility, keep in mind that BBT is not a foolproof method of birth control or pregnancy prediction. If you are using BBT charting to try to conceive, consulting with a healthcare provider to ensure that you are tracking your temperature correctly and to rule out any underlying fertility issues is recommended. If you are using BBT charting as a method of birth control, using additional methods of contraception is important because of the risk of unintended pregnancy even with accurate BBT tracking.
Additionally, seeking medical advice is important if you notice any unusual or persistent changes in your BBT chart patterns or if you experience other symptoms such as irregular periods, pain, or abnormal vaginal discharge, which could be signs of underlying reproductive health issues, such as polycystic ovary syndrome (PCOS), endometriosis, or thyroid disorders. Your healthcare provider can help you identify the cause of these symptoms and provide appropriate treatment options. By seeking medical advice when needed, you can ensure that you are monitoring your reproductive health effectively and addressing any concerns in a timely manner.
Basal body temperature (BBT) charting is a useful tool for tracking ovulation and fertility that can provide valuable insights into your reproductive health. By understanding the various patterns and changes in BBT readings, you can identify when you are most likely to conceive or detect any underlying fertility issues. However, keep in mind that BBT charting is not a foolproof method of birth control or pregnancy prediction. Seeking medical advice is important if you notice any unusual or persistent changes in your BBT chart patterns or experience other symptoms. By following the guidelines for taking and recording BBT readings accurately and by being aware of the various factors that can affect BBT readings, you can use this method of tracking your reproductive health effectively and confidently.
Disclaimer: The information contained in this article is for educational purposes only and should not be used for diagnosis or to guide treatment without the opinion of a health professional. Any reader who is concerned about his or her health should contact a doctor for advice.
This post was originally published on December 28, 2011 and updated on April 20, 2023.
Basal body temperature for natural family planning: https://www.mayoclinic.org/tests-procedures/basal-body-temperature/about/pac-20393026
Weschler, Toni. 2006. Taking charge of your fertility, 10th anniversary edition: The definitive guide to natural birth control, pregnancy achievement, and reproductive health. New York: Collins.
Basal Body Temperature Chart Patterns © 2019 Heather Johnson
Classic Rise © 2019 Heather Johnson
Sloping Rise © 2019 Heather Johnson
Slow Rise © 2019 Heather Johnson
Fallback Rise © 2019 Heather Johnson
Staircase Rise © 2019 Heather Johnson