The temperature shift is the rise in your basal body temperature (BBT) from its lower, pre-ovulatory range (usually between 97.0° and 97.7° Fahrenheit, or 36.11° and 36.50° Celsius) to its higher, post-ovulatory range (usually between 97.7° and 98.3°F, or 36.50° and 36.83°C). The temperature shift typically occurs the day after ovulation and is due to an increase in progesterone after the egg is released. Your BBT will remain in its higher range until you get your period and start a new cycle, at which time your BBT will drop back down to its lower range (1).
Below are some answers to a few common questions about the temperature shift.
How much does my BBT have to rise to suggest that ovulation has occurred (2)?
Your BBT will generally increase at least 0.2° Fahrenheit (or 0.11° Celsius) higher than the temperatures of the previous 6 days and will be sustained at or above this higher temperature for 3 days or more (1).
How many days after the temperature shift will I get my period?
The number of days between ovulation and when you get your period, or the luteal phase (read more about the Phases of the Menstrual Cycle here), varies from person to person and is usually between 12 and 16 days (1). However, an individual's luteal phase usually varies only by a day or two from cycle to cycle. So if your luteal phase is usually 14 days, it may occasionally be 13 or 15 days, but it probably won’t be shorter or longer than that (3). After the luteal phase ends, your body stops producing progesterone, and you have your period (4).
How many days after my cycle begins will the temperature shift occur?
The number of days between the start of your cycle (the first day of your period) and ovulation is called the follicular phase (read more about the Phases of the Menstrual Cycle here). The length of the follicular phase can vary among individuals and from cycle to cycle for each person (1). Though the length of your luteal phase is usually consistent from cycle to cycle, the length of the follicular phase may not be (3). To get an idea of where you are in your cycle and when you are approaching ovulation, pay attention to changes in your cervical mucus as well as your cervical position (1).
What happens to my temperature if I become pregnant?
If you become pregnant, the temperature shift will occur after ovulation as usual, but instead of dropping back down to the pre-ovulatory temperature range at the end of your cycle, your temperature and progesterone levels will remain elevated throughout your pregnancy (4).
What if my temperature doesn’t rise? What if my temperature rises, but the rise isn't sustained?
First, make sure you're accurately recording your BBT. Take your temperature first thing in the morning, before you get out of bed, and after at least three hours of sleep (5). Try to take your BBT within the same one hour period each day throughout your cycle. If you have doubts about the accuracy of your thermometer, try purchasing a basal thermometer made specifically for BBT tracking, and if you're measuring in Celsius, you’ll need a thermometer that measures to at least two decimal points (reads .00 instead of .0 only).
Other factors that can affect your BBT include (1):
- How much sleep you’ve had
- Having alcohol the night before
- Taking certain medications
- Having a fever or being sick
- Recently coming off hormonal birth control
- Approaching menopause
- Being over- or underweight
- And many others
Obviously, you can’t control a lot of these factors, but keep them in mind as you track your temperature and remember that they may alter your BBT.
Another possibility is that you are experiencing an anovulatory cycle, in which ovulation does not occur. Anovulatorion can occur for many different reasons, including high levels of stress, eating disorders, a prolonged, strenuous exercise program, and hormonal imbalances (6). Tracking your fertility signs can help clarify whether your fertility signs are inaccurate or missing; check with your doctor to help you to determine if you are truly anovulatory and what the cause may be.
To learn how to record a temperature shift on your Kindara chart, click here.
- Weschler, Toni. (2015). Taking Charge of Your Fertility: The definitive Guide to Natural Birth Control, Pregnancy Achievement, and Reproductive Health. p62, 89, 102, 218
- Martinez, A., van Hooff, M., Schoutc, Erik., van der Meer, M., Broekmans, F., Hompes, P. (1992). The reliability, acceptability and applications of basal body temperature (BBT) records in the diagnosis and treatment of infertility. European Journal of Obstetrics & Gyneocology and Reproductive Biology, 121-127.
- Fehring, R. J., Schneider, M., & Raviele, K. (2006). Variability in the phases of the menstrual cycle. Journal of Obstetric, Gynecologic, & Neonatal Nursing, 35(3), 376-384.
- Planned Parenthood
- Mayo Clinic
- Hamilton-Fairley, D., & Taylor, A. (2003). ABC of subfertility: Anovulation. BMJ: British Medical Journal, 327(7414), 546.