The temperature shift is the rise in your Basal Body Temperature from its lower, pre-ovulatory range (usually between 97.0 and 97.7 degrees Fahrenheit, or 36.11 and 36.50 degrees 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 your body’s production of progesterone following ovulation. 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.
Here are some answers to a few common questions about the temperature shift:
How much does my BBT have to rise to confirm that ovulation has occurred?
If ovulation has occurred, the BBT will generally shift at least 0.2 degrees Fahrenheit (.11 degrees Celsius) higher than the temperature of the previous day, and will be sustained at or above this higher temperature for three days or more.
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, varies from woman to woman and is usually between 12 and 16 days. However, a woman’s personal luteal phase usually varies only by a day or two. So, if your luteal phase is usually 14 days, it may occasionally be 13 or 15 days, but it probably won’t ever be shorter or longer than that. After the luteal phase ends, your body stops producing progesterone, and you have your period. Your BBT will then shift back down to its lower range.
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. The length of the follicular phase can vary from woman to woman and from cycle to cycle for each woman. So, whereas the length of your luteal phase is usually consistent from cycle to cycle, the length of the follicular phase may not be. To get an idea of where you are in your cycle and when you are approaching ovulation, pay attention to changes in your cervical fluid as well as your cervical position.
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 its pre-ovulatory range at the end of your cycle, your temperature and progesterone levels will remain elevated throughout your pregnancy.
What if my temperature doesn’t rise? What if my temperature rises, but the rise isn't sustained?
First, make sure you are 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. 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 (such as Wink, a highly accurate basal thermometer that automatically syncs your temperatures to the Kindara app). If you are measuring your temperature in Celsius, you’ll need a thermometer that measures to at least two decimal points.
Other factors that can affect your BBT include how much sleep you’ve had, if you’ve had alcohol the night before, if you’re on any medications, if you’ve been traveling, if you’re sick, if you’re breastfeeding, if you’ve recently come off the pill, if you’re approaching menopause, if you’re underweight…et cetera! 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. Without ovulation, it is impossible to get pregnant. There are a number of possible reasons for an anovulatory cycle, including high levels of stress, eating disorders, a prolonged, strenuous exercise program, and hormonal imbalances. Tracking your fertility signs can help to clarify whether your fertility signs are inaccurate or missing, and a doctor can help you to determine what the cause of an anovulatory cycle may be.
To learn how to record a temperature shift on your chart, click here.