Tracking basal body temperature (BBT) is a relatively simple method of fertility awareness that only requires having a very sensitive thermometer. The method involves you keeping track of your daily body temperature and provides a relatively easy way to find out if and when you’re ovulating.

What is Basal Body Temperature (BBT)?

Basal body temperature (BBT) is a person's at-rest temperature i.e. during sleep. It is usually taken immediately after waking up before any physical activity occurs that would change the temperature. BBT changes based on a number of factors, including hormones.

What is the BBT method for tracking fertility?

Women tend to have lower temperatures before ovulation, and slightly higher temperatures once ovulation has occurred. A woman’s BBT usually increases slightly by around 0.2ºC/0.4ºF after ovulation. Women are at their most fertile during the couple of days before this temperature increase. 

Regularly charting this pattern of low temperatures followed by higher temperatures (a biphasic temperature pattern), may help to estimate a woman’s most fertile days and when ovulation will occur.

Why track BBT?

Basal body temperature can be used predict your fertile window or alternatively as a method of contraception alongside other methods. It’s easy to do and is a relatively inexpensive method without any side effects.

BBT is slightly lower during the first half of the menstrual cycle (follicular phase) due to the presence of the higher estrogen levels. Once ovulation occurs, progesterone is released to prepare the uterus for implantation of a fertilised egg. It is the progesterone that causes the rise in BBT and it stays raised through the second half of the menstrual cycle (luteal phase).

The shift in temperature after ovulation is slight and will rise by around 0.2ºC/0.4ºF to

0.6 ºC/1ºF  compared to the BBT of the previous days. If pregnancy doesn’t occur, levels of progesterone will fall, leading to menstruation and a drop in BBT.

The basal body temperature method can also be used to detect pregnancy. If a pregnancy is detected, levels of progesterone will stay high and therefore BBT will also stay high. Following ovulation, a rise in basal body temperature that continues to last past the expected menstruation date may be an early indicator of pregnancy.

How to measure and chart BBT 

• The first step to charting your basal body temperature is getting a chart to track your temperature. You can use a physical chart, an online fertility calendar or use a fertility app, many of which are free.
• Ideally you need a thermometer that measures temperatures to at least one-tenth of a degree (e.g. 98.6, if measuring in F) and one-hundredth of a degree (e.g. 37.00, if measuring in C). This could be a regular digital thermometer or a special basal body temperature thermometer.
• You should take your basal temperature at the same time (plus or minus 30 minutes) every day. The best time to take this temperature is immediately after waking up before getting out of bed.
• Try not to get up, sit up, walk around, or go to the bathroom before taking your temperature. Sleep with the thermometer within arm’s reach of your bed and take your temperature the minute after you wake up.
• Make sure you have had at least three to four hours of uninterrupted sleep before taking your temperature in the morning. If you were up all night it may make your reading less accurate.
• You may consider taking your temperature orally, or vaginally or rectally if you have trouble determining a pattern or change in your basal body temperature. For the most accurate results, always take your temperature using the same method.
• Plot your temperature readings on a chart. Record your daily basal body temperature and look for a pattern to emerge. You can assume ovulation has occurred when the slightly higher temperature persists for at least three days. The temperature typically increases only slightly by about 0.2ºC/0.4ºF to 0.6 ºC /1 ºF It may take a few cycles to determine when this rise occurs each month.
• Figure out your fertile window. As sperm can live in your body for up to 5 days, you are most likely to get pregnant during the 5 days before and 2 days after ovulation. If you are hoping to become pregnant, have sex during this time. If you want to avoid pregnancy, do not have unprotected sex until three to four days after ovulation.

Interpreting your chart

Your temperature may rise and fall as your cycle progresses, but you should notice temperatures are overall higher after ovulation (a biphasic pattern). With basal body temperature charting, you're looking for an overall pattern, as opposed to a temperature spike. However in general you  should notice that temperatures are on average lower in the first half of your cycle and then higher after ovulation.

Once you see higher temperatures for three days in a row it is likely that ovulation occurred on the day before the first high temperature. Some women may even notice a dip in temperature on the day of ovulation. Not every woman sees this but if you see this consistently it is likely a sign that ovulation occurs on that day.

Signs you may not be ovulating

Charting your BBT can help you to see whether you are ovulating. After several months of tracking, you may see some of the below signs which might indicate that you’re not ovulating. Talk to your doctor if you are trying to conceive and you see any of the following:

-           Irregular length cycles

-           Very long or very short cycles

-           No rise in temperature (no biphasic chart)

-           A very short luteal phase (second half of your cycle) which is less than 9 days.

Benefits and Effectiveness of the BBT method

Using the basal body temperature method is beneficial as it has no side effects and is inexpensive. Some women choose this method because they do not want to take medications or hormones for birth control. In some cases, people use the method for religious reasons.

However, the basal body temperature method may not always be effective in accurately predicting ovulation in all women, including those with irregular menstrual cycles. Also some outside factors can affect the accuracy of the BBT - drinking alcohol, being ill or having a fever, stress, medications, breastfeeding, and oversleeping or being in a different time zone so that the BBT is not taken at the same time.