Charting Your BBT (Basal Body Temperature)

Basal body temperature (BBT) is the temperature of your body at rest. By charting your temperature every day, you can see patterns in your menstrual cycle and hopefully determine when you ovulate and if you’re pregnant. Taking your temperature first thing in the morning -- before you get out of bed, eat, drink, or use the toilet -- will give you the most accurate reading. To chart your body temperature, purchase a special BBT thermometer, which records temperatures to the tenth degree and is the most precise thermometer for the job.


The Dos and Don'ts of Basal Body Temperature Charting


A woman in bed reading her temperature on a thermometer.


Do take your temperature at the same time every day. You may want to set your alarm clock to wake you up each morning at the same time for accuracy.


Do take your temperature consistently. You can take your temperature orally, rectally, or vaginally. Whichever method you choose, make sure you are consistent with the method.


Do keep a chart beside your bed. Keep your printable JustMommies fertility chart beside your bed with your thermometer and pen ready. It’s important that you take your temperature while your body is at rest. Rummaging around the house looking for your thermometer or a pencil will get your temperature rising.


Do take your temperature after at least three hours of restful sleep. Try to get a good night's sleep before taking your temperature. If for some reason you did not get a lot of sleep or you woke up frequently through the night, you should still take your temperature --just make a note on the chart of your abnormal sleep patterns.


Don't get upset if you oversleep or forget to write a temperature down. The purpose of charting is to help you determine when you ovulate and to hopefully help you have a baby! If you forget to chart one day, don’t give up on the process. Make a note of what time you took your temperature or if you skipped a day and start over again the next morning.


Don't drink, smoke, or eat anything before taking your temperature. Hopefully, you won't be smoking if you are trying to conceive. Eating or drinking can affect the temperature in your mouth if you are taking your temperature orally.


How to Detect Ovulation on Your Basal Body Temperature Chart


A fertility chart.


During the first part of a woman's menstrual cycle, known as the follicular phase, basal body temperatures will be lower than normal. Right before ovulation, there’s a chance you will have a slight drop in temperature. This will be followed by a sharp rise in temperature after you ovulate. Not all women will have a drop in temperature before ovulation--but if you do notice your temperature drop, you should start having intercourse then. By the time you notice the rise in temperature, you will have already ovulated.


Charting works best when done for a few months. What you are looking for is a rise in temperature of about 0.4 degrees or more after ovulation. If you have ovulated, your temperature will remain high for three days or more.


Determining a Biphasic or Luteal Phase and Pregnancy


You may want to draw a coverline on your chart to help you see if your chart is biphasic (has two phases). The easiest method for drawing a coverline is the “three over six” method. To draw a coverline using this method, look at the six days before you ovulate. Take the highest temperature during these six days and draw a horizontal line 1/10th of a degree higher than your highest temperature. Notice how the temperatures before ovulation are lower and the temperatures after ovulation are higher? This is typical of a normal biphasic chart.


After ovulation, your temperatures should stay above the coverline for at least 12 days. If your temperatures fall below your coverline, this may be an indication of a short luteal phase or a luteal phase defect.


Another thing many charting women look for are signs that they are pregnant. If your temperature remains high for 18 days after ovulation, this could be an indication of pregnancy. You may also see a third rise in temperature occurring about seven to 10 days after ovulation. This is called a triphasic chart. Sometimes this can be a seen with pregnancy, but it’s not a definite sign; women can have triphasic charts and not be pregnant.


If you’ve charted for several cycles and have not achieved pregnancy or are concerned about your ability to conceive, consult your doctor. Bring your charts with you as they will be helpful in determining what path to take with your fertility.