Basal body temperature (BBT) is the temperature of your body at rest. You could say it is the temperature of the body when you wake up, but before you get off the bed.

BBT charting refers to your recording the temperature of your body each day when you wake up on a chart, to see the changes in the temperature over the days.

BBT charting is a useful tool because of its ability to confirm ovulation. Not all women ovulate 14 days before their next period. For many women, the time between ovulation and their period (the luteal phase) is shorter. A woman needs to have at least 10 days in this luteal phase in order to allow enough time for the fertilized egg to reach the uterus and get implanted.

The goal of BBT charting is to find out if you are ovulating and help you time intercourse. If you see a chart with two clearly defined phases (biphasic chart), that’s a good sign. (A biphasic chart is one which clearly shows two periods of normal and higher temperatures.)

Take your temperature first thing in the morning before you get out of bed. If you use a glass thermometer, make sure you shake it down before going to bed.

Try to take the temperature at as close to the same time each day as possible.

It is best to take your BBT after a minimum of 5 hours sleep, and at least 3 hours at a stretch.

You can take your temperature orally, vaginally, or rectally, but with the same method for the entire cycle. We recommend that you take your temperature vaginally only for the charting purpose.

You should try to place the thermometer the same way each day (same location of your mouth, same depth vaginally and rectally).

Plot your temperature on your chart each day, for the entire menstrual cycle.

Some women are known to have a temperature drop when they ovulate. Your chart will tell you this. If you see this drop, it is a good idea to have sex in case you are ovulating.

A temperature shift of at least 0.4 degrees over a 48-hour period indicates ovulation.

After you see a temperature shift for at least three days, or at the end of your cycle, you can draw a line to identify your follicular phase and luteal phase temperatures. You should be able to see a clear shift and draw your line between the highest follicular phase BBT and the lowest luteal phase BBT.

Chart for a few months to understand the pattern.

If your temperature stays up for 18 days or more after ovulation, you should test for pregnancy.

If your chart shows very irregular ups and downs in BBT, what does that mean?

1. Most likely, you are not taking your BBTs consistently at the same time or in the same way every day. Perhaps you sleep erratically.

2. You are taking your BBTs orally and you sleep with your mouth open.

3. You are not ovulating.

If being more consistent, or switching to taking your BBTs vaginally or rectally doesn’t help, you should go to the doctor to see if something may be causing you not to ovulate, like your hormone levels.

Sometimes two eggs are released from the ovary with a gap of 10 days. But keeping track of such irregular ovulation will require you to go for more sophisticated methods of conception planning like frequent ultrasound scans while our interest here is low tech, low cost do at home methods.