Preconception 101

Lesson 2: Charting Basics

Basal Body Temperature

Basal body temperature (referred to as BBT) is the temperature the body is at rest. BBT can tell a woman a lot about her fertility cycle. Charting temperatures helps a woman to know when she has ovulated, and possibly even if she is pregnant. It can also tell a woman if she has infertility.

The first two weeks the temperature in a normal cycle will range anywhere from 96.0 to 98.0 F. This is due to the hormone estrogen. After ovulation occurs, the temperature has what is called a shift upward, usually about four-tenths a degree. This is indicating that the egg has been released and usually by the time you see the temperature shift, the egg has either been fertilized or has died. Progesterone is the hormone released after the egg has moved into the fallopian tubes. The higher temps will stay this way for the length of the luteal phase, the time from ovulation to menstruation. This will vary from woman to woman. Usually it averages around 12 to 14 days. In order to effectively chart your temperatures, you need to have a good basal body thermometer.

A fever thermometer is not good to use because it won’t tell the temperature to one tenth degree like a basal body thermometer will because every tenth of temperature has significant meaning. In my experience, I think the digital ones are as good as the glass ones. It’s very important to take your temperature every morning at the same time having had at least three hours sleep prior to taking your temperature. Take your temperature before rising and before moving around very much, as these things tend to affect the true temperature. Be sure to start your chart on the first day of your cycle, the first day of actual bleeding.

A good rule of thumb to use if you wake up earlier or later than your scheduled time is to add one-tenth a degree for every half hour earlier you awaken and to subtract one-tenth a degree for every half hour later. This is because the basal body temperature will creep up as the day gets started.

There are factors that can affect your BBT, such as sleeping with your mouth open, having your feet outside the covers, having it too cold or too warm in the room, snuggling with your partner, and/or being sick. If any of these occur, note it on your chart.

You will not really know when ovulation occurs until after the fact when you see the temperature shift upward. Have a good chart handy to record the temperatures on every day. A good chart will have areas to record other fertility symptoms as well, which I plan on covering in upcoming sections.

Once you see the temperature shift - look at the last six temperatures that were taken right and draw a line one-tenth a degree above the highest of the previous six. This is called your coverline. Your temperatures should stay above this line post ovulation. If pregnant, the temperatures will stay high. The rule for pregnancy is eighteen days of high temperatures indicates pregnancy. Usually when the menstrual cycle is due the temperatures may start to drop down a day or so before the bleeding starts.

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