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Basal Body Temperature

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Basal body temperature (referred to as BBT) is the temperature the body is at rest. The BBT on fertility charts can tell a woman a lot about her fertility cycles.
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Charting temperatures on fertility charts 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 a problem, such as not ovulating – called anovulation and low progesterone and thyroid dysfunction. Low progesterone in pregnancy can cause miscarriages and preterm labor.

First of all, let me explain what the temperature is suppose to do in a normal cycle. The first two weeks or so the temperature is in the low range – anywhere from 96.0 to 98.0 – though some women will be a little higher. This is due to the hormone estrogen. After ovulation occurs the temperature has what is called a shift upward, usually about .4. 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 the .1 degree like a basal body thermometer will. In my experience, I think the digital ones are as good as the glass ones – however I have read conflicting advice on this. It’s very important to take your temperature every morning at the same time. And it’s very important that you have slept at least three hours prior to taking your temperature. Take your temperature before rising, 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 is if you wake up earlier or later than your scheduled time, is to add .1 degree for every half hour early you awaken and to subtract .1 for every half hour you awaken late. 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 weeks.

Once you see the temperature shift – look at the last six temperatures that were taken right and draw a line .1 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 18 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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