Basal body temperature (referred to as BBT) is the temperature of the body when it is at rest. The BBT on fertility charts can tell a woman much more than she realizes about her fertility cycles. Charting temperatures on fertility charts helps a woman to know when she has ovulated, for some – the time of conception and possibly even if she is pregnant.
A woman’s basal body temperature can even call attention to problems such as not ovulating – called anovulation. Other alerts by BBT are low progesterone and thyroid dysfunction.
Low progesterone in pregnancy is a major cause of miscarriages and one of the main factors of preterm labor.
So let’s take a stab at explaining what the temperature is supposed to do in a normal cycle:
- The first two weeks or so the temperature should be in the low range – anywhere from 96.0 to 98.0 – though some women will be a little higher. Even 96 is on the low end. This is due to the hormone estrogen. Estrogen is a “cooling hormone” where progesterone would be considered a “warming hormone”.
- Once ovulation occurs, the temperature has what is called a shift and in this case, it moves upward, usually about .4 degrees. This shift indicates 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 in a sense, leaving other hormones behind.
- Progesterone is the hormone released after the egg has moved into the fallopian tubes and the first temperature shift will happen whether or not pregnancy occurs. These higher temps will stay this way until near the end of the luteal phase – the time from ovulation to menstruation. This will vary from woman to woman. A healthy luteal phase will average around 12 to 14 days.
In order to effectively chart your temperatures, you need to have a good basal body thermometer. Any household fever thermometer won’t work well because it won’t tell the temperature to the .1 degree as 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. Some will say that only an analog thermometer will do best.
4 Tips For Taking Your BBT
- 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. Basically, keep the digital basal thermometer on the nightstand so you can roll over and take it in hand without much movement. It is amazing how small movements can affect temperature.
- Be sure to start your chart on the first day of your cycle – the first day of actual bleeding. Day one of the cycle is the first day of bleeding. A cycle is from first day to first day again next month.
What if I wake up early when taking my BBT?
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.
What are other factors other than movement that affect BBT
Other factors that can affect your BBT include 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 every day. A good chart will have areas to record other fertility symptoms as well, which we cover in other articles on Beyond Fertility.
What is a Coverline and how is it used?
OK so pay close attention. Once you see the temperature shift – count back and look at the last six temperatures, that were taken correctly, 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 coverline following ovulation. Here is where your coverline is so important. If pregnant, the temperatures will stay high. You won’t know what is considered high for you, if you have not been tracking your norms and your changes.
The rule of thumb is 18 days of high temperatures indicates pregnancy. In most cases, when the menstrual cycle is due, temperatures will drop down about a day or so before the bleeding starts.
Basal Body Temperature (BBT) has been considered the most common, non-invasive, ways to track fertility but there are other methods such as charting cervical changes, saliva fertility, and urine testing. Read more about these subjects here in our Beyond Fertility Blog and purchase fertility tracking gadgets such as a BBT Thermometer in Beyond Fertility Shop.