Basal Body Temperature: What Factors Can Affect Your BBT?

What factors affect your basal body temperature? If I go to bed late, or drink wine or coffee, or smoke a few cigarettes, can this affect my BBT?

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Toni Weschler, MS

Toni Weschler is the author of the popular book Taking Charge of Your Fertility. By offering a simple and effective means of identifying... Read more

One of the reasons you take your temperature (basal body temperature, or BBT) first thing upon awakening is so that you won't have many variables that can influence it. This allows you to rely on it for a means of identifying your fertile phase.

If you have an occasional temperature that is artificially high due to reasons such as fever, a restless night's sleep, or alcohol consumption the night before (coffee the night before does not seem to affect temps the next morning, unless it prevents you from getting at least three hours of consecutive sleep before taking your temps), you may cover the outlying temperature with your thumb when you are interpreting your chart. Note the outlying temperature as you would any other, but then draw dotted lines between the temperatures on either side, so that it doesn't interfere with your ability to decipher your chart.

In order to get an accurate BBT reading, though, it is important that you take your temps first thing upon awakening, about the same time of day, after at least three hours of consecutive sleep and before doing anything, including talking on the phone, drinking water, getting up to urinate, or (horrors!) smoking a cigarette. Of course, I won't even go down that road, because you've already heard it a zillion times before, you should absolutely not smoke if you are trying to get pregnant, or are already pregnant.

In order to properly interpret your chart, you need to be able to draw a coverline. To draw the coverline, remember the general concept that you are always eyeing the last six temperatures at a time. The first day you see a temperature of at least .two degrees higher, than the highest of the last six days, you assume you've had a thermal shift. Then count back six temperatures, highlight them and draw the coverline one-tenth of a degree above the highest of those temperatures.

Once your temperatures have been above the coverline for at least three days, you can be virtually positive that you have ovulated. Post-ovulatory temps will usually remain high for about 12 to 16 days, with 10 days being the minimum in order for a fertilized egg to have enough time to burrow into your uterine lining.

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