BBT Chart: How do I Take My Basal Temperature

10. Question: What’s involved in BBT Charts? And how do I take a “basal” temperature reading?

Answer: Your BBT Chart is the anchor of your ovulation calendar, the archimedean point for knowing the best time to get pregnant. That’s because, by bbt charting you are doing two smart things: First, you are confirming that your cycle is healthy and that you are indeed ovulating each month. Secondly, by knowing when you ovulate each cycle, you can accurately determine when you will likely “O” in the future. As a corollary of these two functions, if you have cycle irregularities, hormonal imbalances, or potential infertility obstacles, a bbt charting record may alert you to certain issues and can certainly help your doctor gain better insight into possible diagnoses or treatments (including the use of fertility drugs or supplements).The purpose: The whole point of bbt charting is to measure an increase in body temperature that takes place about 48 hours following the moment you ovulate. Once the egg is expelled from the follicle/corpus luteum, the luteal body that once hosted the egg starts to manufacture an important reproductive hormone called progesterone. One of the effects of this surge in progesterone is the “heating up” of your entire organism – and this thermal shift is quite measurable using a basal thermometer. In most cases, women will detect a temperature increase of typically between 0.4° F and 1.0° F, though it could be a bit more. This rise is indicative that ovulation has just taken place as progesterone will not be produced in volume until the ovum leaves the ovarian follicle/corpus luteum.

How do take my BBTs? The only part that’s a drag about taking your bbts is that you have to do it the very first thing in the morning. And by the very first thing, I mean the very first thing – no moving around, no peeing, no drinking water, no getting out of bed. Your special basal thermometer should be next to you on your nightstand, ready and waiting. Basal refers to the word base, or baseline, so this means your base temperature before getting up, a kind of standardized thermal benchmark, if you will. Also, you need to be coming off a fairly decent night’s sleep – at least four hours worth of solid shut-eye. In a sense, your body starts the day “calibrated” for a bbt measurement. Ideally, try to take your bbts at the same time every morning. The minute you wake up, pop the thermometer in your mouth and take a reading. Don’t drink, eat, pee, brush teeth – or anything – until you have established your day’s bbt result.

You’ll need a special bbt thermometer that displays changes of 1/100 degree. We have these available here at Once you start with one thermometer, do not use another! Its the relative accuracy of one thermometer that tells you what you need to know. Absolute accuracy, while fine and dandy, is less relevant than relative accuracy – because its the relative changes you monitor throughout your cycle that tell you when your thermal shift takes place.

The biphasic bbt chart: A biphasic bbt chart represents the before and after of ovulation, so to speak. Imagine a simple graph that represents each day of a hypothetical 30 Day menstrual cycle. Cycle day 1 is designated as the start of your period, the first day of menstrual bleeding. Given a 30-day cycle, the first 15 days of your bbt chart will display a flat line composed of lower temperatures (relatively speaking). This is the cool, estrogen-dominant phase of your menstrual cycle. Once you ovulate (let’s say on cycle day 15), you should see a stark and definitive temperature increase – a spike in the line – with you bbt temps shooting upwards like stock-market numbers during the dot-com boom. The temperature spike should last just a few days and then your line will flatten out up there at a higher temperature for the rest of your cycle. That’s because the corpus luteum will continue to manufacture progesterone during the hot phase of this cycle, the 15 days after you O. If you do not achieve a pregnancy, you can expect to see your hot phase temps cool down around the time of your period. Once you experience menstrual bleeding, you’ll know that this particular cycle is over and its time for a new cycle day one!

Important: The BBT thermal shift will be measurable about a day after you ovulate. In effect, your thermal shift does not predict when you ovulate, it only confirms the fact after the fact. So, your temperature increase will come a day or so after ovulation. Your bbt chart, then, is not an ovulation prediction device in the formal sense of the word; what it does provide however is a more general, wide-angle perspective on your cycle and menstrual/fertility health.

BBT Charts and Pregnancy: In the case of pregnancy, the hormone hCG will tell the corpus luteum to keep pumping out the progesterone. In the case of pregnancy, it is theoretically possible to detect that you have conceived if your bbt temps stay high or do not fall when you typically expect them to. Following implantation, for example, the bbt line may dip slightly and then rise again. This could indicate you are pregnant. That said, there are many variables when it comes to bbt charting and many vagaries and vicissitudes as to when and how fast your bbt temps drop, so many TTC experts do not promote using your bbt chart as a pregnancy test. Better to pee on a stick.

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