Basal temperature is a way to uncover what happens during the cycle and to find out the most fertile day for a woman with just one tool: the thermometer. Measuring basal temperature can help a woman get to know her body better and can also make it easier to become pregnant when you are trying for a baby. In addition, many women also find out they may be pregnant through temperature. But how does basal temperature work? It involves dealing with the temperature variations that the hormones from different phases of the cycle provide. Well, to better understand basal temperature, we need to know three basic principles:

  1. Follicular, ovulatory and luteal. In the follicular phase, the predominant hormones are estrogens, which stimulate the follicles and also make the endometrium thicken for a possible implantation of the zygote. In the ovulatory phase, that’s when LH kicks in; it causes the egg to be released from the follicle, thus ovulation itself occurs. And in the luteal phase, the corpus luteum formed by the follicle that housed the egg becomes a cyst that produces progesterone.

  2. Progesterone keeps the body at a higher temperature than in the other two phases of the cycle, so if the temperature is higher, we can say that ovulation has already happened.

  3. A lot of discipline and willpower is needed, since basal temperature has to be the first thing done in the morning, before getting out of bed or moving around much.

Writing down your temperature daily is mandatory, and to better understand your temperatures, there are websites specialized in charts like Fertility Friend, which is all in English, and Mama&co, which is in Portuguese. Both mark ovulation days if detected. There are also mobile apps for this purpose. The most used is Ovu View, available on Google Play and the App Store. But there are others like Period Calendar, Period Tracker, and Love Cycles or sites like Fertility Friend.

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There are two places where the temperature is most reliably measured: in the vagina and in the mouth. But some people sleep with their mouth open, in which case oral temperature measurements are unreliable. If you are not sure if you snore or sleep with your mouth open, it is best to measure vaginally. The armpit can also be used, but it is not the ideal place to get a 100% correct temperature. The best thing is to try to measure vaginally. The thermometer—for measuring basal temperature, the thermometer can be the regular type, digital or mercury, the kind you buy at a pharmacy to check children for fever. There are also special thermometers for basal temperature, but these are rarer and usually sold outside Brazil, where basal body temperature is taken more seriously by doctors and is sometimes recommended. Temperature can also be measured with ear thermometers, the kind that give results in just 2 or 3 seconds. Remember, to safely interpret the chart, you should log all your symptoms and also do the Billings method alongside. These two methods together are great for anyone trying to get pregnant.

See also: Basal Temperature