Basal body temperature (BBT) is your core temperature after being at rest for a long period of time, such as a night of sleep. Most practitioners use the BBT to spot the temperature drop and surge that happens prior to and soon after ovulation and to evaluate the lengths of the follicular and luteal phases of the menstrual cycle. It is an excellent tool to support other ovulation detection methods such as the OPK test and menstrual cycle tracking. There are many things, extending far beyond ovulation timing, that can be told about a person’s health by looking at their basal body temperature. This can be the key to overall fertility enhancement. Here we will describe and discuss ways that you can look at your BBT and understand deeper patterns within your cycle that might be contributing to your fertility.
BBT is measured when you first awake before getting out of bed. It is measured each morning either orally or vaginally with a thermometer and the temperature is recorded, either with a paper chart or an application on a tablet or smart phone. The temperature does not have to be taken at the exact same time every day but should be taken at the moment you wake up even if you decide to stay awake in bed for some time afterward.
In an ovulatory cycle, people generally observe average, relatively low temperatures during the first part of the cycle or what is called the follicular phase, then a slight decrease before ovulation and a slight increase at ovulation leading into the luteal phase. Temperatures will stay slightly elevated in the luteal phase relative to the first part of the cycle until menstruation starts, when it will drop again, or conversely will continue to increase if pregnancy occurred. You can find a sample BBT chart at the end of the book in the resource section.
It always amazes clients when they come in with their BBT chart and we can analyze together when they traveled, when they had insomnia, when they maybe had too many glasses of wine, etc. The reason we are able to see these things in the chart is because a person's body temperature is very sensitive to all stimulation that might cause it to fluctuate out of its preferred balanced temperature, which, without outside influence, doesn't fluctuate much. This is also what makes it possible to analyze the BBT in regards to overall health.
Tracking your BBT for a few months can provide insight into more than just ovulation timing, there are deeper patterns within menstrual cycles that help create what is called your fertility map. Many apps and technological fertility tools that do not use the BBT give you an approximate time of when you're ovulating based on your cycle length. We have found, however, that just charting length is not sufficient for optimizing your fertility awareness and many people are missing their actual fertility window.