Basal body temperature (BBT) is the lowest temperature that your body reaches during each day. This “resting” temperature is usually reached during deep sleep and then increases as you wake up.
BBT is the oldest method of fertility monitoring and was first used in the 1930s. It is a well-established and trusted method of ovulation detection.
Women’s BBT patterns are biphasic i.e. they consist of two different phases during the menstrual cycle. The temperatures taken in the phase before ovulation are lower than those taken in the phase after ovulation. The average temperature prior to ovulation is around 0.2ºC lower than after ovulation, although this varies from woman to woman and cycle to cycle. This small increase in BBT will occur within 48 hours of ovulation and will remain elevated until your next period.
During the first phase of your cycle, high levels of estrogen in your body keep temperatures low. Your raised BBT after ovulation is due to the level of progesterone in your body. Progesterone is secreted by the corpus luteum (the empty egg sac that the egg emerges from) and indicates that you have ovulated. This is why measuring BBT is a reliable method to confirm that ovulation has occurred.
Traditional methods of recording your BBT involve taking an oral, rectal or vaginal measurement at the same time each morning. These measurements must be taken as soon as you wake, as even the act of waking or getting out of bed will increase your temperature, leading to inaccuracies in your data. As a result, these methods tend to generate “noisy” data, which can be confusing when trying to identify such a small change in temperature. Most of these methods use the 3 over 6 rule – ovulation can be confirmed if you see 3 high temperatures following 6 low temperatures. However, confirmation that you have ovulated 3 days after the event is too late to take action, as the egg only survives for around 1 day!
Although BBT is a trusted method of ovulation detection, it can only confirm ovulation after the event. DuoFertility uses your BBT combined with other factors such as cervical mucus, and mood, all of which are related to ovulation. Together with the guidance from our Fertility experts, this allows us to more accurately predict when you are ovulating up to 10 days in advance, giving you plenty of time to plan for the best time to try to conceive. DuoFertility is the only assisted fertility wearable to incorporate BBT and other factors with Fertility expert analysis to help you conceive naturally.