OvuSense logo

Basal Body Temperature

Understanding your body’s menstrual cycle is important at any point in your life. Recognising how your cycle works and the rhythm it falls into helps you understand your body and also helps to identify health problems early if they cause a departure from this routine. If you’re trying to get pregnant, it becomes more important than ever.

You can only get pregnant in a relatively tight window of time around when you ovulate, determined by how long an egg remains fertile once ejected from the ovaries, and how long sperm can survive in a woman’s body. With a lifespan of roughly five days for sperm and between twelve and twenty four hours for the egg, that means each cycle you have a roughly six day window in which you can conceive.

Identifying when that window is means pinning down with as much precision as possible when you’re going to ovulate. If you can do that, you predict when you’re at your most fertile and give yourself the best possible chance to conceive.

One of the ways you can do this is by measuring your basal body temperature. It’s a term you’re going to encounter a lot as you learn more about fertility issues and ovulation tracking but what does it mean?

Bodies at Rest

Your basal body temperature is the lowest temperature your body reaches when it’s at rest – meaning while you’re actually asleep.

This means it can be quite challenging to measure! Measurements of your BBT are taken orally, but you can only do so when you first wake up, and even if you do it while still lying in bed without moving, that will still give a reading a little higher than your true basal body temperature.

Why is it Important?

Tracking your basal body temperature allows you to get a relatively accurate map of your menstrual cycle and specifically ovulation each month. Once you know what your average body temperature is, you can look for deviations from the average that show you when ovulation is happening!

The Signs

Your basal body temperature often (though not always) takes a small drop the day before you ovulate, and then rises in the 72 hours from ovulation. Normally, you’ll find that it’s 0.2 degrees higher for that three days than it was for the week before ovulation, making it a good indication of when you’re in that all important window to conceive.


Unlike BBT monitoring, OvuSense uses a vaginal sensor, along with an app, to monitor true fluctuations of progesterone throughout your cycle. This method has been clinically proven to give a more accurate temperature reading than for someone using BBT monitoring alone. This also accounts for those with irregular cycles by using up-to-date data to predict the current month's fertile window, unlike BBT monitoring which projects previous month's data forward. This allows OvuSense to provide and eight day fertility window prediction at the beginning of your cycle which is correct 96% of the time and 24 hours in advance of your ovulation!

Find out more about core temperature technology and fertility here