BBT Explained: What Is Basal Body Temperature?

You might have heard about BBT or basal body temperature, but what do these terms actually mean? And, more importantly, why do they matter? Here at Natural Cycles, we’re on a mission to break down the language of science so every woman can understand her body a little better. So what is basal body temperature? How does it change throughout the menstrual cycle? And what’s the link between body temperature and tracking fertility. Read on to find out….

Woman lying in bed with her eyes shut taking her temperature with a basal body thermometer

What does BBT mean? 

BBT stands for either basal body temperature or basal body thermometer. These two things go hand-in-hand, or rather, thermometer-in-mouth, i.e. a basal body thermometer measures basal body temperature. 

What is BBT?

So that’s what BBT means, but what is basal body temperature you might well ask? Well, It’s more precise than normal body temperature which can be measured with a regular digital or mercury thermometer. Basal body temperature is the body’s lowest resting temperature which is best measured first thing in the morning, just after you’ve woken up. You need a specific thermometer to measure basal body temperature, this shows two decimal places.

Why measure basal body temperature?

Most people are used to measuring body temperature only when they are sick. In fact, there is a key link between fertility and body temperature. This is because right after ovulation happens in the menstrual cycle, there is a noticeable rise in basal body temperature. This slight, yet significant, shift cannot be picked up by a regular fever thermometer, so women who want to track their fertility use a basal body thermometer to identify their ovulation.

Once ovulation is identified through BBT, this information can be used to work out a woman's fertile window. This info helps us understand our cycles better and with this knowledge, we can do one of two things: plan a pregnancy, or prevent a pregnancy. 

However, because every woman is different and her cycle is unique too, it’s not as simple as working out one ‘fertile window’ which applies to every woman. There are many other factors, such as period length and sperm survival, to take into account. This is why Natural Cycles, an intelligent app, is a popular birth control method for many women, and it can also be used to plan a pregnancy too. The app learns your unique fertile window and lets you know when you can and can’t get pregnant.

Normal BBT

Since every woman is different, we should avoid saying what normal BBT is, and there are conditions, such as hypothyroidism that can affect body temperature regulation. However, before she ovulates, a woman's BBT averages between 97°F (36.1°C) and 97.5°F (36.4°C). After the egg cell is released, BBT increases to 97.6°F (36.4°C) to 98.6°F (37°C). Basal body temperature will drop again if she doesn’t become pregnant and a new cycle starts when she gets her period.

What else affects basal body temperature?

It’s not just ovulation that can cause a shift in BBT. If you haven’t had enough sleep, have a hangover, or are sick/on medication, all these factors can play a part in a rise of basal body temperature. If you are using Natural Cycles to track your cycle, then you will be able to add this info in the app as an 'excluded temperature’.

Basal body temperature when pregnant

In early pregnancy, the presence of the hormone progesterone causes basal body temperature to stay high. This stops the uterine wall from shedding and helps with the implantation of the fertilized egg cell. If you’re measuring BBT in early pregnancy you might notice that the temperature stays high as it does in the days following ovulation.

Tracking BBT and finding your fertility

Understanding the link between temperature and ovulation has long been used as a fertility-awareness based natural family planning method, as it’s key to finding fertility both if you are looking to plan a pregnancy or naturally prevent pregnancy. However, manual charting of BBT is time-consuming and prone to human error. 

Natural Cycles is the first app to be FDA cleared in the US and CE marked in Europe for use as contraception. The app’s algorithm uses basal body temperature data to learn your unique cycle and can predict and confirm ovulation and identify the fertile window. Using Natural Cycles to either plan or prevent pregnancy makes BBT tracking easier than doing it by hand, and since the algorithm does the calculations for you, the risk of human error is reduced. 

Thousands of women worldwide have already made the switch to Natural Cycles and are saying goodbye to hormonal birth control once and for all. Whether you’re looking to start a family or simply want to try a hormone-free birth control option, Natural Cycles is here for you. Are you ready to take the next step on your fertility journey?

Discover Hormone-Free Birth Control

Learn more


16k ratings in the App Store

Discover Natural Cycles° today

Jen on the roof terrace at Natural Cycles headquarters.

Written By

Jennifer Gray

A writer with a passion for women’s health, Jennifer Gray has years of experience writing about various reproductive health topics including birth control, planning pregnancy, women’s anatomy, and so much more.

Jack in a suit and tie holding a microphone and giving a presentation.

Scientifically Reviewed

Jack Pearson

With 10 years of experience working in the field of fertility, Jack Pearson is Natural Cycles’ in-house expert. As Medical Affairs Manager, he dedicates his time to conducting groundbreaking research and educating healthcare professionals.

Keep Reading: Latest Articles