What is a Fever?

A fever is your body’s natural response to an infection. In this post we'll dive deeper into what causes a fever, take a look at the symptoms of fever, and how to measure your temperature. We’ll also touch on other factors that can affect our body temperature and the role sex hormones play in this.

woman lying in bed measuring her temperature with a thermometer

What temperature is a fever?

The CDC considers a fever to be temperatures of 100.4 Fahrenheit or 38 Celsius and above. Fever can last for a few days and there can be a number of different bacteria and viruses that can cause fever. These include the flu and tonsillitis. Depending on the infection, a fever will often go away by itself when the body has successfully fought the infection.

What causes a fever?

When our immune system senses the body is under threat from infection, it heightens its defenses to try to protect us. One way it does this is by raising the body’s temperature. This creates an environment that makes it harder for the virus or bacteria causing the infection to survive and spread.

What are the symptoms of a fever?

A high temperature is the main symptom we associate with fever. However, you may also experience feeling too warm, cold or shivery. It’s common to feel sweaty and clammy. Children with a fever may also have flushed cheeks.

How to measure a fever?

You can measure a fever using a thermometer. There are several places you can measure your temperature on your body. Most people using a digital thermometer measure fever in the mouth, although the ear, armpit, and rectum are also common. Make sure the thermometer is clean before use and read the instructions carefully before you measure.

Fever and COVID-19

Since fever is a common symptom of COVID-19, many people are choosing to take their temperature every day to monitor for fever. If you are worried you have the virus, please follow your government’s guidelines in order to protect yourself and others. If you want to read more advice about COVID-19 you can find info from The World Health Organization here. At Natural Cycles we have also added specific COVID 19 trackers to our app these make it easy for users to document their symptoms.

What else can cause our body temperature to rise?

Besides having a fever, there are a few things that can cause our body temperature to fluctuate. You are more likely to notice a change in body temperature if you’re measuring basal body temperature (BBT), as this is done with a thermometer that shows two decimal places. You can still check for fever with a basal thermometer - in fact, you’ll get a more specific temperature reading.

Drinking more than a couple of glasses of wine can cause our body temperature to rise, as can smoking and drug use. You might notice a change in body temperature after meals, as your metabolism is linked to your temperature. A change in sleep pattern can affect our resting body temperature too, as can exercise and increased stress levels. Your temperature readings may also vary due to your sex, age or the time of day you measure. 

Did you know that women’s body temperature changes depending on where she is in her menstrual cycle? This is because there is a link between ovulation and temperature. A fluctuation in sex hormones causes our temperatures to rise after ovulation. 

Measure every day and learn your unique cycle 

Thousands of women around the world are already measuring their temperature every day with Natural Cycles using a basal body thermometer. Our app uses temperature data paired with an algorithm that learns your unique cycle and can identify where you are in your cycle and when you are fertile. That’s right, you can’t get pregnant every day of your cycle! You can use Natural Cycles to plan a pregnancy or as hormone-free birth control depending on your goal. 

Want to learn more about a hormone-free future?

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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.