Taking your baby’s temperature and fever

A true fever is particularly worrying in the first few months of a baby’s life. That’s because the infant's immune system is immature and not as effective in fighting off infections as it will be after three or four months of living outside the womb.

So it’s important to learn how to correctly take your baby’s temperature and understand what signifies a true fever.

What is considered a fever for my baby?

According to the NHS:

• In children under five, a fever (high temperature) is a temperature over 37.5C (99.5F) 
• In children five and over, a fever is a temperature of 38C (100.4F) or over

Learn what your baby’s normal temperature is by taking it a few times when he or she is well.

What causes a fever in babies?

There can be many causes for a baby’s fever. Infection is a very common one. Or the infant might be over-bundled with clothes in a relatively warm environment¬ - the rule of thumb is to dress your baby in just one more layer of clothing than you are comfortable in.

Most commonly, fever is caused by an infection. The body’s immune system senses the “foreign invader” - such as bacteria or a virus - and sends a chemical message to the brain’s temperature centre instructing it to crank up the heat inside the body. This has many advantages:

• Some bacteria and viruses don’t like the higher temperature and are then more easily destroyed by the immune system.
• The higher body temperature helps the body to ward off the infection.

In this way, fever is actually our friend: it lets us know that an infection may be brewing and it helps to fight off the infection.

When to worry about your baby’s fever

Any fever in the first months could indicate a significant infection requiring immediate attention. All children under three months old with a true fever need a medical evaluation.

But fever is only part of the story. An even more important question is: does your newborn look unwell? If your baby appears to be sick in any way - irritability, listlessness, lethargy, poor feeding, respiratory distress, rash, vomiting, diarrhoea, etc – seek medical advice, even if the temperature is not high enough to be called a fever.

How to measure your baby’s temperature

There are many technical skills to learn with your new baby,  but one of the most important is being able to take an accurate temperature.

Digital thermometers are quick to use and accurate. Always use the thermometer under the armpit with children under five following the manufacturer's instructions.

Ear (also called tympanic) thermometers are convenient and quick and accurate when used correctly.

Strip-type thermometers held on a child’s forehead are less accurate as they only give skin temperature not the body temperature.

If you have older mercury glass thermometers around the house, do not use them. They are no longer on sale because they can break with the risk of small pieces of glass and mercury poisoning.