Which thermometer for a baby: the answer in three lines
Before the age of two, the standard thermometer is an electronic thermometer with a flexible tip, used rectally. This is the position of the French National Authority for Health and the French National Health Insurance Fund. Forehead and ear thermometers, which are faster and more comfortable, are not reliable enough for infants and only become acceptable after the age of two, as a supplementary option. So if you are wondering which thermometer to buy for your baby before birth, the answer is one device costing less than ten euros.
The rest of this article explains why, compares the five types of thermometers, details the corrections to apply depending on the measurement site, and reminds you what should lead you to seek medical advice.
Why reliability matters more than comfort
Fever is defined as a core temperature above 38 °C. In an infant under three months old, this figure alone warrants immediate medical attention, whatever their apparent condition. A measurement off by a few tenths of a degree can therefore change the decision: a thermometer displaying 37.7 °C when the rectal temperature is 38.2 °C could mean missing a necessary consultation. Conversely, an overstated measurement may send a family to the emergency department unnecessarily.
This is why recommendations favor the measurement site closest to core temperature: the rectal route. It is essential for children who are not yet able to keep a thermometer under their tongue or hold their arm tightly against their body.
The five types of thermometers reviewed
Electronic thermometer with a flexible tip
This is the standard option from birth to five years old when used rectally. It measures in thirty seconds to one minute, beeps when finished, and its flexible tip limits the risk of injury. It can also be used under the armpit, with a correction, and in the mouth after the age of five. Typical price: 5 to 15 euros. Choose a model with a genuinely flexible tip, waterproof for cleaning, and a battery that is easy to replace.
Infrared ear thermometer
It measures the eardrum's radiation in one second. In infants, the ear canal is narrow and often blocked with earwax: the probe does not correctly target the eardrum, making the measurement unreliable. The French National Health Insurance Fund considers it unreliable before the age of two, and the pediatric emergency department at Trousseau Hospital advises against using it in children. After the age of two, it is an acceptable alternative for a quick check, to be confirmed rectally if in doubt.
Forehead or temporal thermometer, with or without contact
Very widespread since 2020, it is appealing because of its speed and non-contact operation. Its reading depends on perspiration, ambient air, a lock of hair, or a hat removed shortly beforehand. The French National Health Insurance Fund indicates that temporal readings are approximately 0.2 °C lower than rectal readings, while forehead readings are highly inaccurate. It is suitable neither as an infant's first thermometer nor for deciding whether to seek medical attention.
Gallium thermometer
It replaced the mercury thermometer, which has been banned from sale in France since the late 1990s because of its toxicity. Made of glass and battery-free, it is used under the armpit or in the mouth, never rectally in a baby, as the glass could break. The measurement takes several minutes. It remains reliable but is not very practical with a moving child.
Liquid-crystal strips and pacifier thermometers
Forehead strips are formally discouraged by the French National Health Insurance Fund because of their lack of reliability; Trousseau Hospital cites a margin of error of 50%. Pacifier thermometers take an approximate oral temperature from a child who must suck for several minutes without letting go. They are gadgets, belonging in the same category as those described in our guide to essential baby products and gadgets.
Which measurement site at which age?
- From birth to 2 years: rectal measurement as the first choice. As a second choice, use the armpit, confirming with a rectal measurement if the result is close to the threshold.
- From 2 to 5 years: rectal measurement as the first choice, ear measurement as the second, and armpit measurement as the third.
- After 5 years: oral measurement, followed by ear measurement, then armpit measurement.
Fever thresholds change depending on the measurement site. With rectal and ear measurements, fever is considered to be above 38 °C. With oral and armpit measurements, it is above 37.5 °C. Under the armpit, a widespread practice is to add 0.5 °C to the reading to estimate core temperature; this is what the Trousseau emergency department indicates. The French National Health Insurance Fund prefers using a lower threshold rather than applying a correction. In either case, an armpit reading close to 37.5 °C should be checked rectally.
How can you take a baby's rectal temperature without causing injury?
Lay the child on their back with their legs bent toward their stomach, or on their side. Apply a little petroleum jelly or cream to the tip, then insert it no more than two centimeters, without forcing it, holding the thermometer between two fingers. Wait for the beep. Remove it, read the result, and clean it with soapy water or alcohol. The French National Health Insurance Fund recalls a frequently overlooked point: rectal measurement carries a risk of ulceration if the procedure is rough or repeated too often. Do not take the temperature every hour, and never use the thermometer to stimulate a bowel movement.
Wash your hands before and after. Reserve one thermometer for rectal use, and a second one, or the same one after thorough disinfection, for other measurement sites.
Medical thermometer and bath thermometer: two different objects
A bath thermometer measures water between 30 and 40 °C, with an accuracy of half a degree, which is sufficient for aiming for 37 °C. A medical thermometer measures the body to the nearest tenth of a degree. The first never replaces the second, and vice versa: a medical thermometer immersed in bath water is not designed for this use and may be damaged. Our article on bath thermometers explains what distinguishes reliable models. Some bathtubs include temperature measurement directly: our foldable freestanding bathtub continuously displays the water temperature, so you do not need to buy an additional accessory.
How many thermometers do you really need?
One electronic thermometer with a flexible tip is enough for an infant. A second device, ear or temporal, may be useful after the age of two for quick nighttime checks, provided you never rely on it to decide whether to seek medical attention. If you are preparing your maternity bag, the electronic thermometer belongs in it; the others can wait.
Check the battery twice a year and replace the device if it displays inconsistent values from one measurement to the next. A simple test: two rectal measurements taken five minutes apart should not differ by more than 0.2 °C.
When a reading should lead you to seek medical attention
The French National Authority for Health recommends immediate medical attention in the event of fever in an infant under three months old. At any age, seek medical attention without delay in the event of breathing difficulties, unusual drowsiness, or a bulging fontanelle. The same applies in the event of paleness or mottled skin, weak crying or whimpering, or a stiff neck. In the absence of these signs, fever can be managed at home: offer drinks frequently, do not overdress the child, and do not overheat the room. Use only one medication, paracetamol, while respecting the weight-based dose. Cool baths and wet wraps are no longer recommended.
Two common causes of fever in infants are covered in dedicated articles: bronchiolitis and ear infections.
In summary
- Before the age of two, only one thermometer matters: electronic, flexible tip, rectal use.
- Ear and forehead thermometers are fast but inaccurate in infants; they are acceptable after the age of two for checking purposes.
- Forehead strips and pacifier thermometers should be avoided.
- Fever is above 38 °C rectally; before the age of three months, seek medical attention without delay.
- A bath thermometer is a different object that measures only water.
Sources
- French National Health Insurance Fund, ameli.fr, “How to take a temperature correctly,” updated September 10, 2026.
- French National Authority for Health, “Management of fever in children,” quick-reference sheet, October 2016.
- Pediatric Emergency Department of Armand-Trousseau Hospital (AP-HP), “Measuring temperature,” childcare information sheet, accessed September 15, 2026.
