There are different ways of taking a baby's temperature. Of these, the rectal approach is the most accurate. It is also the most risky. Babies are prone to squirming and thrashing around, especially when they are upset. This would make even the most experienced parent nervous about sticking an instrument up the baby's bottom and keeping it in place long enough for an accurate temperature to register.
The reason why the rectal route is more accurate than other methods is because it is the most internal and insulated from the external atmosphere. The next most accurate would be under the tongue, or sublingual, but nobody in their right mind is going to attempt to hold a thermometer under a baby's tongue for the three minutes it would take to get an accurate reading. The other two sites are under the armpit, on the forehead and in the ear.
Babies, especially during their first few weeks of life, have immature immune systems and are especially vulnerable to infection. The most common cause for fever in babies is infection. A rectal temperature of 100.4 constitutes a fever and medical attention should be sought as soon as possible. In the meantime, the baby should be kept cool.
Rather than wait until it is a panic situation and make a garbled phone call to your baby's doctor in the middle of the night, it is a good idea to discuss their fever action plan at a routine visit. Discuss the pros and cons of the various types of thermometer and ask what constitutes a fever using each type. Then, go home and practice a few times a day to get the baby used to the idea and to give yourself an idea of what is your baby's normal temperature.
When the doctor's instructions have been followed to the letter and the temperature remains stubbornly elevated, there is no choice but to summon the professionals. If you have already alerted your doctor to the problem, don't be shy about phoning again to give a progress report.
The best advice anyone can give a parent of a young child is to not be afraid or feel embarrassed about phoning medical help in the event of an infant with a fever. After it's not about the parents' guilt, vanity, pride or any other emotion. It's about protecting the health of a helpless child.