Any discussion of this topic needs an immediate disclaimer: the short answer is to call the doctor whenever you are worried. No list can cover every possible scenario, and most, like this one, will cover the most obvious situations. As parents, you know your child better than anyone else. Your instincts should never be ignored. If you are worried, call, regardless of how silly you feel the question is. You may end up getting a serious problem detected earlier. At the least, your concerns will be addressed and you will have learned something in the process. That stated, here are a few situations to watch out for:
Fevers are taken very seriously in young infants. If your child feels warm, is acting fussy, or seems off, take his or her temperature rectally. You should be comfortable taking a temperature this way, as it's the most accurate way to detect a fever in a young infant, and fever can sometimes be the only sign that an infection is brewing.
Any temperature above 100.4°F or 38°C is considered too high and should be explained. If your infant has been well bundled and you suspect he is overheated, remove some of the layers, wait 20 minutes, and retake the temperature, but don't ignore even the slightest fever in the first few months of life.
Refusing to Eat
Newborns need to eat every few hours: as often as every one and a half hours for breastfed babies and only slightly less often for bottlefed babies. During the first month of life, the interval between feeds should not be longer than four hours, day or night. An infant who seems hungry and tries to eat but then pulls away from the breast or bottle may be having trouble breathing or may be in pain during sucking. An infant who isn't interested in eating way beyond his or her usual interval is just as worrisome. Not only is loss of appetite is an early sign of illness, but it can lead to fatigue, weakness, and dehydration. Your pediatrician should hear about either of these situations.