It is good idea to call the doctor when you feel something is wrong with your child or you are feeling anxious. There are a few signs which are of special significance in the first 8 to 12 weeks.
FEVER: A fever in newborns is more worrisome than in older children. Call if your newborn has a rectal temperature greater than 100.4° F. to take the temperature, shake the rectal thermometer down until the reading is less than 97° F, apply a small amount of Vaseline® to the tip, and insert it in your baby's rectum a little less than an inch. Don't force it. Take the temperature while your baby is face down on your lap. Leave the thermometer for two and one half minutes, than read.
COUGH: Another reason to call the doctor is the presence of a cough. If a cough occurs at multiple times during the day, or especially during sleep then you should call your doctor. Sneezing is normal; a persistent cough isn't.
Although there are no absolute guarantees, some signs let you know your infant is not likely to have a serious problem. A reassuring sign is a good appetite. Children just don't eat very well when they are sick. The presence of smile is also very reassuring. However, you may not be able to use this because most babies don't smile responsively until 5 and half to 6 weeks. Before that, they may smile when their stomach is full, or in their sleep. Many babies don't begin doing this non-social smiling until around 3 weeks.