1 If your baby is younger than 3 months old, contact the doctor for any fever. 2  If your baby is 3 to 6 months old and has a temperature up to 102 F (38.9 C) and seems sick or has a temperature higher than 102 F (38.9 C), contact the doctor. Mild fevers are common and usually harmless, but keep an eye on the thermometer. 2  If your baby is younger than 3 months old, contact the doctor for any fever. 3  If your baby is 3 to 6 months old and has a temperature up to 102 F (38.9 C) and seems sick or has a temperature higher than 102 F (38.9 C), contact the doctor.