Fever in New Born Fever in a newborn during the first month of life should always be taken seriously. Babies younger than 28 days have developing immune systems and may not show obvious signs of infection. A temperature of 100.4°F (38°C) or higher in a newborn requires prompt medical evaluation, even if the baby otherwise appears comfortable. Understanding the possible causes, warning signs, and appropriate response can help parents act quickly when their baby needs medical a