Allergens. Your baby might get hives in reaction to something he eats, once he's eating solids. Or he might react to something you ate if you're breastfeeding him. The most likely foods to offend are nuts, eggs, shellfish, fish, milk, strawberries, tomatoes, and certain food additives. (Some of these foods, like milk, are not suitable for babies, but your baby might still react to them in your breast milk.) Babies who have developed an allergy to cats could also break out in hives when they touch the kitty. Your baby could even get hives from something in the air, like pollen.
• Drugs. Antibiotics and some other medications might cause your baby to break out in hives.
• Insect bites and stings. If your baby's allergic to bees or fire ants, for example, he could develop hives in reaction to being stung or bitten.
• Viral infection. Your baby might get hives when he has a cold, a stomach virus, or another viral infection.
• Temperature. Sometimes cold temperatures can cause hives. The same goes for a sudden change in temperature — such as when your baby's skin is quickly warmed up after being cold.