What is neonatal thrush? Neonatal thrush, also called intrastomatitis, is a common oral disease that usually occurs within the first two weeks of life. Symptoms include pain, stinging, and drooling in the mouth, as well as small sores, usually white or yellow spots. The ulcers initially appear on the tongue, lips, and inside of the mouth and can spread throughout the mouth. Parents need to detect and treat it promptly to prevent the infection from spreading. Causes of Newborn Oral Thrush Newborn oral thrush is caused by the fact that the immune system in the developing baby\’s mouth is not strong enough and is susceptible to bacterial infection. breast milk