Hello, This can be a defect in the enamel while tooth was forming. It can be a deficiency or a medication or illness affecting the enamel structure that was not present when other teeth were forming. These tooth buds form diring pregnancy. These are teeth that come in first and if a baby if left with a bottle with milk or juice acids, the enamel can be weakened and destroyed. It can appear as you describe. The tooth will decay and appear dark. If left untreated, the teeth can become infected. Root canal and crown placement would be an option if enough tooth remained. Early treatment could be a bonding, but your description sounds like a full crown would be needed if possible. If teeth need to be removed, temporary removable teeth can be made. You should have your child seen by a dentist. A pedodontist specializes in dental care for children. It may be 4-5 years until the adult teeth erupt. The baby teeth are important to keep the correct room needed for the adult teeth and help keep the sequence in order. There are speech and cosmetic concerns. Options are best discussed with the dentist after an exam to diagnose your child s dental health. Other teeth may have decay no visible to you. Thank you for your inquiry. I would be glad io answer any additional questions. I hope the information provided is helpful to you and you are able to visit the dentist soon.