Scientists don't know what causes stuttering. Their best hypothesis is that it's caused by a combination of genetic, and environmental factors. What we do know is that stuttering is not the result of bad parenting. But there are ways in which parents can help their child through this period of stuttering.
Since scientists suspect that stuttering may be exacerbated in part by stress, one way to help a child who is stuttering is to create a calm home atmosphere. So although it may seem helpful to tell your child to slow down or start over, doing so actually adds more stress by making them feel that they're doing something wrong. Instead, provide a helpful example by speaking slowly and calmly and maintaining eye contact. And don't be afraid to talk about the stuttering with your child.
In many cases, stuttering disappears after a short while. If it doesn't, and your child's stuttering seems to be getting worse, the best bet is to seek the help of a speech-language pathologist.