Urinary-tract infections (UTIs). These can occur when your child's not urinating regularly enough, or because of poor hygiene (usually in girls -- make sure you stress wiping from front to back from the start!). Prevent UTIs by making sure that your child gets plenty of fluids and doing the wiping yourself (or having a trusted grownup do it) for the first few months.
Constipation. Sometimes children will hold in bowel movements rather than go in the potty. When they do, the stool gets drier and harder, and more difficult to squeeze out. This makes pooping painful, which can make the child even more hesitant to do it. To avoid this, make sure your child's diet includes plenty of fiber. If he's really holding stool in for a long time, it may be a sign that you should let him poop in a diaper for now.