Why is a retainer needed? Do teeth move after orthodontic treatment?

The objective of a retainer is to keep your child's teeth in perfect alignment after braces are removed. Usually, when braces are first removed, your teeth will all be in perfect alignment, but your gums, bones, etc. will not have completely shifted into their new positions. The retainer holds your teeth in position until your gums, bones etc. settle in to their new positions. Also, in the case of a child, they are still growing after the braces are removed. Sometimes, your child's mouth will grow unevenly. If so a retainer can be used to make sure the child's teeth stay in perfect alignment as your child grows.