Summarize this article in one sentence.
Exercising helps build up your core muscles. That, in turn, can help with your constipation. You don't want to suddenly jump into an exercise routine you're not used to, especially if you weren't exercising much before your pregnancy. Start with just fifteen minutes of exercise three times a week. Over time, you can gradually increase the amount. Swimming is a good way to begin exercising, partially because your increased weight won't be an issue in the water. Ask the instructors at your local pool if they offer any prenatal swimming classes. Talking a walk every day can help keep things moving. Plus, it can help you keep from gaining too much baby weight. Of course, you're probably not going to try to play football while pregnant. However, you also shouldn't do sports like judo, kickboxing or trampolining whilst pregnant due to the risk of injury.

Summary:
Understand why it helps. Start slowly. Try daily walks. Skip contact sports.