During pregnancy I highly recommend eating lots of fruits and drinking adequate amounts of non-carbonated liquids like water and natural non-sweetened juices. I agree with the above recommendation that Watermelon is excellent in preventing constipation.
But if it gets to the point where you need assistance moving your bowels, a time tested safe and natural way to relieve constipation during pregnancy is a gentle old-fashioned cleansing enema. Not the small disposable chemical squeeze enemas they sell in drugstores and supermarkets, but an enema using warm water from an enema bag.
Try to take at least a quart, but better, 3 pints of warm water (103 degrees F at preparation time). I would not recommend letting yourself go more than 48 hrs without a BM, but preferably once a day.
If you have never had an enema before or would like more information, please follow up here or email me. An enema properly administered can be practically without discomfort if taken before you are too constipated. Enemas are only uncomfortable when you are severely constipated, or if you hang the bag too high and let the water in too fast, or if the water temperature is too hot or too cold.