Have you noticed any blood at all in your stools or on the toilet paper? During pregnancy you can get hemroids/varicose veins up higher in the rectum. Sometimes it has nothing to do with straining to go pooh - but it's just because of the extra weight and pressure from the baby.
Eat lots of fruits and veggies and drink plenty of water. A glass of prune juice or tomato juice per day may also help you loosen your stools.