First of all make sure you are getting enough water, drink like 8-10 glasses of pure water daily.
Do you eat fruits and veggies?
Try a tbsp of psyllium based laxative everyday. It is cheapest in bulk at the health food store. I like to mix it diluted apple juice, mix it quickly and chug it down. Follow with a glass of water.

First cut out as much processed food as possible. If the food doesn't look similar to the way it was when picked. don't eat it.
Then take Dr. Christophers Lower Bowel formula (safe during pregnancy) start out taking 2 capsules 3x/ day. If after one day you aren't having at least 2-3 bowel movements per day then increase the amount by one capsule 3x per day, EVERY DAY until you get results. Since this is a whole food formula it is safe to take as high a dosage as needed, for example taking 5 capsules 3x per day or more is fine. YOU MUST BE CONSISTENT and continue for at least 9 months until the bowels are able to work on their own (2-3 movements per day) This is an awesome product and it works. You will not become dependant on it and will eventually be able to cut down on the dosage.
ALSO, if you are taking a prenatal vitamin, it is probally making you constipated due to the high amounts of iron. Try switching to a whole food source of vitamins such as Garden of life Perfect Food or Living Multi. And if you need extra iron, Flora makes an awesome natural iron called Floravital. You'll need to go to a natural foods store to get these.