Urgh I had this big time with my second daughter. To the point I had stabbing pains in my stomach and I seem to be heading the same way this time too.
My midwife told me to drink a lot of water, leafy greens, high fibre food like whole wheat pasta/bread/rice and cereals, raisins, prunes, fruit and laxatives (there's a special type for pregnant women, ask your pharmacist). It's so horrible when you're pregnant and can't get a quick fix lol.