are you taking a calcium supplement? Pretty much all calcium supplements have magnesium in them to help absorb the calcium. The added benefit of magnesium is that it helps you poop :) If you are not taking one right now you could start. Look for one that says calcium/magnesium. My midwife wanted me to do liquid, but I'm not sure if it being in liquid form is that important. It always helps me. Extra calcium in pregnancy is so good for you too. It helps with leg cramps and is good for your bones and the babys.
If that doesn't work try a clorophyll supplement. Man that worked for me! I followed the dosage on the bottle and had to back off after a couple of days because it was making me go too much. It also helps with iron absorption. I took that after testing low on iron.
To lifeguard- you probably don't want to keep taking metimucil. I was told (by the receptionist at my dr.'s office before I switched to a midwife) to take it, but then my dad who is a pharmacist looked up its safety in pregnancy and kind of freaked out and made me stop taking it after 2 days. :( I'm not sure what it is about it but I trust my dad so I stopped.
I also have a friend who said that a bowl of oatmeal and a walk every morning helped her.
Oh, one more very natural suggestion that pretty much always works for me. Drink a huge glass of water in the morning before you eat any food. It sounds weird, but every morning I remember I poop right away. Sounds too simple to work, but it does.