There are a number of things that can cause/contribute to constipation for pregnant women--all of them able to be successfully addressed.
1. This is usually the worst culprit--iron supplementation. While taking some iron can be beneficial/necessary during pregnancy, the form of iron is very important. Ferrous sulfate is the worst, especially if you are ingesting more than about 20mg a day. The problem is that some iron supplements are not well-absorbed, and they bind in the bowel....the iron can't get into your body, but your bowels won't let much out! Ferrous gluconate and ferrous fumarate are not as bad for the bowels, but still can cause constipation. The least constipating forms of iron are herbal and plant based formulations, taken in doses of no more than about 30mg a day. Floradix is one product---there are others. I have found that whatever form of iron is used, it can be very helpful to supplement with Yellow Dock, which can be purchased as a powdered herb capsule, or in tincture form. Some of the herbal formulas contain YD already, btw. Yellow Dock has compounds that greatly aid the absorbtion of iron, and it also tends to loosen the stool a bit and promote good muscular activity in the bowels. I have used, and have clients that have successfully used YD by itself for iron deficiency in pregnancy--tho it only has a small amt of iron in it, it is just very effective in helping iron absorption from your food.
I won't go into all the possible forms of supplemental iron available, nowadays there are so many. I would look into this, though. Excess calcium supplementation can also cause constipation, but is not so likely as iron to cause the degree of constipation as you have mentioned. Making sure to get enough vit c can aid the absorption of both iron and calcium, tho--so that is something else to consider along with the forms of iron and calcium you might take. Often, the more generic brands of prenatal vitamins have the worst forms of iron and calcium where absorption is concerned. They also tend to have far more iron than you need--and too much iron ingestion in any form is not good.
2. You need plenty of fiber from both whole grains and fresh fruit/veg. Take a look at your diet, see if maybe you just weren't getting enough of these things.
3. Water/clear fluids--make sure you are getting plenty every day, at least 1/2 gallon. If it's hard for you to remember to drink enough, then get yourself a 1liter drinking bottle, fill it in the morning and bring it everywhere you go (even from room to room when you're home. Make sure you drink it all--even by little sips at a time--by midday, then refill it and drink the rest by bedtime.
4. Exercise--really really really helps with bowel efficiency and all body system functioning. Walking for 30min a day, or most days, can help a lot...or any other form of exercise you are willing to do. Chasing toddlers doesn't count! Nor does housekeeping. Needs to be 30 min of focussed exercise, just for you. Does not have to be aerobic/exerting, but should be as brisk as you can comfortably manage, not just a lazy stroll.
5. Calories--women who are not eating enough will get constipated. Review your intake, be sure it's enough--get help figuring this out if needed. When insufficient calories are moving through, the pregnant body tends to hold onto food, trying to wring every last everloving bit of energy and nutrients from it--leading to constipation.
Constipation--or at least, bowels that are slower moving than usual--is not so uncommon in pregnancy for various reasons including hormone action that tends to relax the smooth muscle of the bowel. However, it need not--and really, should not--be as much a problem as you have described. The above things--sometimes just one of them, sometimes a couple together--have never failed to ease constipation in the women I've worked with. Hopefully, your hbmw can help you sort through these possible issues, and help you make this pregnancy far different than the last one--which should make the next birth far less painful than your last one.