Same as the ladies have mentioned above really........I have IBS and have always had severe constipation all my life, now made even worse with pregnancy and even worse with iron supplements! I ditched my pregaday iron tablets given to me by the hospital as they made me feel awful, and I bought some Spatone liquid iron supplement as it's not as strong and I am able to tolerate it. For the constipation I use lactulose and Fybogel, both avail on prescription from the doctors so you can get it for free, but also both available to buy over the pharmacy counter (in the UK). Another one you can buy off the shelf is Califig, a liquid mixture of figs and senna, it's more gentle than Senacot, my doctor said that's fine to take, but I find the lactulose/fybogel combination works best. I can take lactulose staight off the spoon, it's just really really sweet and syrupy and I have to have a few gulps of water after!
Good luck with it.