Are you drinking loads of water? eating lots of fruit?
Are you taking any supplements like iron that can cause constipation? Maybe stop them if you are. If you are on a mutli with lots of iron, i'd just switch to straight folate for a while. Might help.

Other than that, i'd ask the pharmacist or dr.... I'm not sure what you can take when you are preg. Coloxyl works a treat but again, i don't know the recommendation when preg.