i've tried a ton of prescription brands of prenatals, and they've all caused me to be constipated. now, i take prenatals without iron (from GNC), and then take an iron supplement called ironchel (also from GNC) which is really easy on the system and doesn't cause constipation.
also, my one proven cure for constipation is a bowl of quaker oat squares or shredded wheat (any flavor.... strawberry is my favorite), DRY, eaten with a big glass of ice water. eat it once a day. it's never failed me!