There are a couple prenatals that don't have corn that I've found. The one I have right now is Vitanica Maternal Symmetry. I got it at my local health food store. It says "This product contains no: preservatives, binders, artificial colorings or flavorings, sugars, starch, lactose, salt, corn, wheat, milk,derivatives, eggs, peanuts, tree nuts, fish, or shellfish." Picked it up for just shy of $40 with 180 capsules. However, it says to take a total of 6 pills a day which I didn't realize until later. I can't spend $40/mo on vitamins which sent me looking for another.
There is one other I have come across so far, I'll put the link below. It doesn't state on their website specifically that they're corn free, but they do state that they are hypoallergenic. I haven't emailed or called to see if any of the ingredients are corn derived or not though. Hope it helps.
Their website: http://www.supernutritionusa.com/simply_one_prenatal.php
A seller that states it is corn free: http://www.vitacost.com/super-nutrition-simply-one-prenatal-triple-power-multivitamin