When I had this same question (whether prescription or OTC vitamins
were the same), I took the bottle of OTC
pills and the bottle of prescription ones to my ob-gyn and asked her. She just flipped the bottle over and compared the number of international units of each vitamin, and said "they look comparable to me." In other words, all she was concerned with was the amount of the various vitamins in the prenatal formula. She didn't act like the vitamin's brand was the issue.