Because the FDA doesn't approve OTC prenatal vitamins, there's no regulation over the quality/grade of ingredients.
Many pharmacy chains are striking deals with prenatal vitamin manufacturers to provide Rx vitamins free. They are FDA approved and have standards they're required to meet with the FDA for the manufacturing process as well as safety mechanisms in place while they're on the market.
I'd recommend asking your OB/GYN's office if there are any stores in your local area that offer Rx prenatal vitamins. Since you're trying to conceive, you're likely working with your OB's office anyhow, and they are the best place to ask for advice.
Good luck - each person experiences each pregnancy differently, so we can only comment from our own experiences. I never had issues with nausea, swallowing or certain foods during either of my pregnancies.