Can pre natal vitamins cause a stomach ulcer?

Unlikely: Prenatal vitamins to my knowledge haven't been associated with stomach ulcer formation. Having said that, many women have adverse reactions to many prenatal pills. Most commonly women complain of nausea or a bad feeling in their stomach. Pregnancy, however, can be associated with new onset or worsening heartburn. See your doctor if you're worried about an ulcer, that's a serious condition.