Why do prenatal vitamins contain significantly less calcium than multivitamins?

They're supposed to be taken instead of multivitamins, correct? I'm not pregnant, but I thought about taking them because they have more b vitamins than regular multis. I'm on a medication, metformin, which depletes b vitamins, so I need more. I thought about prenatals instead of a b supplement since I want to cut down on how pills I'm taking. Why do prenatals contain such little calcium, though? Wouldn't pregnant women require MORE calcium?