Today I switched from my regular woman's multivitamin to Spring Valley prenatal vitamins. So far, so good. I hear it's not uncommon for prenatal vitamins to cause nausea, either due to the iron content, folic acid, or the binding in the capsule.
Folic Acid it essential prior to pregnancy to deter birth defects, yet the body has a hard time absorbing it naturally. Iron is needed due to the increase in blood supply, and calcium needed to replenish the calcium taken from the bones as the baby grows.
400mcg of folic acid, 30mg of iron, and 1,000mg of calcium are essential. I get plenty of calcium, so I am not so concerned if my multivitamin contains 100% recommended value. More often than not, it will not because of the bulkiness of the mineral. A glass of milk is 300mg of calcium.