Studies have shown no clear and overwhelming benefit to supplementing all women in pregnancy with vitamin C, vitamin E, magnesium, calcium, and iron. Certainly, in some cases of malnutrition or an absorption disorder a woman would greatly benefit from something she is lacking, but this should be done with her health care provider on a case-by-case basis.
It seems the message most American women get is that taking a prenatal vitamin will ensure they have everything they need for a healthy baby. They are often instructed to "watch their weight", but not given much education about what is truly unhealthy to eat. Often, diet is given little attention until a problem has already surfaced and there is little time to correct the issue. Ideally, women who are planning on getting pregnant will be eating a very healthy, varied diet BEFORE they conceive. If a prenatal vitamin was a cure-all, our pregnant women and babies would be a whole lot healthier.
Then there's the point that too much of any one thing is usually not a good idea unless there's a clear reason. Our bodies try to keep all chemicals within ranges. For example, too much iron can be very constipating, too much calcium can lead to heart disease, and too much potassium can result in stomach upset.
There's also quite a difference in the quality of prenatal vitamin. The cheapest ones probably aren't doing much of anything for you. In order to make them cheaply, the nutrients are in a synthetic form that is not in a form readily absorbable by your body. The highest-quality ones are food-based, so your body actually has the correct enzymes and knows what to do with it. Think about how much dehydrated vegetables there are in a few prenatal vitamins. Couldn't you eat more than that every day?
The point of this is not to tell you not to take prenatal vitamins (I take them), but rather to remind women that diet is the first and best line of defense in a healthy pregnancy and life. I'm afraid some women are taking their vitamins and then eating junk food all day. Prenatal vitamins are there mostly as a back-up for those who are not getting proper nutrition either because of a poor diet or genetic metabolism problems, but does not lead to as equally good outcomes as great nutrition does. Ask yourself: how's my diet?
To read more about individual supplements from previous posts: