Do you mean prenatal vitamins? It is a good idea for any woman of childbearing age to take prenatal vitamins. Prenatal vitamins contain folic acid, which when taken prior to pregnancy can help prevent birth defects such as spina bifida. Prenatal vitamins also help to grow super healthy skin, nails and hair.
It's not necessarily bad, but they are probably not helpful. The one thing that could be of concern is the iron in prenatal vitamins; these levels tend to be higher than regular vitamins. Too much iron is really hard on one's liver and can potentially be lethal. If you eat a well rounded diet, no vitamins are necessary.