Most women who plan to get pregnant want to do everything they can to ensure that they're as healthy as possible prior to conceiving. This helps to provide for a healthy pregnancy. Neural tube defects, the second most common group of debilitating birth defects, can be reduced with adequate folic acid intake. These conditions develop within the first 28 days of pregnancy -- often before a woman knows she is pregnant. As a result, folic acid should be taken as soon as you try to conceive to help prevent these defects.
When you become pregnant, your growing baby will place demands upon your body for nutrients and energy. As a result, you'll not only need a few hundred more calories each day than you require before pregnancy, you'll also need more vitamins and minerals. Because you can "stock up" on certain vitamins and minerals, ensuring that you start pregnancy in the healthiest possible state, you may wish to consider supplementing even before you conceive.
Prenatal vitamins are designed for women who are pregnant, and in some instances, for women who are planning to become pregnant. They contain many of the same vitamins and minerals found in a daily multivitamin, but contain higher levels of folic acid and iron. Your pregnant body will need iron to build new red blood cells, and your baby uses folic acid to produce the neural tube, which becomes the brain and spinal cord.
Prenatal vitamins are probably safe for adults, but that they may not be appropriate for adults who aren't pregnant or planning to become so. If you're actively trying to conceive, or planning to start trying in the near future, there's no health reason not to take prenatals -- and in fact, the vitamins will help you start your pregnancy in a healthy way.
There are two disadvantages associated with taking prenatals for any length of time before you get pregnant. The first is that they're very expensive, and the second is that it's impossible to know how long it will take you to conceive. As such, you could end up taking the vitamins for a very long time, and paying a premium in the process. Women who are trying to conceive should use, at a minimum, a folic acid supplement containing 800 to 1,000 mcg of folic acid during the entire period of time you're trying to conceive.
In the end, if you're actively trying to conceive, you should at least be taking folic acid, and it's quite safe to use a prenatal. If you're not yet trying to conceive, you can use a prenatal for a few months prior to trying, but the benefits don't warrant more than a few months of prenatal vitamins before you actually start trying to become pregnant. It's never a bad idea, however, for women of childbearing years to take a folic acid supplement on its own, just in case.