If you are planning on getting pregnant you should start them 3 months or more before you get pregnant. Some women don't know they have started trying. ;) In that case, start as soon as you know you are pregnant.
How long should you take prenatal vitamins for?
I HIGHLY recommend that you continue to take your prenatal vitamins for as long as you are breastfeeding or a year. Whichever is longer.
What should ALL pregnant women be taking?
If you are able to keep down supplements during your pregnancy I would recommend you take a high quality prenatal and an EPA/DHA or fish oil.
These supplements are HIGH QUALITY and have not only a prenatal multivitamin but also contain your EPA/DHA (fish oil) (OrthoMega) supplements.
OrthoMega is the HIGH QUALITY fish oils that I recommend. If you are talking a different prenatal and need to supplement with fish oils this is an amazing NON-fishy option!