I agree with every one above. You should use pre natals as soon as you start trying to Conceive and should call the doctor as soon as you get a positive preg test.
If you are TTC you should start them right away. As soon as you get the positive test call your dr to at least get the prenatals.
I believe 3-6 months before you plan on conceiving is the best time to start prenatal vitamins and continue after pregnancy if you are going to breastfeed.
Ideally you would be using prenatal vitamins before you concieve as this helps provide the right nutriens for you and your body before you concieve,however its not important if you didnt,you can start taking them at any time.so start now if your pregnant or trying.and yes make an appointment with your doctor.depends how your doctors do things.here in the uk,as soon as we find out we make an appointment.