Ideally, you should switch your prenatal vitamins about 6 months before trying to conceive. This will ensure that your body has the necessary nutrients it needs for a healthy pregnancy. Additionally, many prenatals contain folic acid which is essential for preventing birth defects and neural tube defects in developing babies so starting them early can help give baby the best start possible. Finally, consider speaking with your doctor or a nutritionist to determine if any other supplements might be beneficial during this time as well.