Taking a pre natal supplement is the best way to ensure that you are getting all the correct vitamins and minerals during pregnancy to support your health and the growth and development of your baby. Most pre natal supplements contain everything you need, but always do your research first to make sure they contain all your essential nutrients. 
If you have decided to start trying for a baby I would recommend taking a folic acid supplement straight away. Ideally all woman of childbearing age should supplement with 400mcg of folic acid every day regardless if they are trying for baby or not as neural tube defects occur in the first month of pregnancy - which is often before many woman even know they are pregnant.
Other vitamins and minerals to take into consideration are…..
Calcium - Calcium deficiency can occurs during pregnancy when the mother is not getting adequate amounts through diet and supplement form, when this happens the body will leech the calcium from the mothers supply to support the growth of the baby, this can lead to dangerous conditions like preeclampsia. Calcium is an important supplement for the development of the babies bones and teeth during pregnancy.
Iron - Iron deficiency anaemia can occurs during pregnancy when you don’t get enough iron through diet or supplement form. This can lead you feeling tired and weak and could also result in a low birth weight of the baby.
Vitamin D - Vitamin D deficiency can occur during pregnancy, when not getting enough naturally through sun exposure, food or supplement form. Vitamin D is important in supporting the development of the babies bones and teeth.
Iodine - Iodine is needed for the babies brain and nervous system development.
Omega-3 -Omega-3’s help with babies brain and eye development.
By taking your prenatal supplement and having regular check ups/blood tests/ urine samples done by your Doctor can make sure that you are getting all the required nutrients and not becoming deficient.