Folic acid is important for development during 1st 3 months, and after that the vitamin and mineral supplements found in the prenatals are important to supplement mom's nutrition more so than baby's, as baby gets everything it needs as a priority from our bodies, anyway. I.E. if we are malnourished we will suffer before baby does. If you miss taking prenatals in last months of pregnancy, your baby won't, be negatively effected, but you may, because Baby will leech nutrition from you. Take care.
Because they provide DHA and Folic Acid both very vital in the growth and development of your baby throughout your pregnancy and more so in the first few weeks. And actually... prenatal vitamins should be taken by all women who are in their child baring age and take no precautions against pregnacy giving even an unexpected fetus the healthy enviornment it needs to grow.