When you are looking at prenatal development you are looking at two factors. First the genetic factors, these are the factors that are predetermined by the genetic material brought by the mother's egg and the father's sperm in the form of DNA. Genetics can determine if the development of the fetus occurs normally or if it does not. Some common examples of when the fetus does not develop normally (when defective genes are present on the part of the mother or the father) are Downs Syndrome and Sickle Cell Anemia.
It is also possible for the environment within the womb to have an effect on prenatal development. This can include tertogen's introduced into the womb and hormones that can effect the development of the fetus. A teratogen is a chemical that can negatively effect the physical development of a fetus. Some common examples of teratogens include alcohol or nicotine.
These are physical effects that can effect psychological development after the infant is born as these factors can effect the child's ability to learn and process information from thier environment.