That is a great question and one that is best answered by your primary care doctor
. Unfortunately, based upon the limited history you have provided, it is difficult to know your age, gender, or other medical comordbidities to best answer your question, and so again, it is best that you see a medical professional. With regards to your question, undeveloped brains can form in utero and have several grades depending on the severity of the brain and how undeveloped it is when you are born. Some people have very minor issues with brain formation during birth and so live relatively normal lives. Others who have more severe forms of undeveloped brains may have mental retardation or cerebral palsy, and at the most severe form, may be a stillbirth. Other causes of underdevelopment of brain after birth include thyroid dysfunction, lead poisoning, and vitamin deficiencies, and so the best way to figure out what is the potential cause of the mental illness
you are concerned about is to have yourself or your child evaluated by a medical professional as they will be able to answer your questions best, after performing a thorough history and physical examination.