Some could be due to hereditary reasons but what we are afraid of is babies with low birth weights because they are not growing well, also know as intra uterine growth restriction/retardation (IUGR). Babies not growing well could be due to the mother having a medical problem. For example, high blood pressure or badly controlled diabetes, which can affect the growth of the baby. Sometimes, it's due to he lack of oxygen for many reasons - it could be an umbilical cord being twisted or the placenta is not healthy so less blood flow to the baby, the mother having an infection that gets transmitted to the baby. Other reasons include chronic alcoholism, chronic smoking and use of recreational drugs. These are some common reasons in addition to the baby having structural or chromosomal abnormalities.