What is intrauterine growth restriction, and what happens when this diagnosis is made? First of all, intrauterine growth restriction is usually defined as an estimated weight that is less than the tenth percentile for gestational age. It can have lots of different causes. One of the most common causes is the gestational age is just wrong. Sometimes the estimated due date is calculated incorrectly, if that can be confirmed, then sometimes we have to change the due date. Another cause of the baby measuring less than the tenth percentile is just the normal variation in size of a normal baby, that's called constitutional growth restriction or constitutionally small baby. Sometimes that happens because the parents aren't that big. Sometimes it happens because the baby is just not destined to be a great big baby, but sometimes fetal growth restriction, also called intrauterine growth restriction, can have more significant causes. One of the major categories of causes of growth restriction is congenital or genetic. For example, babies with Down's Syndrome are more likely to be small than babies who don't have Down's Syndrome. Another major category is growth restriction because the baby is not receiving enough oxygen and nutrients from the placenta. We're usually able to use ultrasound and some of the other techniques that we have at our disposal to sort out which is which, but if your doctor makes that diagnosis and suspects that growth restriction is due to abnormal fetal oxygenation, most likely your doctor will want to refer to a specialist with additional experience and training in evaluating such conditions.