Genetic & biological. Down syndrome, fragile x, and fetal alcohol spectrum disorders together, about a third. A cause is found in 75% of cases; of those, a genetic cause of is found in over 66%. Infections, substance abuse and other pregnancy problems, about 10%. Bleeding, pre-eclampsia, placental damage and labor/delivery problems, < 10%. 5% after birth (e.G, traumatic brain injury, meningitis).