Doctors or nurses prick your baby’s heel to take a tiny sample of blood and then use this blood to test for many diseases. According to the U.S. Department of Health and Human Services, all 50 states require testing for at least two disorders: phenylketonuria and congenital hypothyroidism. But many states test for up to 30 different diseases: All of these problems are impossible to spot without a blood test. And if left untreated they can cause mental retardation and even death. The March of Dimes recommends that all newborns be tested for at least 29 diseases.