In addition, ELBW infants are more likely to have immature digestive systems and are at risk for digestive tract problems such as necrotizing enterocolitis (NEC). NEC is a disease characterized by infection and decreased blood flow to the intestines. NEC is a major cause of devastating illness and death in this vulnerable preterm population. Leftover blood samples, urine, stool, and breast milk samples will be collected, frozen, and stored. In the event a baby develops a digestive tract complication they will be studied in a laboratory to identify markers that indicate a healthy digestive tract versus illness.