The definition of a 'premature' or 'preterm' baby is one that is born before 37 weeks. There are different levels of prematurity and these carry their own risks. Very premature babies, born before week 26, are at most risk and are sometimes known as micro preemies. A baby born at 37 weeks or more is known as a 'term' baby. Generally the earlier your baby is born the higher the risk of health problems.