A preterm birth is a birth that takes place more than three weeks before the baby is due. In other words, a premature birth is one that occurs before the start of the 37th week of pregnancy. Normally, a pregnancy usually lasts about 40 weeks. Premature birth gives the baby less time to develop in the womb. Premature babies, especially those born earliest, often have complicated medical problems. Developing baby goes through the major growth in the final stages of pregnancy. If the baby is born earlier, there will be a higher risk of disability.