Premature birth is the result of the precipitous ending of pregnancy before 37 weeks. The causes are various, some dependent on the mother (previous abortions, muscular insufficiency of the upper uterine cervix, inflammatory processes of the sexual organs), and on with the foetus (improper foetal position, anomalies of placental attachment). The smaller a baby is at birth, the less are its chances of survival (especially if the necessary medical attention is unavailable), the more specialized are its nutritional requirements (need for more calories, poorer fat absorption, more prone to iron-deficiency anaemia), and the more it is prone to mental deficiency. Every week a baby is born prior to 39 weeks increases the likelihood of the need for breathing support and admission to newborn intensive care. Babies born at 32 weeks to 33 weeks are about six times more likely to die within their first year than full-term babies (40 weeks); those born at 34 weeks to 36 weeks have nearly three times the risk. Additionally, those who survive have a much greater risk of sudden infant death syndrome (SIDS) and lifelong medical complications such as cerebral palsy, visual and hearing problems and mental retardation.
Prematurity is a leading cause of infant death in the USA and is directly responsible for its relatively high infant mortality rate. In 2000, more than 450,000 US babies were born prematurely (defined as before the 37th week). In the 1980's, approximately 2.5 to 6% of all American births were premature. In the 1990's the rate rose to 10%. overall a 23 percent increase since the early 1980s.
Noisy, tiring jobs where the mothers have to stand for 4 to 6 hours a day increase the premature birth rate by 80%; standing for more than 6 hours triples the risk. Lifting, carrying and other types of physical exertion are associated with a rise of 40%, and loud noise with a rise of greater than 50%.
Bacterial vaginosis in the mother increases the risk of premature delivery by 6%, and increases the risk of low birth weight in the baby by 40%. 16,000 premature American births a year could instead occur at full term if this infection were cured. Smoking, having had an earlier miscarriage or a previous premature baby, or being black are other factors that are associated with premature delivery.
Although there have been major improvements in treatments for premature babies, doctors simply don't know why many babies are born prematurely. and have not been able to stem a rising tide of premature births. There are a number of potential causes but no clear, single biological reason for the increase.