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.