Causes of premature birth
There are many reasons for premature delivery, but despite the research to try to predict why a pregnant woman might go into premature labor or why her membranes may rupture weeks before the due date, doctors are still unable to prevent the vest majority of preterm births.
Indeed, doctors don't know exactly what triggers labor itself, let alone what the exact mechanisms are that set this trigger off too early. One theory focuses on the role of hormones secreted by the baby, the mother, or the placenta, while another indicated that the levels of a specific proteins that is present in the vagina and cervix will increase markedly when a woman is about to go into labor.
It appears that infection plays a part in 20-40 percent of all premature births, and if you have already had a premature birth but there was not a clear medical reason for the early birth, then statistically you are more likely to have another premature birth.
However, babies sometimes have to be delivered early for medical reasons. These include preeclampsia, high blood pressure, diabetes, placental conditions, bleeding.