Most of the time, doctors
don't know why babies are born early.
When they do know,
it's often because a mother has a health problem during pregnancy, such as:
Diabetes (high blood sugar)
Hypertension (high blood pressure)
Heart or kidney problems
An infection of the amniotic membranes or vaginal or urinary tracts
Other reasons why a baby may be born early include
bleeding, often due to a low-lying placenta (placenta previa) or a placenta that separates from the womb (placental abruption)
having a womb that isn't shaped normally
carrying more than one baby (twins, triplets, or more)
being underweight before pregnancy or not gaining enough weight during pregnancy
mothers who smoke, use drugs, or drink alcohol while pregnant