Problems with the uterus can cause premature birth.
These include : malformation of the uterus, distension (especially for multiple pregnancies), opening of the uterus, detachment of the placenta and placenta previa (where the baby is right next to the opening of the uterus and this causes the placenta to detach itself).
Infectious diseases such as urinary infections, diabetes, high blood pressure caused by pregnancy and the bacterial disease listeriosis can also cause premature birth. This is why proper and regular check-ups are so important.
If you have the slightest concern, go and see your doctor. Don't worry about wasting your doctor's time: better to re-assure yourself than to end up with complications.