There is no way of knowing which women will experience preterm labor or deliver their babies preterm. But there are factors that place a woman at higher risk for preterm labor or birth like Certain infections, such as bacterial vaginosis and trichomoniasis, Shortened cervix and previous history of preterm birth. I suggest you get a hysterogram done after you start normal menses. The timing of this special x-ray should be just before your menses so you must make sure you are not pregnant again, before the x-ray. For preventing a pregnancy, the best method is abstaining from sex in that cycle; it is also important you do not have sex and take antibiotics before and after the x-ray to prevent infection. This x-ray will give information regarding any congenital malformation in shape of the uterus and of the weak cervix .After all tests are done; you can attempt a pregnancy preferably 6 months later. However, even if no cause was found for the preterm labour this time, you must be under the strict supervision of an obstetrician - because there are slightly higher than normal chances of a repeat preterm labor. I am giving you this information so that you take adequate precautions as will be advised by your obstetrician when you are pregnant again.