Women who have had a previous premature birth are at higher risk of premature labor, especially those with multiples. It is recommended that they be offered some form of progesterone to prevent another preterm birth. Also, previous preterm birth may cause cervical weakness,that could further be the reason for another preterm birth.
What is know is that the uterine cervix is a dynamic anatomical structure that serves during most of gestation as a barrier between the fetus and its intra-uterine environment and the vagina as the portal to the outside world. During that time it is a firm structure that predominantly consists of collagen, but in the prelude to parturition the collagen is degraded and the cervix becomes soft and pliable enough to dilate. Imperfections in the process and/or timing of cervical ripening do occur, given the occurrence of preterm labor and dystocia in labor.
Infection and inflammation are causally related to preterm labor and cervical ripening. This relates to the cervical properties, as the chance of preterm delivery is inversely related to the length of the cervical canal, which contains mucus with antibacterial properties. If the mechanical and/or antibacterial properties of the cervix are anatomically or functionally impaired, for example by intra-uterine exposure to diethylstilbestrol, or by surgery or trauma to the cervix, the remaining strength of the cervix may be insufficient to retain the pregnancy.