Cervical function
Gradual softening and effacement of the cervix occur in the weeks before labour. Cervical ripening involves a breakdown of collagen, changes in proteoglycan concentration, and an increase in water content that occur in response to increased local prostaglandin release or partial antagonism to progesterone receptors (i.e.
action of mifepristone) (29).The role of the cervix in maintaining pregnancy remains undefined and is probably multifactorial, with two key roles: (a) prevention of ascending infection and (b) physical support to keep the pregnancy in utero. Maintenance of a healthy mucus plug and adequate length to the cervix may act to prevent ascending infection that triggers production of local inflammatory cytokines and prostaglandin release.
The quality of strength of the cervix to support the pregnancy in situ against gravitational pressure is also required to prevent premature cervical dilatation. Recognized cervical weakness, called cervical insufficiency, causes women to suffer recurrent mid-trimester loss usually with a history of painless dilatation of the cervix. On transvaginal screening of cervical length (CL), funnelling is a feature associated with cervical weakness, and the membranes can be seen prolapsing through the endocervical canal of the cervix. Contributing to the argument for the function of the cervix being related to its length, women who have excisional cervical surgery for cervical intraepithelial neoplasia (CIN) or cervical cancer have an increased risk of sPTB. In the treatment of CIN, the volume/ length excised has been shown to correlate with the pregnancy duration (30). However, women with CIN have higher baseline risk for sPTB, suggesting cervical function is not be entirely related to length. (30)