Assessing the cervix

The cervix remains closed because of its rigidity due to the collagen fibres that make up the bulk of cervical stroma. Cervical connective tissue consists mainly of collagen and a matrix of large proteoglycan molecules.

Cervical changes are due to:

A uniform means of assessing the cervix is the Bishop’s score (see Table 38.1). The Bishop’s score assesses the favourability of the cervix for induction of labour by assigning points to each of the five cervical features and adding these points. With a low score (0–3), there is a high risk of a failed induction, resulting in caesarean section (>20%), compared with a score of 8 or more, where the failed induction rate is <3%. With a high score, the cervix is said to be ‘ripe’.

Mechanism of labour

The exact mechanism of labour is still unknown, although it is thought that the production of fetal cortisol increases placental oestrogen and prostaglandin production, and sensitises the myometrium to oxytocin. Other factors such as progesterone, relaxin and prostacycline dominate early in pregnancy to inhibit contractility. Prostaglandin E₂ and F₂-alpha are synthesised by the decidua and amnion, and may sensitise the myometrium to oxytocin.

Induction of labour

Uterine rupture can occur with any agent that enhances uterine tone. The sensitivity of the myometrium to prostaglandin and oxytocin rises as gestation increases. However, the myometrium is relatively insensitive to oxytocin before term.

Cervical favourability (Bishop’s score) is an indicator of the myometrial sensitivity to oxytocin. Prostaglandin may augment the action of oxytocin and lead to uterine rupture.