I agree, there are no randomised controlled studies of obstetric outcomes following excisional versus ablative cervical treatment, and thus no direct comparison of obstetric morbidity associated with each procedure. However, the association between cervical surgery and preterm labour is a significant clinical issue that warrants further research and debate. Social deprivation and smoking are risk factors for both pre-invasive cervical disease and preterm delivery, thus it is difficult to separate the issue of causation from association, especially with retrospective studies.
The current literature does provide evidence to support all the necessary criteria for causality, temporality, strength of association, dose–response effect, consistency and biological plausibility. The association of preterm birth with excisional cervical surgery was only found in pregnancy subsequent to, and not prior to, surgery.1 The association between cervical surgery and preterm delivery and its consequences is strong.2,3 A clinically useful way to express this association is that a single treatment with cold-knife conisation or laser conisation resulted in about one perinatal death in every 70 pregnancies,3 and large loop excision of the transformation zone (LLETZ) was associated with a lesser risk, of two perinatal deaths in 1000 pregnancies.3 The more cervical tissue excised the greater the risk of preterm birth; cold-knife cone biopsy was associated with a higher risk than LLETZ,2,3 and two treatments were associated with a higher risk than one.1 The association between excisional cervical surgery and preterm delivery has been consistently reported in over 30 cohort studies.2–4 It is biologically plausible that excising some of the cervix would weaken the physical and immunological barrier to infection ascending from the vagina into the fetal–placental unit, which is commonly seen and in preterm labour.
Thus, there is a body of data that supports excisional surgery as a potential cause of preterm delivery. It is therefore sensible and logical to plan treatments for pre-invasive disease that not only minimise the risk of failed treatment but also minimise the risk of obstetric morbidity.