Increased risk of preterm birth after treatment for CIN

Meta-analyses published a few years ago in the Lancet and the BMJ showed an increased risk of adverse obstetric outcomes after treatment for cervical intraepithelial neoplasia (CIN).1 2 Since then many studies, from large national linkage to small cohort studies, have confirmed the association. The cause remains unclear; however, potential mechanisms include anatomical changes,3 cicatrisation of the cervix,4 immunological factors, and alterations of the cervicovaginal flora. In a linked research study (doi:10.1136/bmj.e5174), Castanon and colleagues suggest that the increased risk of adverse pregnancy outcomes may not be attributable solely to the treatment itself but to common risk factors that also predispose to precancerous cervical conditions.5 This retrospective linkage study from 12 quality assured British colposcopy centres evaluated outcomes derived from a sample population of 18 441 births. With a comprehensive analysis aimed at optimising control for possible confounders, the authors again found an association between treatment for CIN and preterm birth, but to a smaller degree than previous estimates.

The differences in the size of the treatment effect across studies may be partly explained by the choice of control population, because women with CIN may have …