Objective To determine whether large loop excision of the transformation zone (LLETZ) is associated with an increased risk of preterm birth (PTB) and to determine whether the volume of tissue excised is important.

Methods This retrospective cohort study was undertaken at West Middlesex University Hospital between 2001 till 2009. Women with a history of LLETZ that subsequently fell pregnant were included. Suitable cases were identified by linking the pathology register for histological cone samples with the maternity birth register database. The primary outcome was PTB (<37 weeks). Secondary outcomes included the volume of the excised cone and its correlation with the risk of prematurity.

Results A total of 28 354 women delivered over the study period. Our cohort consisted of 135 women who delivered after LLETZ. The rate of PTB for the untreated group was 5.4% (1529/28 354) as opposed to 12.6% (17/118) for treated women. The risk for PTB increased almost threefold among those who had a LLETZ (relative risk 2.93, 95% CI 1.76 to 4.89). The number needed to harm was 28. The mean volume of excised tissue in those who delivered preterm was greater than those who delivered at term (5.73 cm³±4.96 cm³ vs 3.66 cm³±2.81 cm³; p=0.01).

Conclusion LLETZ predisposed women to PTB, and larger excision volumes increased this risk. Larger studies are required to determine if larger excision volumes are associated with an increased risk of ascending infection or alterations in the tissue composition resulting in PTB and subsequent poor perinatal outcomes.