How many preterm births in England are due to excision of the cervical transformation zone? Nested case control study.

BACKGROUND: Preterm births (as a proportion of all births) have been increasing in many countries. There is growing evidence of increased risk of preterm birth following excisional treatment of the cervix. We estimate the number of preterm births attributable to excisional treatments with a length of 10 mm or more in England. METHODS: Case-control study nested in a record linkage cohort of women with a histological sample at 13 hospitals in England. We combined observed age at first excisional treatment in our cohort with the weighted distribution of excision length from the case-control study to estimate the length distribution by age at first treatment among the cohort. The number of births after excision for each 5-year age group was estimated using national fertility data; published absolute risks of preterm (<37 gestational weeks) and very preterm birth (<32 weeks) were applied to these to estimate the number of preterm births per 100 women treated. Excess preterm births were estimated assuming all treatments were small. The attributable risk of preterm birth following excisional treatment in England was estimated. RESULTS: The majority of first excisional treatments at colposcopy were small (47.5%) or medium (39.1%), 9.5% were large and 4.1% were very large excisions. 4.0% of women treated before birth had more than one excisional treatment. Thus based on our cohort of 10,711 treated women and the length of treatment observed in the case control study we estimate an excess of 240 preterm births (including 57 very preterm) or 2.2 (including 0.5 very preterm) per 100 women treated. At a population level (for England) we estimate that 39,101 women aged 20-39 would be treated each year and that these treatments will lead to an excess of 840 preterm births (including 196 very preterm) in England each year. CONCLUSIONS: Assuming associations between preterm birth and treatment for cervical disease are causal; we estimate that an excess 840 (2.5%) preterm birth in England each year are due to excisional treatments of 10 mm or more. Those that go on to become pregnant should be closely monitored during antenatal period to reduce their risk of preterm birth.