Twenty-one studies were included in the review (n=496,129); one prospective cohort study (n=213) and 20 retrospective cohort studies (n=495,916).
Statistically significant heterogeneity was found for the risk of perinatal mortality when all procedures were pooled (p=0.031), and in studies of laser conisation (I2=67%, p=0.082) which were not pooled. The same was noted for risk of extreme preterm delivery following large loop excision of the transformation zone (results not shown) which were not pooled.
All models and continuity corrections had similar pooled estimates.
Cold knife conisation was associated with a significantly increased risk of perinatal mortality (seven studies) RR 2.87 (95% CI: 1.42, 5.81), severe preterm delivery (gestation <32/34 weeks) RR 2.78 (95% CI: 1.72, 4.51, five studies), extreme preterm delivery (gestation <28/30 weeks) RR 5.33 (1.63, 17.40, four studies) and low birth weight <2000g RR 2.86 (95% CI: 1.37, 5.97, one study).
Laser conisation was described in one study. This was associated with a significantly increased risk of low birth weight <1500g RR 10 (95% CI: 1.19, 83.84) and <2000g RR 3.50 (95% CI: 1.06, 11.53). Ablation by radical diathermy (one study) was significantly associated with severe preterm delivery RR 2.54 (95% CI: 1.65, 3.89) and extreme preterm delivery RR 2.15 (95% CI: 1.11, 4.18) and low birth weight <1500g RR 2.53 (95% CI: 1.62, 3.95) and <2000g RR 2.04 (95% CI: 1.45, 2.87).
Serious adverse pregnancy outcomes were not significantly associated with large loop excision of the transformation zone or ablative treatment with cryotherapy or laser.
Previous treatment with cold knife conisation, laser conisation or diathermy was estimated to result in approximately one perinatal death in 70 pregnancies and large loop excision with two deaths in 1000 pregnancies.
After cold knife conisation and diathermy, the NNTH was often <60 for severe and extreme preterm delivery and low birth weight. After large loop excision, the NNTH was greater than 100 for delivery <32-34 weeks and birth weight <2000g; and for a birth weight <1500g, the NNTH was greater than 500.