Cone biopsy shortens the cervix and is associated with preterm birth in later pregnancies

Research question Does vaginal ultrasonography help predict preterm birth among women who have been treated for cervical intraepithelial neoplasia?

Answer Possibly. Vaginal ultrasonography may help predict preterm birth among women who have had loop electrosurgical excision.

Why did the authors do the study? Some evidence exists that treatments for cervical intraepithelial neoplasia, such as cone biopsy, are associated with preterm birth. If so, a shortened cervix could be to blame. These authors wanted to find out if measuring the cervix in mid to late pregnancy could help predict preterm birth among women who have had cervical procedures.

What did they do? They compared three groups of pregnant women: 132 who had had loop electrosurgical excision, cold knife conisation, or cryotherapy for cervical intraepithelial neoplasia, 63 who had had a spontaneous preterm birth, and 81 who had had neither. The authors measured the women's cervices by using vaginal ultrasonography between 24 and 30 weeks of pregnancy and followed them up until the birth. They looked for differences in cervical length and incidence of preterm birth between the three groups and then did a multiple logistical regression analysis to find out which cervical procedures were independently associated with preterm birth. Finally, they estimated the predictive value of a short cervix for preterm birth among women who had had loop electrosurgical excision.

What did they find? Women who had had any cervical procedure had a shorter cervix mid-pregnancy than low risk controls (mean length 3.54 cm, 3.69 cm, 3.75 cm among women with loop excisions, cold knife conisation, and cryotherapy compared with 4.21 cm among controls; P < 0.05 for all procedures). Women with a previous preterm birth also had a shorter cervix (3.78 cm).

The two types of cone biopsy, but not cryotherapy, were independently associated with preterm birth (odds ratio 3.45 (95% CI 1.28 to 10) for loop excision and 2.63 (1.28 to 5.56) for cold knife conisation). Among women with a previous loop electrosurgical excision, a cervix less than 3 cm on vaginal ultrasonography predicted preterm birth with a positive predictive value of 53.8% (7/13) and a negative predictive value of 95.2% (59/62). The authors were unable to estimate the predictive value of vaginal ultrasonography for women who had had cold knife conisation because of the small size of their sample.

What does it mean? These data add to growing evidence of a link between cone biopsy and preterm birth and are reassuring for women who have had cryotherapy. The strongest findings were for women who had had loop electrosurgical excision, whose risk was three times higher than that of controls. Measuring the cervix of these women in pregnancy may be useful, if only because women with a cervix over 3 cm long seem unlikely to deliver early. Note, however, that the authors had no data on social class and were unable to account for the effects of this important confounder.

Crane JMG, et al. Transvaginal ultrasonography in the prediction of preterm birth after treatment for cervical intraepithelial neoplasia. Obstet Gynecol 2006;107: 37-44

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