From 1967 to 2003, 0.7% of the births in the population studied occurred in women who had undergone a cervical conisation before the index pregnancy and 2.6% after. Births after a cervical conisation were more common in older women and with higher birth orders (table 1). The proportion of preterm birth (delivery before 37 weeks’ gestation) was 17.2% (95% confidence interval 16.6% to 17.8%) in women who gave birth after cervical conisation, 6.7% (6.5% to 6.9%) in women gave birth before cervical conisation, and 6.2% (6.2% to 6.3%) in women who did not have conisation.
The relative risk of premature delivery in women after a cervical conisation compared with women who did not have cervical conisation increased with decreasing gestational age (table 2). Also, the risk of late abortion was higher after a cervical conisation. The relative risk decreased slightly after adjustment for maternal age and birth order (table 2). The same pattern was observed according to birth weight (data not shown).
Table 2 Numbers and proportions of preterm deliveries with relative risks (95% confidence intervals) in births of women with cervical conisation and no cervical conisation by gestational age in Norway, 1967-2003
Births in women without cervical conisation and with conisation after delivery had similar distribution according to gestational age, whereas delivery at lower gestational ages was more common in women with cervical conisation (fig 1). The distribution according to birth weight showed a different pattern (fig 2). Birth weight in women who gave birth after conisation was lower than in those who had not had conisation. The relative risk of a preterm birth, however, was lower when compared with women with a conisation after delivery, particularly the relative risk of delivery at 24-27 weeks, which was reduced from 4.3 to 3.0 (table 2).
Fig 1 Births before and after cervical conisation or with no cervical conisation by gestational age, Norway 1967-2003
Fig 2 Births before and after cervical conisation or with no cervical conisation by birth weight, Norway 1967-2003
Infants born to women who had a conisation after delivery were lighter than those born to women without a conisation. In women with no cervical conisation, z scores were on average 0.004 (95% confidence interval 0.002 to 0.005) compared with −0.04 (−0.058 to −0.023) in births after a conisation (data not presented). The lowest z score −0.135 (−0.144 to −0.127) was found in births before a conisation.
During the study period, the excess risk of a preterm delivery in women who underwent cervical conisation decreased, particularly the risk of delivery before 28 weeks (fig 3).
Fig 3 Relative risk of preterm birth in various gestational age groups in women who gave birth after cervical conisation compared with births to women with no cervical conisation by year of birth, Norway 1967-2003
In women aged under 25 at the time of treatment, preterm delivery was no more common than in older women (table 3).
Table 3 Preterm delivery (<37 weeks) by maternal age at delivery and at treatment with cervical conisation, Norway 1967-2003. Figures are numbers (percentages) of births
The population attributable risk percentage of preterm delivery attributable to cervical conisation before 28, 33, and 37 weeks of gestation was 2.0%, 1.7%, and 1.2%, respectively.