Variation in preterm birth rate and the role of short cervical length across two populations: A comparative cohort study

Objective: To examine variation in preterm birth (PTB) rates between nulliparous women from two different populations and to investigate the contribution of short cervical length to any observed variation. Study Design: A comparative study between two cohorts of singleton nulliparous women from the Netherlands and Chicago, USA was performed. Women with a non-anomalous singleton pregnancy who underwent routine transvaginal cervical length measurement between 16+0 and 21+6 weeks of gestation with a known pregnancy outcome were selected. Fetuses with congenital anomalies, intrauterine fetal deaths and deliveries before 24+0 weeks were excluded. Total, spontaneous and iatrogenic PTB rates were calculated for both populations and compared with univariable and multivariable logistic regression. Results: We included 3409 women from the Netherlands and 3334 women from Chicago. The median cervical length was slightly lower in the Netherlands compared with Chicago (42 vs 44 mm, P<0.0001), but the rate of cervical length of ≤25 mm was comparable (0.9% vs 0.8%, P=0.69). The total rate of PTB prior to 37 weeks was comparable between locations (8.0% in the Netherlands vs 7.3% in Chicago, P=0.27) but PTB prior to 32 weeks was higher in the Netherlands (1.3% vs 0.5%, P=0.002). Multivariable regression revealed that women from the Netherlands, compared with women from Chicago, are at increased risk for spontaneous PTB between 24 and 27 weeks and between 28 and 31 weeks (adjusted odds ratio (aOR) 5.3, 95% confidence interval (CI) 1.5 to 19 and 3.1, 95% CI 1.2 to 8.3, respectively). In contrast, the number of late spontaneous PTB between 34 and 36 weeks did not differ between both populations (4.1% vs 4.3% aOR 1.1, 95% CI 0.87 to 1.5). Conclusions: Despite similar percentages of short cervix, there are marked differences in early PTB rates between the Netherlands and Chicago, possibly indicating different phenotypes of PTB. A high frequency of spontaneous early preterm birth is not always accompanied by an increase in the frequency of a short cervix.