Study Objective: To compare cervical lengths of adolescents and adults in mid-gestation. Design: An analytical, observational, and cross-sectional study. Setting: Public health system in the city of Blumenau, Brazil. Participants: Primigravidae adolescents under the age of 16 and adults over age 20 (n = 40/group) were systematically sampled. Interventions: Cervical measurements were performed between 21 and 24 weeks of gestation through transvaginal ultrasonography using a previously validated method. Main Outcome Measures: Mean cervical length (Mann-Whitney test) and percentage of cervices below 25 mm (Fisher exact test). Results: For adolescents and adults, average uterine cervix lengths were 28 +/- 6.6 mm 33 +/- 4.1 mm (P < .0001), respectively, and the proportion of cervixes below 25 mm were 27.5% and 7.5% (P < .02), respectively. In addition, adolescents had significantly lower gynecologic age, education, and family income than adults. Conclusion: Primigravida adolescents under the age of 16 have shorter cervices than adults, and a higher percentage of adolescents have cervices shorter than 25 mm. This may be associated with the higher risk of preterm birth observed in adolescents and suggests that this population requires special attention in prenatal care.264209211Polyclinic of Reference and Specialty Poet Lindolf Bell