Objective: To estimate whether there are demographic or clinical characteristics that are associated with the likelihood of having a short cervix and whether these characteristics can be used to optimize cervical length screening. Methods: This is a cohort study of women with a singleton gestation without a history of spontaneous preterm birth who underwent routine transvaginal second-trimester cervical length screening. Seven risk factors for preterm birth were compared by cervical length status. A multivariable logistic regression was performed to identify independent risk factors for a short cervix (cervical length 2.5 cm or less). Different prediction models for a short cervix, based on the number of risk factors present, were developed and test characteristics for cervical length assessment for different risk-based screening approaches were calculated. Results: Of the 18,250 women screened, 164 (0.9%) had a short cervix. Maternal age and conception by in vitro fertilization were not significantly associated with a short cervix. However, black (adjusted odds ratio [OR] 3.77, 95% confidence interval [CI] 2.42-5.87) and Hispanic (adjusted OR 1.73, 95% CI 1.10-2.74) race-ethnicity, current tobacco use (adjusted OR 3.67, 95% CI 1.56-8.62), prior indicated preterm birth (adjusted OR 2.26, 95% CI 1.26-4.05), and having a prior cervical excisional procedure (adjusted OR 2.96, 95% CI 1.86-4.70) were independent risk factors for a short cervix. If only women with any of these variables present were offered transvaginal cervical length screening, the specificity increases from 62.8% for universal screening to 96.5% with a risk-based approach. The sensitivity with one variable present to offer transvaginal scanning was 62.8% and with two factors 14%. Conclusion: Limiting cervical length screening to women with at least one of the identified risk factors for a short cervix substantially decreases the number of ultrasonograms for cervical length assessment. However, this strategy results in nearly 40% of women with a short cervix not being ascertained.