Cervical length (CL) measured by transvaginal ultrasound is an effective screening test for the prevention of preterm birth (PTB). The criteria for an effective screening test are all met by CL. It studies an important condition (PTB); it is safe and acceptable by >99% of women; it recognises an early asymptomatic phase that precedes PTB by many weeks; it has a well-described technique, is reproducible, is predictive of PTB in all populations studies so far; and, perhaps most importantly, it has been shown that 'early' treatment is effective in prevention. These two interventions, effective only in specific populations, are ultrasound-indicated cerclage and vaginal progesterone.