Measuring cervical length with ultrasound: evaluation of the procedures and duration of a learning method

To evaluate the learning curve and principal learning steps for a novice ultrasound operator to master the technique for measuring cervical length by transvaginal ultrasound.

The study involved asymptomatic patients with a singleton pregnancy between 21 and 38 weeks attending for a routine ultrasound scan. Two operators new to cervical ultrasound including one with experience in transvaginal gynecological ultrasound were assessed during their training. After an initial lecture session on the technique of cervical measurement, the trainees had one or two supervised practice sessions, validated by two interobserver agreement studies with 30 patients at a time; these compared the level of agreement between trainer and trainee throughout. Five examinations were performed in the first practice session; a second session was planned if the trainee could not perform all five examinations alone satisfactorily, in agreement with four quality criteria predefined for good cervical measurements. The second session lasted until five consecutive examinations could be performed adequately without the trainer's aid. Agreement between the two operators was assessed by intraclass correlation coefficient (ICC). The ICCs were compared with Fisher's Z transformation.

Of the 150 patients included, 2.7% gave birth before term. The trainee with no experience in transvaginal ultrasound required two practice sessions; none of the five ultrasound scans performed in the first session was correct. In the second session, 18 consecutive ultrasound examinations were necessary to obtain five consecutive adequate results. During interobserver studies with the trainer, the ICC progressed from 0.43 in the first study to 0.64 in the second and 0.77 in the third (P = 0.049 between the first and third studies). The trainee with experience in transvaginal ultrasound required only one practice session. The ICC increased from 0.73 to 0.85 in the two interobserver studies with the trainer.

Measurement of cervical length by transvaginal ultrasound examination is a technique that can be learnt rapidly. While roughly 23 supervised ultrasound scans appear necessary for an operator with no experience in transvaginal ultrasound, substantially fewer are required for an operator already familiar with this approach for other indications.