Measurement of cervical length
In the measurement of cervical length by transvaginal sonography:
- The woman empties her bladder and is then placed in the dorsal lithotomy position.
- The ultrasound probe is introduced in the vagina and directed in the anterior fornix. Care is taken to avoid exerting undue pressure on the cervix, which may artificially increase the length.
- A sagittal view of the cervix is obtained and the endocervical mucosa (which may be of increased or reduced echogenicity compared to the cervix) is used as a guide to the true position of the internal os, thereby avoiding confusion with the lower segment of the uterus.
- The calipers are used to measure the linear distance between the triangular area of echodensity at the external os and the V-shaped notch at the internal os.
- Each examination should be performed over a period of 2-3 minutes. In about 1% of cases the cervical length may change due to uterine contractions and in such cases the shortest measurement should be recorded.
Repeatability of measurement. Transvaginal measurement of cervical length is highly reproducible and in 95% of occasions the difference between two measurements by the same sonographer or by two different sonographers is about 4 mm or less.