It is extremely important to assess the cervical length as a shortened cervix puts the mother at a high risk for for preterm delivery.
As the pregnancy progresses the length changes.
Saloman et.al (2009) developed a model for singleton pregnancies after assessing 6614 cervical length measurements across multiple gestational ages. These results are shown in the graph and table below.
Reference: Salomon, L.J., Diaz-Garcia, C., Bernard, J.P. and Ville, Y. (2009), Reference range for cervical length throughout pregnancy: non-parametric LMS-based model applied to a large sample. Ultrasound Obstet Gynecol, 33: 459-464. https://doi.org/10.1002/uog.6332