The 'virtual' cervical internal os: Diagnosis during the first trimester of pregnancy

Objective: To determine the appropriate time during gestation for assessing the cervix for possible incompetence by ascertaining the gestational week at which the sac reaches the level of the internal os. Methods: Three hundred and eighteen women with an intact singleton pregnancy at 5 to 15 weeks' gestation underwent endovaginal sonographic examination to measure the distance between the gestational sac and the cervical internal os. The change in location of the gestational sac in the endometrial cavity over time, and the earliest gestational week at which the gestational sac reached the level of the internal os, were calculated and analyzed with one-way analysis of variance. Results: The distance between the gestational sac and the 'virtual' cervical internal os decreased from a mean of 4.72 cm at 5 weeks to 1.71 cm at 13 weeks. This distance decreased significantly during the early first trimester (weeks 5 to 7) (P = 0.004), but remained almost unchanged from 12 to 15 weeks. Only at 12 weeks' gestation could the entire cervical length be determined and the configuration of the cervical internal os assessed for incompetence. Conclusions: Transvaginal sonography is a good method for evaluating the cervix during pregnancy. The location of the 'virtual' internal os can be determined in the first trimester using the urinary bladder as a reference point.