Introduction: Vaginal examination is the gold standard in assessing the progress of labor. Vaginal examination is not only to assess the patency of the cervix, but also to assess the progress of the leading part of the birth canal, the position and presentation of the fetus, the length of the cervix and the capacity of the pelvis. However, the majority of vaginal examinations were limited to assessing the opening of the cervix. Vaginal examinations can be painful, unpleasant and intrusive to mothers. They are associated with the risk of infection, especially in cases where they are done frequently. The rise of routine interventions in childbirth is causing concern around the world, and they are turning to alternative, less intrusive methods to assess the progress of labor. The red line or "red line", "purple line" or purple line as an alternative method for monitoring the progress of labor was first noticed in 1990. The red line appears during labor and increases as labor progresses parallel to the opening of the cervix. The direction of movement of the red line is from the anus towards the intergluteal line to the sacro-coccygeal joint like a thermometer where the column moves upwards. Research objective: The primary objective of this research is to investigate the association between the length of the red line and the patency of the cervix. And the secondary goals are to determine in which stage of labor the red line appears and to determine the appearance of the red line in relation to the opening of the cervix. Subjects and methods: A prospective longitudinal study was conducted at KBC Rijeka, at the Clinic for Gynecology and Obstetrics, delivery room department. 30 respondents participated in the research and signed their consent to participate in the research. The sampling method is a systematic sample according to inclusion and exclusion criteria. For the purpose of this research, a questionnaire was created for data collection. Results: The red line occurred in 96.67% of cases. The correlation between the opening of the cervix and the increase in the length of the red line is positive, very high at the level of r=0.91 and statistically significant with a risk of less than 5%. The most precise match between the opening of the cervix and the red line begins in the active phase of the first stage of labor. On a sample of 30 subjects, 92 measurements of the opening of the cervix and the length of the red line were performed. The average number of vaginal examinations per subject is 3. The red line first appears when the cervix is 4 cm wide. Conclusion: The research showed that the red line is present and that there is a positive correlation between the openness of the cervix and the red line. In most cases, it has high sensitivity and specificity and can be used as a non-invasive complementary method for clinical assessment of the course of labor.