Two minutes is sufficient to characterize the viscoelastic properties of the human lower birth canal during the first stage of labor.

Background: The lower birth canal is the final constriction through which a fetal head must pass for delivery. Unfortunately, injuries to the lower birth canal tissues occur in up to 19 % of first-time vaginal deliveries due to the 300 % stretch required.

Methods: This is a secondary analysis of data from 56 healthy nullipara recorded by a lower birth canal dilator during the first stage of labor. A four parameter Fractional Zener rheological Model was used to characterize the canal viscoelastic properties during the first stage of labor. We tested the (null) hypothesis that the Model constants identified during the initial 20 s ramp-and-5-min-hold dilation from 40 to 55 mm will not accurately predict the final hoop tension after 60 min of slower dilation.

Findings: The null hypothesis was rejected in that when the four Model parameters were calculated for all nullipara from the 20-s-ramp-and-5-min-hold data, the relative fit error was 8 ± 4 %, and the relative prediction error after 60 min of dilation was 10 ± 5 %. Furthermore, when the Model constants were instead calculated from the 20 s ramp and only the initial 90 s of the 5-min-hold, the error was still acceptable: 13 ± 4 % for relative fit and 13 ± 18 % for relative prediction.

Interpretation: Two minutes is sufficient to characterize canal viscoelastic properties during the first stage of labor and identify those with stiffer tissues at higher risk for a pelvic floor injury during delivery. This could form the basis for a clinical test predicting injury.