Pratt dilators: resistance at 9 mm is an instrumentation artifact

A previous study reported frequent resistance of the cervix at 9 mm during dilation and interpreted the resistance as the beginning of a tear of the internal os. A more recent identical study did not indicate such tears. Measurement of the dilators revealed that the set used in the previous study had uneven increments in diameter with a wide variation between size 25 and size 27 dilators. Increments of newer dilators are more even, and newer dilators reveal a more even progression of cervical resistance with increasing diameter.