Simplification from a "half-model" cervix to a "quarter-model" cervix. Above is the "half-model" superimposed on a balloon and cervix. The half-model approximates the dilation and effacement in reaction to the balloon as a series of linkages, pin joints, sliding joints, springs and dampers. Below is the further-reduced "quarter-model", which captures the same effects but with fewer components. The mathematical model described in this paper uses the "quarter model". It has one quarter the cervix mass, one quarter the compliance, and bears one quarter the traction force.