Evaluation of midpelvic contraction.

Of the pelvic parameters, IS, which represents the narrowest pelvic dimension is of great value in determining the midplane capacity; WS's Sum, which represents two dimensions of the posterior segment, only concerns the space available in the posterior segment and, therefore, can explain the occurrence of easy delivery in patients with short I.S. but long P.S.M.; Area, which gives two dimensions of the whole midpelvis, not only stresses the space available within the whole midpelvis, but can also indicate whether there is any shortening of the two diameters or not. WS's Sum is consequently superior to I.S. and Area is superior to both WS's Sum and I.S. in the evaluation of midplane capacity.