"A depiction of Friedman's curve, based on data from Friedman, E. A. (1955). ""Primigravid labor; a graphicostatistical analysis."" Obstet Gynecol 6(6): 567-589. and Friedman, E. A. (1956). ""Labor in multiparas; a graphicostatistical analysis."" Obstet Gynecol 8(6): 691-703"