Aim : To develop a cervical dilatation curve for spontaneous vaginal delivery in Japanese primiparas. Material and methods : This is an observational study. Data were retrospectively collected from delivery records. The subject sample included 483 Japanese primiparas who delivered by spontaneous vaginal delivery at the study institution between January 2012 and June 2015. For statistical analysis, a cervical dilatation curve was created using a smoothing spline function, which was compared with

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... the Friedman's labor curve. In addition, a prediction model for delivery progress of Japanese women was developed using nonlinear regression. Results : (1) A new cervical dilatation curve for Japanese primiparas was created. The active labor period was longer in the cervical dilatation curve for Japanese women compared with that in the Friedman's labor curve. (2) As a new indicator for the management of labor and delivery in Japanese women, we developed a four-parameter logistic approximation curve model formula. Conclusions : The cervical dilatation curve for Japanese primiparas showed a sigmoid pattern with respect to the Friedman's labor curve and resembled the four-parameter logistic model.