Changes in pelvic shape among Japanese pregnant women over the last 5 decades.

ABSTRACT

AIM: Pelvic shape affects the progression of labor. The gynecoid-type pelvis has been the most common in women, but a previous study reported that the anthropoid type has increased in Japan. The purpose of this study was to examine (i) the current prevalence of different pelvic shapes in Japanese pregnant women; and (ii) the changes in the prevalence of the pelvic shapes over the past 50 years. MATERIAL AND METHODS: We conducted a cross-sectional study using descriptive analysis for the prevalence of the pelvic shape and a historical comparison to assess the changes in the proportions of pelvic shapes with Japanese data from the 1960s to the 1980s. We reviewed a total of 517 delivery records from May 2010 to August 2012 at a rural Japanese hospital where prenatal X-ray pelvimetry had been routinely performed as local practice. We analyzed the readings of the digital imaging data in pregnant women who underwent X-ray pelvimetry. RESULTS: We identified the X-ray data in 326 Japanese pregnant women. The prevalence of the anthropoid, gynecoid and flat types was 151 (46.3%), 142 (43.6%), and 33 (10.1%), respectively. There were no differences in the maternal characteristics according to the pelvic shapes. The prevalence of the anthropoid-type pelvis has increased by approximately 40% and that of the gynecoid type has decreased by 20% since the 1960s. CONCLUSION: The prevalence of the anthropoid-type pelvis have significantly increased in Japan. Identifying a determinant of this phenomenon requires further research.