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

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.

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.

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.

The prevalence of the anthropoid-type pelvis have significantly increased in Japan. Identifying a determinant of this phenomenon requires further research.