Female pelvis types and age differences in their distribution
Keywords: pelvic typology, narrow pelvis, pelvimetry
AbstractThe aim of this study was a statistics-based exploration of a typology of the female pelvis. The research sample included 172 females aged from 18 to 69. For measurements, the three dimensional CT images of pelvis were used. A cluster analysis was performed on anteroposterion and transverse diameters of the pelvic inlet and the midplane. The results revealed three clusters representing gynecoid, “narrow”, and intermediate types of female pelvis. The distribution of pelvic types in age groups indicates a tendency for the “narrow” pelvis to be presented more frequently in the group of younger females.
Variability and typology of the female pelvis is a traditional topic in anthropological studies [3, 4, 13, 17]. The best known classification of the female pelvis was suggested by Caldwell and Moloy in 1933  and it was based on the pelvic inlet shape. In the frame of this classification, four main pelvic types were suggested: gynecoid, android, anthropoid, and platypelloid. The development of this classification resulted in the identification of the mixed types and subtypes based on the width of the pelvic outlet . As a result, more than twenty subtypes were suggested that complicated their analytical implication. In addition, the critics of this classification addressed the subjective impression in the judgments of pelvic shapes without a well established statistical base .
A tendency of increase in cesarean section is observed in the last decades [2,5, 9, 15]. In Latvia, the number of cesarean section delivery is growing from 3.9% in 1980 to 23.7% in 2010 . A narrow pelvis is one of the factors increasing the risk for cesarean section . On the one hand, there is a solution for the narrow pelvis problem from the obstetric perspective. On the other hand, a more detailed analysis is needed from the anthropological perspective because of possible evolutionary trends in the human body in general and in the pelvic shape in particular. The aim of this study was a statistics-based exploration of a typology of the female pelvis.
A well recognized anthropological tendency of the last century is the secular trend in growth. Previous studies demonstrate an increase in the mean height about 1–2 cm per decade in different European countries [6, 8]. An investigation of external body parameters of Latvian women also demonstrated significant changes in the period of 70 years. The women’s height increased for 6 cm, shoulder breadth increased for 0.6 cm, and the hip breadth increased for 2.9 cm . Based on the relationship between the lesser pelvic parameters and height observed in previous studies [7, 11, 13], it is possible to expect that parameters of the lesser pelvis also changed during the last 6–7 decades.
It should be noted that the female pelvic cavity has a cylindrical shape with the narrowest place in the midplane between two ischial spines (the bispinous diameter). The obstetric importance of the pelvic inlet and the midplane was emphasized in anthropological studies [3, 4, 16, 17]. In a typical female pelvis, a longer diameter of the inlet (the transverse diameter) and a longer diameter ofthe midplane (the anteroposterior diameter) are placed perpendicularly. Therefore, a fetal head rotates from a transverse position in the pelvic inlet to a sagittal position in the midplane. A narrowing of the pelvic cavity in the midplane causes this rotation. Stalberg at al.  demonstrated that a narrow pelvic midplane is an important reason for the emergency cesarean section. In addition, an inadequate proportion of the pelvic inlet also causes cesarean section .
Therefore, both the pelvic inlet and the midplane are highly important from the anthropological perspective and need to be included in a statistics-based exploration of the female pelvic typology. Changing body parameters allow to expect age differences in a distribution of pelvic types between younger and older females. As a result, two research questions were posed for the presentstudy:
1. What female pelvic types could be detected on the basis of the measures of the inlet and the midplane of the lesser pelvis?
2. How does the distribution of female pelvic types among age groups differ?