Upper and right side of the PCA, the obliquely elongated inlet was represented (roi, loi, rias, and lias).

The inlet with large pectineal angle plotted to the negative part of the first component and the left side of the PCA. The inlet variables explained most of the variability of the pelvic shape rather than the variables related to the midplane, outlet, curvature of the birth canal, or pelvic balance in the standing position (Figure 3).

Environmental factors working at early childhood and adolescence were not associated with any specific female pelvic shapes (Figure 4).

      As displayed in Figure 4A, the females acquiring erect posture before 14 months were not clearly separated from the other ones. In addition, the females with intense activity did not show a clear discrimination from the others (Figure 4B). Furthermore, the subjects with nutritional diet overlapped the other females (Figure 4C). Figure 5 illustrates the distribution of the subjects according to the three environmental factors (Figure 5).

Demographic and health factors are considered in Figure 6. The females were labeled according to their socioprofessional conditions as depicted in Figure 6A. The results revealed no separation in the six socioprofessional conditions of the subjects. In this regard, there were some overlaps among these socioprofessional conditions. Four types of spinal disorders, including six sacral variability scoliosis, protruding disc, and the partial fusion at the L5-S1 level, were identified in the study population.

There was no distinction between the females without spinal disorders and the others females in the PCA. The females without any spinal disorders overlapped the other females with spinal disorders (Figure 6D). In Figure 6B, the subjects were labeled according to their birthplace. The PCA discriminated between two groups of females, namely those reporting Africa as the birthplace and the ones with North Europe as the birthplace. In this respect, Africa birthplaces were in the lower and left side of the PCA, whereas North Europe birthplaces were in the upper and right side of the PCA. Furthermore, Mediterranean birthplaces overlapped both Africa and North Europe. The females born in America belonged to the North Europe variability, whereas the two females born in Asia were between the African and North Europe variability.

Figure 7 illustrates the distribution of the participants based on demographic and health factors.

Discussion

The results of this study did not support the previous classification described by Cadwell and Moloy in 1993 (2), which is in line with the most recent studies (5, 6). Among all the factors considered in this research, birthplace was the only effective factor that played a role in pelvic shape. In our study, the first component discriminated the sagittally elongated inlet, while the anterior space was sagittally elongated in the positive part. The negative part of the first component discriminated the sagittally reduced inlet with a wide anterior space, contributing to the opening of the pectineal angle.

The positive part of the second component discriminated the transversally elongated inlet, midplane, and outlet. These anatomical traits are shared “gynaecoid” or “android” types according to the classification of Cadwell and Moloy (2). Considering this classification, the “anthropoid” type was described as sagittally elongated, with a wide subpubic angle (1, 2). If such a shape existed, the PCA would present a tighter cluster for “subpubic angle”, “inlet anteroposterior”, and “obstetric conjugate” (1). Therefore, our study questioned the relevance of the anthropoid type as a specific pattern of pelvic variability.

Thes results are consistent with those of a previous study performed by Bouhallier (2002), investigating pelvic shape variability in a large sample of pelvises from osteological collections using PCA (6). In the current study, PCA discriminated the transversally elongated pelvic with the second component and round-shaped inlet with the first component. Furthermore, “gynaecoid”, “android”, and “platypelloid” types were represented as pelvic patterns; however, this was not the case for “anthropoid” (6). In our study, most of the investigated factors did not play a role in the formation of pelvic shape. This is not in line with the study conducted by Abitbol in 1996. In the mentioned study, the anthropoid pelvis was encountered more often when the acquisition of erect posture was delayed beyond the age of 14 months, whereas a platypelloid pelvic was more frequent when this acquisition was before 14 months (1).

It is worth mentioning that we did not find the anthropoid shape in our study. This could explain the inconsistency between our findings and those reported by Abitbol (1996). Moreover, the effect of biomechanical load on adult pelvic shape in early childhood was complex. The age of the first step is an interesting variable since a member of the patient’s family, usually the mother, has a vivid recollection of this age (1).

However, it does not reflect the biomechanical load on the pelvis at this stage of life accurately. Before the beginning of erect posture, the baby attempts to stand up leaning hands against a surface, such as a wall or a chair. After this age of erect posture, the child is able to experience a range of locomotor behavior, from bipedal to all-four locomotion. The beginning of erect posture cannot be summed up to the age of the first step but consists of successive psychomotor acquisitions. These successive acquisitions should be accurately investigated to analyze the relationship between adult pelvic shape and early childhood history.

In this study, physical activities did not play a role in the formation of pelvic shape. This finding was inconsistent with the results obtained by Abitbol, where android pelvis was supposed to be affected by the strenuous activities performed during early childhood and adolescence (1). However, Abitbol’s definition of “strenuous activity” is difficult to replicate in other studies.

Physical activity was assigned three degrees of nil (0), moderate (+), and intense (++); however, the way of allocation was not specified. Abitbol also recorded early strenuous physical work and heavy lifting; nonetheless, the females included in our study had not experienced such traumatic events, which could explain the difference between our findings and those of Abitbol.

Kelley and Angel (1987) found that nutritional factors affect the pelvic brim index (19). In our study, the qualitative aspect of the diet was not related to a specific pattern of pelvic shape. Kelley and Angel (19) showed that nutritional stress and starvation are associated with the platypelloid pelvis. According to our results and those of previous studies, the quantitative aspect of diet rather than the qualitative one can affect growth during childhood and inlet pelvic shape.

Huseynov et al. (20) showed that the shape of female pelvis is round near the time of maximum fertility; thereafter, it returns to an ontogenetic trajectory similar to the male development. Our study failed to find such a developmental shape change. However, we excluded postmenopausal females, and only women of childbearing age were included. The age range considered in our study was probably limited (i.e., 22-42 years) to identify this complex developmental trajectory, previously described, during the female’s lifetime.

Geographic origin seemed to be the only factor explaining the two patterns of pelvic shape in our study. The Mediterranean was an intermediate geographic region between Africa and North Europe and seemed to have an intermediate range between African and North European variability. In 2002, Baragi et al. studied the shape of the pelvic floor area of African American and European American women and found that posterior pelvic floor area was 10.4% smaller in African American women (21). This finding was in agreement with our results.

Geographical effect on pelvic variability has been demonstrated in a recent study performed by Betti and Manica (2018) (22), pinpointing the ancient population history as the main cause of variation. For Betti and Manica (22), the shape of the pelvic bone is related to the effects of ancient demographic events, such as founder effect or migration, on contemporary genetic diversity. The effects of demographic past history are also called “neutral population history” and are correlated with geography since the origin of human expansion in Africa.

For Betti and Manica, the neutral population history shaped the human pelvic bone (22). Our results showed that geographic origin could affect pelvic variability, which is in line with the results reported by Betti and Manica (22). However, if neutral evolution had been responsible for the observed pattern of morphological diversity, we would have expected a larger African variability than European and Mediterranean ones. Since only 9% of women were born in Africa, it is required to perform another study with a greater sample size.

The strength of the study lied in its use of PCA in a modern obstetric samples, where environmental, demographic, and health factors were identified with a reproducible questionnaire. Some of the limitations of this study included the use of landmarks rather than metric variables, which improve the analysis of anatomical variability. Therefore, a morphometric analysis is required to confirm our results so as to generate “log-size’’ and ‘‘log-shape’’ variables, as suggested by Mosimann and James (23). This generation can decrease the skewness of data and analyze the contribution of size and shape in the total variance.

Conclusion

The pelvic shape variability of our samples was not explained by the four categories previously proposed by Cadwell and Moloy (2). Our results are consistent with the findings obtained by Kuliukas et al. (5). Based on our results, midwifery teachers are recommended to be more cautious about adherence to the classification of Cadwell and Moloy (2). None of the factors related to early childhood, adolescence health, or demographic data explained the pelvic shape variability. Geographic origin seemed to be related to different pelvic shape patterns, suggesting the effect of neutral population history on pelvic variability.

Acknowledgements

We gratefully acknowledge the two anonymous reviewers for their helpful comments.

Conflicts of interest

The authors declare no conflicts of interest.