Childbirth is biomechanically challenging for humans, as mothers must deliver a relatively large-headed neonate through a narrow birth canal. Previous work has indicated that a narrow birth canal in humans is a result of an anatomical compromise for efficient bipedal locomotion and the ability to give birth to large brained infants. This previous work also suggested that difficult and potentially risk delivery of altricial humans was both evidence of and the solution to the tight relationship between infant size and the maternal birth canal in humans. This is known as the obstetric dilemma (OD). More recent work has complicated the OD and pointed to ecologic context as an influence on pelvic dimensions, suggesting that the mismatch between the maternal birth canal and the size of the human infant may have arisen with the transition to agriculture. This thesis begins with the theme of OD and uses it as a platform to examine the developmental trajectory of the bony pelvis in growing women in the context of childbirth. This project investigates associations between pelvic dimensions, biological and biosocial variables in living girls (n=286) from southern England to clarify the factors affecting the birth canal throughout growth and development.

Dual energy x-ray absorptiometry (DXA) can be successfully used to collect pelvic dimensions measurements from healthy, living women. The same pelvic dimensions were collected from magnetic resonance (MRI) and DXA images. DXA measurements all had a higher technical error of measurement than MRI though were within acceptable error limits. Bi-iliac breadth was most closely matched in DXA and MRI measurements. Pelvic measurements collected from DXA scans represent the same biological variation at those collected from MRI scans.

Growth curves of pelvic dimensions show that bi-iliac breadth increases in a similar manner to shoulder breadth, whilst the pelvic canal increases in breadth most noticeably between 11 and 17 years of age. Peak growth velocity of the pelvic canal occurs between 11 to 12 years of age, whilst peak bi-iliac and shoulder breadth growth velocity occurs at approximately 8 to 9 years of age. The division in growth patterns between the non-canal and canal components of the pelvis suggest differing patterns in canalization and potential for genetic compared to environmental impact on their morphology.

Biological factors that associated with shoulder breadth and bi-iliac breadth were height and thigh circumference, whilst pelvic canal dimensions associated with height and indicators of pubertal development. This suggests that pelvic canal dimensions increase more slowly whilst hip and shoulder breadth increase alongside stature growth in girls. No biosocial factors associated with shoulder breadth or pelvic dimensions, demonstrating stronger associations between biological factors than biosocial factors on pelvic development. These results also underline the unique developmental trajectory of the canal compared to the non-canal components of the pelvis, and suggest that pubertal development indicators (which include localized fat deposits relevant to breastfeeding) align in growth with pelvic canal dimensions.

In summary, the findings of this thesis contribute an important basis with which to consider the origins of childbirth difficulty. The results of this project contribute to scholarly work that is redefining the evolutionary origins of childbirth difficulty, proposing more nuanced origins than the original OD hypothesis. The implications of this thesis are that obstetrically relevant pelvic dimensions have a specific growth trajectory that may be impacted by environmental factors in early life and ultimately contribute to childbirth difficulty in adulthood. Results from this work also suggest that the key period of pelvic canal development occurs ahead of puberty whilst also demonstrating that growth priority for the pelvic canal is linked to indicators of pubertal development including localised pockets of adipose tissue. Methodologically this thesis provides a basis for more widespread use of medical imaging in investigating osteological variation. More broadly the results of this work demonstrate that an evolutionary medicine approach that integrates osteological methods and clinical data provides a richer framework to examine the intersections between growth and reproduction in humans.