Pelvic inlet area is associated with birth mode

Testing the reliability of the rearticulation of osteological primate pelves in comparative morphological studies

The evolution of human pelvic form is primarily studied using disarticulated osteological material of living and fossil primates that need rearticulation to approximate anatomical position. To test whether this technique introduces errors that impact biological signals, virtual rearticulations of the pelvis in anatomical position from computed tomography scans were compared with rearticulated models from the same individuals for one female and one male of Homo sapiens, Pan troglodytes, Macaca mulatta, Lepilemur mustelinus, Galago senegalensis, and Nycticebus pygmaeus. "Cadaveric" pelvic bones were first analyzed in anatomical position, then the three bones were segmented individually, intentionally scattered, and "rearticulated" to test for rearticulation error. Three-dimensional landmarks and linear measurements were used to characterize the overall pelvis shape. Cadaveric and rearticulated pelves were not identical, but inter-specific and intra-specific shape differences were higher than the landmarking error in the cadaveric individuals and the landmarking/rearticulation error in the rearticulated pelves, demonstrating that the biological signal is stronger than the noise introduced by landmarking and rearticulation. The rearticulation process, however, underestimates the medio-lateral pelvic measurements in species with a substantial pubic gap (e.g., G. senegalensis, N. pygmaeus) possibly because the greater contribution of soft tissue to the pelvic girdle introduces higher uncertainty during rearticulation. Nevertheless, this discrepancy affects only the caudal-most part of the pelvis. This study demonstrates that the rearticulation of pelvic bones does not substantially affect the biological signal in comparative 3D morphological studies but suggests that anatomically connected pelves of species with wide pubic gaps should be preferentially included in these studies.

Do neonatal birth weight thresholds for labor dystocia outcomes differ between short and normal stature women?

OBJECTIVE

To determine if there is a correlation between maternal short stature and neonatal birth weight among women with adverse outcomes related to labor dystocia.

METHODS

The medical records of singleton deliveries with adverse obstetric outcomes related to labor dystocia during 2014-2020, in a single tertiary center, were reviewed. Outcomes included at least one of the following: cesarean delivery (CD) due to cephalopelvic disproportion (CPD), prolonged second stage, shoulder dystocia, third- or fourth-degree perineal tear. Maternal short stature was defined as height below the 10th centile (short stature group) and normal stature was defined as maternal height between the 10th and 90th centiles (normal stature group). Maternal and neonatal characteristics were compared between the groups.

RESULTS

A total of 3295 women were included, among them, 307 in the short stature group (9.3%, height 1.52 ± 0.02 m) and 2988 in the normal stature group (90.7%, height 1.63 ± 0.04 m). Evaluating the entire cohort revealed similar neonatal birth weights comparing the short and normal stature groups. A subgroup analysis of women after CD due to CPD (n = 296) revealed lower neonatal birth weights in the short stature group (n = 31) compared with the normal stature group (n = 265) (3215 ± 411 vs 3484 ± 427 g, P = 0.001, respectively). Multivariable linear regression was performed for women who underwent CD due to CPD. After adjusting for obesity and diabetes mellitus, short stature and nulliparity were found to be independently associated with decreased neonatal birth weight (266 g less for short stature, P = 0.001, and 294 g less for nulliparity, P = 0.001).

CONCLUSIONS

Among women with short stature, CD due to CPD occurs at lower neonatal birth weights.

There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form

Compared to other primates, modern humans face high rates of maternal and neonatal morbidity and mortality during childbirth. Since the early 20th century, this "difficulty" of human parturition has prompted numerous evolutionary explanations, typically assuming antagonistic selective forces acting on maternal and fetal traits, which has been termed the "obstetrical dilemma." Recently, there has been a growing tendency among some anthropologists to question the difficulty of human childbirth and its evolutionary origin in an antagonistic selective regime. Partly, this stems from the motivation to combat increasing pathologization and overmedicalization of childbirth in industrialized countries. Some authors have argued that there is no obstetrical dilemma at all, and that the difficulty of childbirth mainly results from modern lifestyles and inappropriate and patriarchal obstetric practices. The failure of some studies to identify biomechanical and metabolic constraints on pelvic dimensions is sometimes interpreted as empirical support for discarding an obstetrical dilemma. Here we explain why these points are important but do not invalidate evolutionary explanations of human childbirth. We present robust empirical evidence and solid evolutionary theory supporting an obstetrical dilemma, yet one that is much more complex than originally conceived in the 20th century. We argue that evolutionary research does not hinder appropriate midwifery and obstetric care, nor does it promote negative views of female bodies. Understanding the evolutionary entanglement of biological and sociocultural factors underlying human childbirth can help us to understand individual variation in the risk factors of obstructed labor, and thus can contribute to more individualized maternal care.