Organ Size and Metabolic Rate Co-Vary with Dimensions of the Obstetric Pelvis: The Metabolic Coordination of Childbirth Hypothesis.

OBJECTIVES: Childbirth complications occur if fetal size is excessive for the maternal birth canal. We hypothesized that young women's metabolic traits (organ and muscle mass, resting energy expenditure (REE)) known to promote fetal growth would correlate with dimensions of the birth canal, and that some of this association would be explained by growth markers (tibia length, height). MATERIALS AND METHODS: We analyzed cross-sectional data on anthropometry, organ mass (magnetic resonance imaging (MRI)), skeletal muscle (dual-energy X-ray absorptiometry), fat mass (4-component model), pelvic dimensions (MRI), and REE (indirect calorimetry) in 68 nulliparous women aged 20-28 years of South Asian ancestry living in the UK. Principal component analysis (PCA) was used to summarize variability in organ/muscle mass and pelvic dimensions (PCApelvis). Associations between metabolic and pelvic traits were assessed using Pearson correlations, and partial correlations adjusting for height or tibia length. RESULTS: Mean height was 161 cm and body mass index 22.3 kg/m2. There were positive correlations between REE, organ/muscle mass, head girth and the first PCApelvis score, which were partly explained by variability in tibia and height. Fat mass was associated with REE and organ/muscle mass but not pelvic dimensions. DISCUSSION: Correlations between metabolic and pelvic traits indicate a degree of coordination: women with greater metabolic capacity to promote fetal growth also have larger pelvic dimensions, partly explained by linear growth patterns. However, body fat in nulliparous women correlates with REE but not pelvic dimensions; hence high adiposity could increase risk of childbirth complications when these women give birth.