Breech presentation is associated with lower adolescent tibial bone strength

Purpose: Breech position during pregnancy is associated with reduced range of fetal movement, and with lower limb joint stresses. Breech presentation at birth is associated with lower neonatal bone mineral content (BMC) and area, but it is unknown whether these associations persist into later life. Methods: We examined associations between presentation at onset of labour, and tibia and hip bone outcomes at age 17 years in 1971 participants (1062 females) from a UK prospective birth cohort that recruited >15,000 pregnant women in 1991-1992. Cortical BMC, cross-sectional area (CSA) and bone mineral density (BMD), periosteal circumference, and cross-sectional moment of inertia (CSMI) were measured by peripheral quantitative computed tomography (pQCT) at 50% tibia length. Total hip BMC, bone area, BMD and CSMI were measured by dual-energy X-ray absorptiometry (DXA). Results: In models adjusted for sex, age, maternal education, smoking, parity, and age, singleton/multiple births, breech presentation (n=102) was associated with lower tibial cortical BMC (-0.14SD, 95%CI -0.29 to 0.00), CSA (-0.12SD, -0.26 to 0.02), BMD (-0.16SD, -0.31 to -0.01), periosteal circumference (-0.14SD, -0.27 to -0.01) and CSMI (-0.11SD, -0.24 to 0.01). In females only, breech presentation was associated with lower hip CSA (-0.24SD, -0.43 to 0.00) but not with other hip outcomes. Additional adjustment for potential mediators (delivery method, birthweight, gestational age, childhood motor competence and adolescent height and body composition) did not substantially affect associations with either tibia or hip outcomes. Conclusions: These findings suggest that prenatal skeletal loading may exert long-lasting influences on skeletal size and strength but require replication.