DescriptionMicrobiome perturbations with antibiotics have been reported to affect bone phenotypes (Cho et al., 2012; Cox et al., 2014; Ohlsson & Sjögren, 2015; Sjögren et al., 2012; Yan & Charles, 2017; Yan et al., 2016). Surgical birth, is another stressor of the normal transfer of primordial microbes to the offspring, and it has been connected to accelerated growth in mice (Martinez et al., 2017). However, the effect of surgical birth on bone phenotypical changes is unclear. We hypothesize that surgical birth, interferes with offspring skeletal development, increasing bone mineral content and density, and that the effect is mediated by the microbiome. To test this hypothesis, we determined the effect of surgical birth and restoration with maternal microbes on bone phenotypes. We separated mice into three groups: control vaginal group, surgical birth group, and surgical birth with maternal microbial restoration group. All litters were fostered. We measured body weight weekly. At 6 and 18-weeks and whole-body composition was measured, as well as bone mineral density and bone mineral content (whole body, both sides of femur, both sides of humerus, both sides of tibia, and spine-vertebrae L1 to L6). Surgically born females had increased weight gain during weeks 4-10 (p<0.05) and showed a non-statistically significant trend to higher spine bone mineral density (p=0.064). Restoration after surgical birth, which indicates a microbiome-mediated effect, normalized weight phenotype but increased whole-body fat percentage (p=0.029) among young females. It also showed a trend to increase whole-body bone mineral content among young males (p=0.054). The non-significant trends in changes in skeletal development should be further examined by to increase the number of animals for greater statistical power. The results supported the hypothesis that microbes interfere with developmental growth.