The developmental environment, including the microbiome (the community of symbiotic microbes that regulates diverse host processes), can strongly influence offspring phenotypes. In mammals, the microbiome is historically thought to establish in the postnatal period after exposure to microbiota in the maternal vaginal tract. However, recent evidence in mice suggests that the placenta is inoculated with microbes originating from the maternal oral cavity and vaginal tract, and that microbes are present in the embryonic gut by late gestation (embryonic day 17). This prenatal inoculation is thought to promote the development of the mucosal immune system in offspring. To determine the source of embryonic gut seeding in the house mouse (Mus musculus domesticus) at embryonic day 17, we collected tissue samples (n = 5 litters) from the placenta and embryonic gut, as well as a suite of maternal samples including the oral cavity, blood, gut, and vaginal tract. Microbial DNA was extracted from each sample and sequenced (V4 region of the 16S rRNA gene); sequence data were processed in Qiime2. Path analysis was used to compare the fit of alternative hypotheses for relationships between embryonic gut microbiota and candidate maternal sources. Future work will address when the prenatal microbiome is initially acquired and how this coincides with microbial shifts in maternal tissues over the course of pregnancy.