In humans, low placental microbiome variation is associated with low birth weight and Proteobacteria and Tenericutes are associated with pre-term births. As current breeding strategies incorporate selection for decreased gestation length, the relationship observed in humans might also exist in dairy cows. Previous research suggests decreased pre-natal fetal growth is associated with mature animal lameness and decreased milk production. Composition of the placental microbiome and its impact on calf prenatal performance is not known. Therefore, we hypothesized that active bacteria within bovine placenta are associated with calf weight. The objectives of this study were to (1) quantify the presence of Proteobacteria and Tenericutes in the placenta, and (2) determine the relationships of these bacterial phyla and classes with calf birth weight and gestation length. Placental cotyledons without exposure to vaginal fluid were collected (n = 4) from multiparous Holstein cows. Total RNA was extracted and qRT-PCR was performed using 5 sets of primers: Eubacteria, Tenericutes, β-proteobacteria, ε-proteobacteria, and δ- and γ-proteobacteria. Pearson correlations were performed comparing bacterial group relative abundance with calf birth weight (kg) and gestation length using the rcorr package in R. Across all cotyledons tested, β-proteobacteria (56.82 ± 35.84%) was most abundant, followed by Tenericutes (3.18 ± 4.82%) and δ- and γ-proteobacteria (0.98 ± 0.97%). ε-proteobacteria was not detected. Both β-proteobacteria and δ- and γ-proteobacteria were negatively correlated with birth weight (R² = −0.96 and −0.97, respectively). There was no correlation between bacterial phyla and gestation length. Similar to results in humans, we observed that Proteobacteria are associated with neonatal weight. Proteobacteria might directly divert placental growth factors intended for the calf, resulting in a lower birth weight, or placental microbiota might indirectly influence fetal growth through yet unknown means.