Recent data obtained in our laboratory suggest that feeding pregnant broodmares with cereal concentrates may affect both mare and foal metabolism in the short and long term. Here, we investigated feto-placental biometry and placental function at term in mares fed with cereals and forage or forage only. Twenty-two multiparous mares inseminated with the same stallion were allocated to 1 of 2 groups from 7 months of gestation: group F (n = 10) were fed forage only, whereas group B (n = 12) received forage and cracked barley until foaling. At 3 and 9 months of gestation, a glucose tolerance test (IVGTT) was performed to evaluate the insulin resistance of pregnant mares. At birth, placentas and foals were weighed and measured. Placental samples were collected above the umbilical cord insertion and snap frozen. An RNA sequencing (RNAseq) analysis was performed on 9 placentas of each group. After normalization, gene levels were analysed using the DESEqn 2 package of R software (https://www.r-project.org/). Enrichment of gene sets was analysed using the Gene Set Enrichment Analysis (GSEA) software using the Kyoto Encyclopaedia of Genes and Genomes (KEGG) and Gene Ontology [GO, biological processes (bp), molecular function (mf) and cellular components (cc)] databases. Gene analysis statistical results were considered significant for P-values < 0.05 after false rate discovery (fdr) correction. The IVGTT results were analysed using a type 3 ANOVA on a mixed linear model with group as fixed effect and age of the mare as random effect. At 3 months of gestation, maternal glucose metabolism was not different between groups. At 9 months, B mares had a higher insulin area under the curve (AUC) after glucose injection than F mares (P < 0.01), without any difference in glucose AUC, suggesting that B mares were more insulin resistant than F mares. At birth, no difference was observed for feto-placental biometry between groups. Gene-level analysis could not discern differences in gene expression between groups after fdr correction. The GSEA analysis, however, showed that 8 gene sets were down-regulated in C placentas (2 KEGG, 2 GObp, 3 GOmf, 1 GOcc) and 193 gene sets were up-regulated (15 KEGG, 144 GObp, 12 GOmf, 22 GOcc) in B placentas. The down-regulated gene sets were involved in neutral amino acids and anion transport, fatty acid oxidation, acetyl coA synthesis, cholesterol and folate degradation, and the up-regulated gene sets were involved in RNA expression, inflammation (activation and recruitment of immune cells, MAPK signalling, complement and coagulation cascades, pro-inflammatory cytokine production and signalling) and in vascularisation (vasculogenesis, angiogenesis and smooth muscle cells development). The results are consistent with the altered function observed in term placentas of women who suffer from gestational diabetes. In conclusion, feeding pregnant mares with cereal from mid gestation alters the placental function at term.