The first foal of a mare is typically smaller and lighter than subsequent foals. A poorer uterine potential to support fetal growth during the first pregnancy could be responsible, but modifications already present in the embryo could also be involved. The aim of this study was to evaluate effects of maternal parity on gene expression in blastocyst’s trophoblast, term placenta, and term placenta histology. Young (4–6 years old) nulliparous (YN), or multiparous (YM) mares were inseminated with the semen of one stallion. Embryos were collected eight days after ovulation in YN (n = 6) and YM (n = 5). Trophoblast was obtained by embryo bisection. Placentas from YN (n = 18) and YM (n = 10) were recovered at foaling, measured, and weighed. Stereology was performed using MercartorPro software on HES-stained paraffin-embedded samples and analysed using a linear model with permutations with R software. Paired-end RNA-sequencing was performed on frozen trophoblast and placenta (Illumina, NextSEqn 500). Differential expression was analysed (DESEqn 2) using a false discovery rate (FDR) < 0.05 cutoff. Gene Set Enrichment Analysis was performed using KEGG and GO BP databases. In trophoblast and placenta, 13,178 and 14,696 expressed genes were identified, only 6 and 2 of which were differentially expressed according to mare parity, respectively. Differentially expressed genes were partly related to molecule transport. Gene pathways disturbed in trophoblast were related to energy and lipid metabolism, transport, and interleukin-1 signalling. Only eight pathways were perturbed by maternal parity in term placenta and were involved in innate immune response. Gestation averaged 5.5 days longer (P < 0.05) in YN compared with YM mares. Lighter placentas were observed in YN, but they produced more kg of foal/cm2 of placental macroscopic surface (P < 0.01). No difference was shown in placenta volume or all histological measurements. In conclusion, term placentas appeared to be smaller than for YM, but similar in morphology and potentially more efficient at supporting the fetus. Maternal parity, however, slightly affected gene expression in both trophoblast and term placenta, indicated by changes related to molecule transport and immune regulation. Due to the importance of these functions in placenta establishment, these results suggest that altered exchanges from the embryo stage with the YN dam could be one cause of reduced foal size at birth.