Diagnosis of early pregnancy in mares is an important component of equine breeding practice, as early embryo loss is relatively common and incurs a substantial economic loss. This is compounded by a short breeding season, placing pressure on breeders to achieve pregnancies early. Furthermore, unlike in other domesticated livestock species, a precise signal or mechanism for maternal recognition of pregnancy has not yet been elucidated in horses. Clinical detection of mare pregnancy typically occurs around day 14 using trans-rectal ultrasonography. As a prelude to the development of an on-farm robust early pregnancy test, we are undertaking proteomic analysis to compare the plasma profiles of pregnant (7P) and non-pregnant (7NP) mares at day 7 following ovulation to identify pregnancy-induced protein biomarkers. Using a batch-mode approach, we have sequenced 66 plasma samples from both 7P and 7NP mares using an Exploris 480 mass spectrometer in an unbiased fashion. Using our established bioinformatical pipelines, we have identified a plasma protein profile of 234 proteins with 13 of these displaying significantly different abundance between 7P and 7NP. Amongst these changes was serpin A6, a member of the serine proteinase inhibitor (serpin) plasma proteins, which was significantly increased in 7P plasma. Members of the serpin family have been identified as being synthesised by the uterus in many species, and serpin A6 is the principal transport protein for cortisol and progesterone, thus supporting a role for this protein in early pregnancy. Other differentially abundant proteins detected in this study may be important in the immunological recognition of pregnancy such as Immunoglobulin lambda light chain variable region, Alpha 2 macroglobulin and Complement C8 gamma chain etc. Overall, this study is providing an important platform to establish a panel of protein biomarkers for the confident detection of early pregnancy in the mare.