Pregnancy loss and neonatal mortality have a significant economic impact on the equine industry. Placentitis is the most common cause of abortion, stillbirths and neonatal infections. The major clinical challenge when managing pregnant mares is the identification of the early stages of infection and inflammation of the placenta, i.e. determining the diagnosis of subclinical placentitis. Early diagnosis of placentitis allows prompt treatment intervention and increases the chances of carrying the pregnancy to complete fetal development. Our study has been designed to identify a combination of clinical parameters that are diagnostic of subclinical placentitis. For our First Hypothesis (H1): Physiological parameters change during the different phases of a healthy pregnancy, we will establish normal clinical, immunological, and hormonal parameters in pregnant mares without placentitis by longitudinally collecting data before, during and after gestation. Physical and reproductive examinations, and peripheral blood samples will be collected from the same mare (n=10) during heat (5 to 7 days prior-ovulation); 30 days post-ovulation; at 90, 150, 210, 240, 270 and 300 days gestation; and 7 days post-parturition. This design will allow measuring changes through time in the same mare, and establish baseline physiologic parameters through gestation among a group of healthy mares. For our Second Hypothesis (H2): Differences from physiological parameters of a healthy pregnancy indicate (subclinical) placentitis, we will measure clinical, immunological, and hormonal parameters that indicate early stages of placentitis (subclinical) when compared to mares with a healthy pregnancy. Pregnant mares will be randomly enrolled and evaluated monthly for signs of placentitis onset. Physical exam, reproductive exam and blood samples will be collected at 90,150, 210, 240, 270 and 300 days of gestation. Given that we will be looking for naturally-occurring placentitis, and the number of placentitis cases can be variable, we will screen at least 50 mares in order to achieve a goal of at least 10 mares in each group in a 2-year study. Mares will be assigned ‘subclinical placentitis’ (affected, expected n=10)’ or ‘healthy pregnancy’ (age-matching healthy pregnancy control, at least n=10) according to the reproductive exam data through pregnancy. By measuring differences in these parameters between normal pregnancy and pregnancy with placentitis, we will identify a combination of markers that can improve the ability to diagnose placentitis as early as possible during gestation.