The objective of this research was to evaluate the effects of supplementing pregnant mares with omega-3 fatty acids and how this may affect the suckling foal. Pregnant mares (n = 18) were randomly assigned to 1 of 3 diets beginning 28 d before their expected foaling date until 84 d after foaling. Diet 1 was a commercial feed fed to meet NRC requirements, based on forage analysis; diet 2 was diet 1 plus a blended fish oil; and diet 3 was diet 1 plus a blend of fish and soybean oil. Blood samples were collected 28 d before their expected foaling date, and at 14 d increments. Placental efficiency was calculated as a ratio of placenta weight:foal weight. Milk samples were obtained at foaling and on remaining blood collection days. Body weights were recorded the same day blood samples were collected. Placentas were weighed and ~5 g sample taken. No differences were found for mare plasma IgG (P = 0.1318), serum insulin (P = 0.3886), plasma glucose (P = 0.2407), or milk IgG (P = 0.1262) concentrations for treatment or time period of sampling. Mare packed cell were not different (P = 0.0885) among treatments; however, decreased (P < 0.0001) as the trial progressed. Mare body weight and body weight change were not different (P = 0.5704; P = 0.08265, respectively) among treatments, or relative to time of sampling. Mare body weight change did differ (P < 0.0001) relative to time. Foal plasma IgG (P = 0.2767), serum insulin (P = 0.4843), or plasma glucose (P = 0.1204) were not affected by treatment or time of collection. Foal packed cell was not different (P = 0.6275) among treatments, however, there was a difference (P = 0.0005) relative to time of sampling. Foal body weight change and total gain were not different among treatments (P = 0.6825; P = 0.8220, respectively); however foal body weight were least (P = 0.0041) for foals consuming diet 1 and greatest for foals consuming diet 3, with foals consuming diet 2 being intermediate. Foal body weight change decreased as the trial progressed (P < 0.0001). Placental efficiency and nitrites were not different (P = 0.1631; P = 0.5604, respectively) among treatments. Research should be conducted to evaluate supplementation earlier in gestation.