Multiple ovulations and resulting multiple pregnancies leading to abortions or births of underdeveloped foals are important problems in Thoroughbred reproduction. The incidence of multiple ovulation during one ovarian cycle per mare in 44 Thoroughbred mares was determined by ultrasonic scanning. Twenty-two mares (50%) ovulated two (27.3%) or three (22.7%) dominant follicles during observed follicular phase. Overall 76 dominant follicles were ovulated, including 45 (59.2%) from right and 31 (40.8%) from left ovary. The mares at the age 6-10 years tended to have higher multiple ovulation rate (63.2°/0) than those below 6 or of more than 10 years old (both age groups amounting to 40%). Multiple ovulation rate was found lower in lactating than in non-lactating mares (-16.7 and 51.7%, respectively). No significant differences in the incidence of single vs multiple ovulation were found between the left and right ovary, age, or reproductive status of mares. The pregnancy on day 17 occurred in 7 (15.9%) single-ovulating mares. Twelve multiple-ovulating mares (27.3%) were diagnosed to be pregnant, including 7 (15%) with single and 5 (11.4%) with twin pregnancy. The pregnancy rate did not significantly differ between single- and multiple-ovulating mares.
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