The probable identification of monozygous twin embryos in mares

Observations based upon ultrasonic examination in early pregnancy of over 6000 brood mares between 1988 and 1995 identified 956 sets of multiple pregnancies. Ovaries of all mares were examined during estrus and up to four days after ovulation, and again at early (13 to 17 days) and later pregnancy diagnoses. By detailed recording of all identifiable follicles and corpora lutea, it was concluded that 18 twin and seven triplet pregnancies resulted from only single and double ovulations, respectively. The apparent incidence of monozygous twin embryos was at least 2.6%.