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Equine Reproduction Basics

from Ovulation to Conception
  • At ovulation the mare sheds an egg from her ovary, and the egg quickly travels into her fallopian tube.
  • The mare is inseminated post-ovulation.
  • Within minutes, the sperm is drawn into the fallopian tube by uterine contractions, not swimming. 90% of mares conceive at this time.
  • After fertilization, the embryo will remain in the tube for five or six days, before moving into the uterus.
  • Following 10 days of moving around the uterus, the embryo becomes more stationary.
  • At about 37 days, the embryo attaches to the wall of the uterus by endometrial cups, which are the beginnings of the placenta.
  • The placenta produces progesterone, to help maintain the pregnancy. Before its development and attachment, blood levels of progesterone support the pregnancy.


  • The highest incidence of early embryonic death occurs before day 11, when the embryo enters the uterus. Often it occurs by day five or six. This happens in about 24% of normal mares.
  • The next highest incidence of embryonic death occurs between days 14 to 40, in about 17% of normal mares.

Factors contributing to early embryonic death:

  • When mares are bred post-ovulation, the egg may not live as long as the sperm, and may have aged and become defective.
  • Stress caused by environment and management may cause lethal stress to the embryo.
  • Hormone deficiencies and imbalances, the uterine environment, and the age of the mare can all be contributing factors to early embryonic death.


  • The national average for first cycle conception using fresh cooled semen is 60%.
  • The national average for first cycle conception using frozen semen is 60-70%.
  • The average number of cycles per conception using frozen semen is 2.5 per mare.
  • The end-of-season conception rate for both fresh cooled and frozen semen is about the same--70-75%.
  • Most mares in good breeding condition will achieve a pregnancy within two cycles, but many settle on one cycle. Few mares take three cycles to conceive.
  • The conception rate with frozen semen is at least as good as, if not better than, the rates of live cover and fresh-cooled breeding. When breeders purchase frozen semen with no guarantee, they are very careful to select an excellent mare candidate and an experienced veterinarian, thus improving their conception rate.