Embryo Evaluation

Information gathered helps determine whether a horse's embryo is likely to survive.

From The American Quarter Horse Journal By Dr. Patrick M. McCue

Embryos are routinely evaluated after collection and prior to transfer into a recipient mare. The evaluation usually takes only a few moments to perform and can provide valuable information as to the probability that a given embryo will survive after transfer. A variety of embryo characteristics are examined including developmental stage, size, shape, color and the number and compactness of cells. An embryo is then given a numeric grade based on the observed characteristics.

Developmental Stage Generally, an embryo's development is directly related to its age. The development of an equine embryo increases each day after ovulation.

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The size of an embryo is also typically related to the age of the embryo. A Day 6 equine embryo is approximately 0.2mm in diameter while day 7, 8 and 9 embryos are about 0.4, 1.1 and 2.2 mm in diameter, respectively. Only small embryos of less than 0.3 mm in size are suitable for freezing, so it is important to flush the embryos before they reach Day 7.

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Grade refers to the quality score of an embryo. A scoring system has been developed that ranges from 1 to 5. A Grade 1 embryo is considered excellent. It has a spherical shape and cells of uniform size, color and texture. Conversely, a Grade 5 embryo would be totally degenerated or dead.

The Difference

During assessment, embryos can sometimes be confused with unfertilized oocytes. Though oocytes typically stay within the oviduct, they are occasionally recovered during an embryo flush. The easiest way to tell them apart is that an embryo is spherical and can roll across the bottom of a Petri dish while examined, while an unfertilized oocyte is flat and cannot roll. Pregnancy rates in recipient mares are correlated with the grade or quality score of the embryo prior to transfer. Grade 1 and 2 embryos have approximately the same transfer success rate, whereas Grade 3 and 4 embryos have significantly lower initial pregnancy rates after transfer and a higher incidence of embryonic loss if a pregnancy is initially established.

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Multiple Foal Production Numbers from Registration

How many foals are mares typically having through the use of embryo transfer under the multiple foals rule? According to AQHA registration records, there have been 6,213* sets of multiple foals born, all foaling years combined (a mare is counted for each set of multiple foals she produced). In that number, there were:

  • 4,830 sets of two foals
  • 1,067 sets of three
  • 253 sets of four
  • 47 sets of five
  • 11 sets of six
  • 4 sets of seven
  • 1 set of nine foals

Those 6,213 sets represent 14,251 registered foals, or an average of 2.29 foals per set. *These numbers are as of February 5, 2009.

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