November 14, 2009 - The Third Trimester

  • November 11, 2009
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Trade Zone: The Third Trimester - Click Here to Download PDF

By Heather Smith Thomas

Each phase of gestation has its risks, and the good news for owners who have pregnant mares in the barn this winter is that most losses will have already happened by now, so they’re out of the woods in some respects. Most equine pregnancy loss occurs during the first trimester, particularly during the first 60 days of gestation; after that, the placenta is developing and producing a hormone called progesterone to safeguard the pregnancy. But even though some of the riskier days are behind, it is important that mare owners remain vigilant and monitor the pregnant mare throughout all of gestation, noting any changes in her attitude or demeanor, and any health problems. Important things to look for, especially during the second half of pregnancy, include weight loss or depression, sudden increase in abdominal size, vaginal discharge, periodic mild colic, and sudden udder development or premature lactation.

A mare’s gestation is roughly 11 months, which doesn’t fall into a neat category of thirds as other species’ gestation lengths do (i.e., humans and cattle at nine months). Defining the last trimester as the final third of pregnancy may be a misnomer, because it may entail 3 1/2 to four months in a mare. “In the French literature, a trimester is three months,” said Dr. Ahmed Tibary, a theriogenologist and professor in the Department of Veterinary Clinical Sciences at Washington State University’s College of Veterinary Medicine. “The last trimester would, therefore, be just the last three months of pregnancy in the mare (the ninth, 10th, and 11th month), rather than the final third,” he explained.

All discussions of nomenclature aside, this final trimester is the time the fetus is growing fastest and the mare’s abdomen generally becomes larger. Tibary says this is also when a lot of the changes in the mare take place to prepare for the safe arrival of a healthy foal.

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