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and its uses

Progesterone is what we call the pregnancy hormone.  It is produced by the ovary.  After the egg is released from the follicle, it collapses to become a crater.  This crater fills with blood and forms a clot. This clot becomes the corpus luetum (CL).  The purpose of the CL is to produce progesterone.  Progesterone prepares the uterus to accept the egg which hopefully is now fertilized.  It prepares the lining to accept and nurture the embryo through pregnancy.

Another important function of the hormone is to suppress activity of the uterus.  By keeping the uterus quiet it cannot contract and dislodge the embryo, thus giving it additional time to signal the mare she is pregnant.  

During the first two to three months of pregnancy, the ovary is responsible for producing progesterone. The size of the ovary increases dramatically as the mare realizes she is pregnant.  The size may increase from a normal of two inches in diameter to as much as six inches.  By the third to fifth month of pregnancy, the placenta assumes responsibility for producing progesterone for maintaining itself. Once this transition is complete, the size of the ovaries will shrink to about an inch in diameter.

Near the end of pregnancy, progesterone, along with estrogen stimulate the udder to develop and start producing milk.  

For the first four months of pregnancy, the ovaries continue to develop follicles.  However, the CL responsible for maintaining pregnancy produces enough progesterone to override the effects of estrogen on the uterus.  If the progesterone level is low in early pregnancy, the uterus may not be able to resist the effects of estrogen. The uterus will start contracting which dislodges the embryo.  This prevents implantation of the embryo, resulting  in its death.  Shortly thereafter, the mare will start showing signs of heat.

The progesterone level in the blood can be measured.  If it is below 4 ng./ml. the mare needs additional progesterone to maintain pregnancy.  This can be provided by oral doses of progesterone in the form of Regumate.  Progesterone can also be given with a weekly injection. These forms of progesterone are continued until the monthly blood test finds the level above 4 ng./ml.  If the blood level is below 2 ng./ml., progesterone will have to be supplemented with Regumate, because the daily administration keeps the blood level more constant.

Mares that we routinely test are:
those in their teenage years
mares that have lost pregnancies during the third and fifth months
mares with a questionable history of carrying a foal
mares that repeatedly cycle too soon for the embryo to implant

Early in the spring we will test a mare to see if she has started cycling. That is when many mares, and especially older mares that are not yet cycling on a regular schedule, will have a very low progesterone level. When the level is low, they will show little sign of heat but just as important may not even show any resistance when teased and mounted by the stallion.

When the mare fails to become pregnant for no good reason, we will often start her on weekly progesterone injections.  She will receive these until she is ultrasound pregnant fourteen days after breeding.  At that time her blood will be tested to see if the hormone level is adequate.  This condition is usually in older mares.  Some of these cannot produce enough progesterone in early pregnancy to keep the uterus quiet while the embryo finds its way to a comfortable place for implantation.  This condition is often the cause of the "every other year mare" that only becomes pregnant and foals every other year.  After being inactive for over six months during the preceding pregnancy, this mare's ovaries are slow to react to the need for progesterone.

Another use of progesterone testing is in the pregnant mare that was left on fescue pasture or hay into the third trimester.  If she has not developed an udder by the last two weeks of pregnancy, we must check her blood for progesterone levels. Fescue will suppress these levels and with low levels the mare will not produce milk.  If her levels are low, she can be supplemented with domperidone to stimulate milk letdown.

We hope this brief description helps explain why we tested your mare for progesterone. Diagnosing low progesterone levels and treating the mare with supplemental hormone until her system begins producing sufficient amounts will result in many more pregnancies that continue through birth.

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