Long Gestations

Do you have information on mares which have had exceedingly long gestations with loss of the foal at birth and premature placental separation? My mare had one pregnancy that resulted in the above problems and is now in her second pregnancy. What are the chances that she will have a repeat incident? How should she be monitored as she nears her foaling date?

Cheryl

There are a number of causes for prolonged gestation and premature placental separation. First of all, gestation length in the mare is extremely variable. Normal gestation length is considered to be 325 to 360 days. Mares which foal early in the year--January through February--carry their foals longer than mares which foal in June and July. So, what you consider prolonged might fall within normal limits.

There are pathological conditions that do result in prolonged gestation. The two most common are fescue toxicosis and degeneration of the uterine lining. The first condition can occur in any mare on pasture that contains fescue grass infected with an endophyte.

The endophyte seems to affect the neuroendocrine system responsible for timing of delivery. Most fescue grasses contain the endophyte because it grows better in drought conditions when infected. Other symptoms that pregnant mares might have if pastured on endophyte-infected fescue pasture is minimal udder development, minimal or no milk production, premature separation of the placenta, an oversized foal, and/or the delivery of a large, weak foal. These problems can be eliminated by management techniques or treatment.

The management technique is to remove the mare from the infected pasture grasses at 285 to 290 days of gestation. If this is not possible, then the drug domperidone can be administered by your veterinarian in the last few weeks of gestation. This drug is not without complications. Mares which are put on this drug will make an udder and will stream milk before delivery. Therefore, a source of colostrum will be needed for the foal at birth.

The second condition, uterine degeneration, usually occurs in older mares (greater than 12 years old) which have had several foals. It also can occur in older mares which were not bred for the first time until their mid- to late teens because they had an extensive show or racing career.

The mare is very different from other animals in that the placenta does not attach until late in the pregnancy, around 60 to 100 days. The pregnancy is maintained by the secretions of the uterine glands. Once the placenta attaches to the mare's uterus, the uterine glands begin a second role--ensuring that the microcotyledon (the placental/maternal attachment) receives nourishment.

If there is degeneration of the uterine glands, then the glands do not produce the correct quantity of "uterine milk" and the pregnancy suffers. The foal grows more slowly in utero, and therefore it takes more time for it to develop.

There is an old, wise saying about delivery: The foal picks the day it is to be born, and the mare picks the time. This is really true. If the foal is not completely developed, the pregnancy continues until the foal has finished developing.

Was this foal weak at birth? Did it have an infection? Was the placenta abnormal in appearance? Is your mare more than 12 years of age? If you answer yes to any of these questions, you must consider your mare to be at high risk of having a problem.

It would be best to have a close working relationship with your veterinarian so that he/she is alerted to the probable day that the mare might deliver. This will ensure that you have immediate help. There are a number of clinics and veterinary hospitals in the country that will foal your mare if she is at high risk of having problems. I hope that this information is helpful, and good luck.

About the Author

Michelle LeBlanc, DVM, Dipl. ACT

Michelle LeBlanc, DVM, Dipl. ACT, is a theriogenologist (reproduction specialist) for Rood and Riddle Equine Hospital in Lexington, Ky. She was previously a professor in equine reproduction at the University of Florida. Her interests deal with mare infertility, embryo transfer, placental infections in mares, and acupuncture in infertile mares. She owns Thoroughbred/warmblood crosses and she competes in dressage.

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