The average gestation length for a mare is usually 11-12 months. That creates a range of 320 days to 362 days. Most fares will foal at some point within that timeframe. Some mares have foaled successfully before or after this average timeframe, however, so the averages listed here are intended to be a framework more than a guideline to determine a foaling date.
Individual mares tend to have their own gestation average as well. Although most mares fit into the average time frames, some mares may naturally have slightly shorter or slightly longer gestation periods for every foal. Unless the health of the mare is at risk, the pregnancy should be allowed to reach its natural conclusion.
Most mares will only produce 1 foal per year. A pregnancy which involves multiples can be quite dangerous. Mares that do experience a twin pregnancy usually do so because they’ve experienced multiple ovulations.
What Happens to the Reproductive System?
When a mare begins to ovulate, there are changes in hormone levels that can change the characteristics of the reproductive organs. These changes help the mare be prepared (or prevented) from conceiving.
The uterus of the horse sees an increased level of estrogen. That causes the uterus to lose its tone and feel heavier. This increases muscular tone around the area. Then the cervix begins to relax until ovulation occurs, when it will reach a peak point of relaxation. High progesterone levels could cause the cervix to close instead.
The reproductive organs become engorged with blood and secretions begin to increase. This process allows for the breeding process or artificial insemination to begin.
When to Check for a Pregnancy in Mares
A mare may not show any visible signs of being pregnant for some time after an ovulation occurs with a successful fertilization. For the first 3 months, the only visible sign of the pregnancy may be the lack of an estrous cycle.
If a pregnancy is attempted, an ultrasound examination is usually carried out within the first two weeks after breeding takes place.
Should the ultrasound be inconclusive, it may take 60-90 days, depending upon the breed, for urine and blood testing to confirm a pregnancy. It may also be possible for a pregnancy to be physically detected by a veterinarian in 6-8 weeks after breeding takes place.
After the 3-month examination, the foal begins to develop rapidly. At 6 months, most mares will look visibly pregnant. As the foaling date approaches, the mare continues to grow. A yellowish fluid, which is somewhat sticky, begins to be produced about 2 weeks before the foaling date. Around the same time, the udder will begin to expand.
With about a week before the foaling date to go, the yellowish fluid from the udder begins to turn into a milk product for the foal. The foal may appear to drop as it prepares for the birthing process. At this point, a mare should be isolated and stalled, with frequent checks provided, to ensure proper health. The foal may appear at any time and the mare may give birth standing up.
Why Are Twins So Rare with Horses?
Double ovulations occur at the same time in most instances, but it is possible for a mare to ovulate two eggs that are several days apart. The sperm from a stallion can survive within the reproductive tract for several days, so even if the ovulation of each egg occurs 7 days apart, it is possible that both eggs will be fertilized from the one sperm deposit.
That means a mare may be having a gestation period for two embryos that are of different ages, attempting to coexist with one another.
It takes an embryo at least 11 days for it to be seen on an ultrasound administered by a veterinarian. Because of the ovulation schedule of the mare, a second ovulation that results in another embryo forming could be missed upon an initial examination.
For that reason, the recommendation of many veterinarians is to schedule an examination, which includes an ultrasound, between 13-14 days post-ovulation instead of 11 days after.
It also puts the mare on a strict deadline. Because multiples can be a life-threatening pregnancy, the second embryo is usually eliminated. The 16th day post-ovulation is the optimal time to remove the second embryo because the two may fuse afterward, which makes removal unlikely and the entire pregnancy could miscarry if attempted.
Multiple ovulations of any number are possible, but anything beyond twins is considered extremely rare. Multiples for Warmbloods and Thoroughbreds have been increasing, however, and may occur in 1 out of every 4 estrous cycles.
It is possible for a mare to carry twins to full-term and to have the offspring and the mare be healthy throughout the entire process. This, unfortunately, tends to be the exception rather than the rule when it comes to pregnancies with multiples.
What Happens if a Foal is Breeched?
A foal is considered to be in a breech position when the hindquarters appear first during the birthing process. When one of the forelimbs are bent backwards during the birthing process, this may also be referred to by some as a breech. When this occurs, there is a high risk of injury to the mare. The foal may also require assistance to be removed from the birth canal in a timely manner so the risk of injury to it is lower.
Many breech pregnancies can be detected before birth with a routine veterinarian examination. If detected, the foal can usually be rotated so that it can be born in the regular position. If the birth process has begun and a foal has breeched unexpectedly, then this is treated as a medical emergency for the mare and foal.
A breech is more likely when a pregnancy includes multiples.
The history of horse breeding goes back as far as humanity has kept records on horses. There may be more than 6,500 years of breeding history that is known, with hundreds or thousands of years that go beyond that. The Bedouins were the earliest people document their horse breeding efforts and written pedigrees of some Arabians go back nearly 700 years.
Yet, over all this time, the gestation period for horses has remained relatively constant. By following good veterinary practices and understanding the individual differences that mares may have with a pregnancy, it is possible to produce a successful foal on a regular basis.
Care may need to be taken for pregnancies which involve multiples, but thanks to modern knowledge and techniques, more mares and their offspring can go on to live healthy and productive lives.