Equine Pregnancy: Why Things Go Wrong

Many horse breeders know that it can be a challenge to get a mare pregnant and keep her pregnant. There are a number of factors that can lead to a loss of pregnancy, making it important for the breeder to take steps to prevent pregnancy loss and/or be prepared to handle such events.

Problems Preventing Pregnancy First, you have to get your mare pregnant. Below are some challenges you might face during that time.

Endometritis is an inflammation of the uterine lining. Mares that have this will very seldom show any clinical signs.  Because of this, it is important to check for endometritis in advance of breeding with enough time to get the problem cleared up. Doing an endometrial culture and cytology is a simple way of detecting this issue.

The timing of insemination with the mare’s ovulation is critical. The ovum, or egg, loses viability six hours after ovulation, with almost no viability by 24 hours after ovulation. Semen viability is highly variable depending on the stallion and if you are using fresh or frozen semen. The majority of semen will maintain its viability for up to 48 hours after insemination, while some may last four to seven days or just a few hours. Mares ovulate 24 to 48 hours before the end of estrus (heat), with estrus lasting anywhere from two to eight days. If insemination happens too early or too late, the mare has little chance of becoming pregnant.

The mare’s body condition score (BCS) at the time of breeding can also prevent pregnancy. The ideal BCS at breeding is 5 to 6 on the 1 to 9 Henneke BCS scale. Mares being too thin or over weight have decreased reproductive efficiency. Prior to breeding, make sure your mare is in good health and good weight.

A mare’s age also plays a role in becoming pregnant. By 18 years old, a mare’s egg quality begins to decline. As she ages her uterus becomes less healthy, compared to a young mare. Vulva conformation issues can additionally become a concern because it can lead to contamination of the reproductive tract.

Choosing to breed at the wrong time of year will furthermore prevent pregnancy. Mares are seasonal breeders, meaning for a portion of the year they are not cycling. May through August is the natural breeding season and when the mare will most likely be cycling. October through November and April through March is when the mare is going through a transitional period and is unlikely to become pregnant because she is not ovulating as reliably. December through February is when mares are anestrus, or not cycling, making chances of becoming pregnant almost nonexistent. Most breeders want to get mares pregnant as close to the beginning of the year as possible and to achieve this they use artificial light and sometimes hormones to encourage the mare to begin to cycle sooner.

Pregnancy Loss So you got your mare pregnant, but she lost the foal before she was full term. Why did this happen?

Most losses occur within the first couple months of the pregnancy. The early embryonic losses are most likely due to embryonic abnormalities, genetic defects, or stress related. Stress related incidents include severe illness, long distance transport, extreme weather conditions, poor nutrition, and too heavy of a work load.

There can also be later term loses. Some common culprits are equine herpes virus, inflammation of the placenta, umbilical cord abnormalities, and twinning. If twins are caught by ultrasound between 10 and 15 days, which is before placental implantation, a veterinarian should be able to successfully reduce the pregnancy to a single embryo, increasing the chances of a fruitful pregnancy.

Parturition Loss You got your mare pregnant and she has carried her foal to full-term and is in the process of foaling, but you still lose the foal. What went wrong?

Losing a foal during the birthing process is usually due to dystocia, or difficulty birthing. The majority of the time, the umbilical cord gets cut off, causing oxygen deprivation to the foal. Dystocia is generally caused by the foal being in the wrong position and the mare being unable to push it out on her own. When this happens it is imperative to get the foal out ASAP. If this happens quickly enough there is a good chance of saving the foal. Dystocia can also occur because the foal is too big to fit through the birth canal. This issue may be able to be avoided by selecting a stallion that is known to produce smaller foals.  

Steps to Take to Be Successful

Here are some tips on how to help your mare have a successful pregnancy.

• Consult with a veterinarian prior to breeding so you have a game plan
• Research the stallion you are going to use. Know his pregnancy success rate for fresh and/or frozen semen (whichever you are using)
• Plan for using multiple straws of semen if you choose to use frozen semen – not all mares will take with one straw
• Have your veterinarian check your mare for endometritis, make sure she is up to date on all vaccines and that she has been dewormed prior to breeding
• Ensure your mare is a BCS of 5 to 6
• Have your vet do an ultrasound 14-16 days after breeding to check for twins
• Schedule a pregnancy check at 60 days after breeding to make sure your mare is still pregnant
• 10 months into gestation get you mare her core vaccines to improve her colostrum quality
• Throughout pregnancy allow your mare to have some exercise, don’t keep her locked in a stall 24 hours

As always, if you have any questions about this topic or anything else, contact your local Extension agent.