The gestation period of a horse is approximately eleven months. For the first several months, it will not be obvious if the mare is carrying a foal. If you suspect your mare is in foal, it is essential she is cared for properly right from the beginning of her pregnancy, for her own health and the health of the foal. Sometimes people feel that nature will take care of things. However, this doesn't always lead to the best outcome.
With proper care early in the pregnancy, potential health problems that could affect the reproductive health of your mare, and the health and life of your foal can be avoided.
Things That Don't Work
There are a lot of folk methods for telling if your mare is carrying a foal or not. Holding a threaded needle, ring or nail tied to a string over your mare's stomach is not a reliable method of determining a mare's pregnancy. Another method involves watching how the mare shakes. If she shakes only her head and neck, and not her whole body, she's in foal. The theory is that she is protecting her foal by not shaking it around.
The lack or presence of a heat (estrus) cycle is not a sure indicator of pregnancy. Some mares will have a heat cycle despite being in foal. Some mares may not show an obvious heat cycle, especially through the fall and winter months.
Pregnancy is impossible to determine early on simply by looking at the mare.
Some mares, especially those who've not carried a foal before may not 'show' very much at all. Others have a well sprung barrel that looks like they are in foal all the time! For months after I bought my mare Trillium people told me I was in for a 'surprise' because she looked like she was in foal.
Even a vet thought she might be in foal. Because I knew she hadn't been anywhere near a stallion for at least a year before I bought her, I was confident there was no surprise foal.
Not all mares show obvious signs of being in foal, even late in the pregnancy. While some mares may look fuller and have milk flowing down their back legs for weeks before foaling, others may not.
How to Tell and Why It's Important
The only way to be sure your mare is in foal and that the pregnancy is a healthy one is to have her examined by a veterinarian, preferably that specializes in equine reproduction. This should be done about fourteen to eighteen days after the mare has been bred. At this point, the veterinarian can determine if the mare is carrying twins. Twin pregnancies are a major cause of re-absorption and spontaneous abortions in horses. If your mare is carrying twins, one of the embryos can be 'pinched off' giving the other embryo a better chance for normal development. It seems cruel, but the chance of a mare successfully producing strong twin foals is very slim.
The mare should be checked again at about forty days to be sure she's still in foal and that there is no uterine infection that needs attention. Because you'll know if the pregnancy is healthy early on and how far it's advanced you will know if things are progressing normally, or if there are signs that aborting is possible.
Feeding and Management
You want to determine early on whether you mare is in foal because you're feeding and management may change slightly. You will need to make sure your mare has the best-quality hay or pasture, salt and minerals. If your pasture grass contains fescues, you may want to remove the mare. You may want to separate your mare from the pasture bullies who could injure her, making the foaling process more difficult for her.
While it's important to maintain a regular vaccination and de-worming medication throughout the gestation period, your mare shouldn't receive these medications for the first three months. Some vaccines and parasite control medications can interfere with fetal development. Your veterinarian is the best resource for advice about which medications are safe to give your pregnant mare at what stage.
Yes, it costs money to have your mare properly examined by a veterinarian, but the cost is small in comparison to the total cost of raising a foal—or losing a foal or the mare. Breeding a mare is not a cheap way of getting another horse. In fact, raising a foal can be one of the more expensive ways of making another horse!
Book Recommendation: Blessed Are the Broodmares