Q. I bred my mare and she has been pronounced safely in foal. The problem is that I still want to ride her this spring and possibly summer but I do not want to do anything that will jeopardize either my mare's health or her pregnancy. What is reasonable?

A. The main risks to pregnancy from more than casual exercise are twofold: trauma to the uterus and its supporting structures from excessive movement and the swinging motion that a heavy fluid-filled uterus makes during exercise and from circulatory stress to the uterus and fetus.

The uterus and ovaries are suspended from the bottom of the spine and the abdominal cavity by strong, broad ligaments. Running in some of these ligaments are the large blood vessels that support the reproductive structures.

As a horse gallops and canters or spins and jumps, stress can be placed on these ligaments and occasionally damage to the blood vessels can occur. If a major artery is stretched and damaged then significant and, rarely, fatal blood loss can occur. Trauma to the fetus from excessive exercise-induced movement is possible but not likely.

Intense exercise that dehydrates the horse or significantly reduces its electrolyte reserves can cause alterations in blood flow to the uterus or can affect uterine muscle function. Either of these problems can lead to complications with fetal development and possible fetal death and abortion.

That being said, the average equine fetus is no larger than a cat up until the sixth month of a normal 340(+/-)-day pregnancy. Historically, a six-month pregnant mare recently won a grade one stakes race and went on to foal normally. Endurance is different in that the swinging motion during exercise is kept up for longer periods of time than a flat race or other short time frame exercise like reining, cutting, jumping or dressage.

It would be advisable to restrict pregnant mares to short (25 miles or less) distances and only for the first trimester of pregnancy. Pace should be what the mare easily tolerates and she should never be allowed to become dehydrated or stressed to the point where respiratory rate or heart rate is significantly elevated. A well-muscled, fit mare does foal more easily though so regular exercise is to be encouraged for as long throughout the pregnancy as the mare remains willing and comfortable.

Endurance competition will likely be beyond the physiological reach of most horses very soon after the first few months of pregnancy but trail rides at low speed with plenty of walking interspersed with more active gaits may be well tolerated and actually beneficial for most horses.

If you pay attention to your mare you will know when she is no longer enjoying the work and should be rested for the rest of the pregnancy. A healthy equine fetus is remarkably tough and pregnant wild horses must gallop on occasion and traverse rough terrain but not for long periods of time with sustained activity. Err on the side of caution and save most of the competition for post-weaning.

Q. My mare always seems to come into heat at endurance rides. She is not excessively hormonal at home but rides seem to put her "in the mood." What can I do?

AThe normal equine cycle is 21 days so the average mare will be in estrus for seven to 10 days, ovulate and "go out," then cycle back in 16 days or so. If your mare is regular then you may be able to plan to attend rides around her cycle but that is rarely possible and few mares are that regular.

For some mares the stimulation of transport, the novel ride environment and the presence of many other horses is enough to stimulate hormonal triggers and they may show estrus behavior even if they are not typically supposed to cycle at that time.

These mares may be difficult to handle at rides. They may be nervous, aggressive or otherwise unfocused, resulting in poor performance. Cycling mares are more commonly associated with cases of tying up and, even if well-behaved, a mare in heat may disrupt the concentration and focus of other competition horses around her.

One method of control for mares is the use of progesterone. This is the hormone produced by the body that maintains a pregnancy. Progesterone essentially stops the cycle of the mare and allows the uterus to provide a stable environment for the developing fetus.

Use of progesterone in performance horses is a case of using chemicals to fool Mother Nature. By giving your horse progesterone in its many forms you are essentially trying to convince the horse's body that it is pregnant which shuts down the cycle and keeps your mare stable emotionally and physiologically. Progesterone is most commonly given as Regumate, which is a daily oral product. This drug can be absorbed through the skin and can also disrupt human reproductive cycles so care and caution should be exercised when handling and administering Regumate. There is an injectable progesterone form and an implantable form as well.

Progesterone is considered a legal drug for competition when used in mares for estrus suppression. AERC's drug rule explicitly lists Regumate as an allowable substance in Appendix C of Rule 13. In international endurance competition a report of its use must be filed under USEF and FEI rules. Side effects to progesterone use are minimal though you will need to allow the mare a full season to a full year after stopping progesterone use for her to clear her system, begin cycling normally and be able to breed successfully. Correct use of progesterone in performance horses, however, can greatly help some horses keep their heads in the game.