Although pregnant women are encouraged to exercise in most cases, it is not known whether pregnant horses should be exercised or not. It was determined that exercise prior to and on days immediately after fertilization was detrimental to embryo recovery rates in horses used as donors for embryo transfer. Also, it was determined that exercise prior to 16 days of pregnancy was detrimental. However, a study performed on mares that were 9 months pregnant did not find any elevations in fetal heart rate and the authors concluded exercise did not induce stress in the fetus.
Many times owners call our office and ask if it is okay if they ride their pregnant mare and how much exercise is too much. We really don't know the answer to that question. I usually tell people that light riding is okay up until eight months of pregnancy but that is just my opinion and there is no scientific evidence to support that recommendation. However, I have never seen a problem in any pregnant mares from light exercise early in pregnancy. A recent study will help answer part of this question as a group of veterinarians from Florida and Clemson exercised eight mares from day 16 to day 80 and analyzed hormones and ultrasound appearance of the pregnancies. Mares were exercised 30 minutes for the first two weeks and then 45 minutes at a long trot and canter with breaks every 5-10 minutes for the duration of the study. Results indicated exercising pregnant mares from day 16 to day 80 of pregnancy did not appear to be harmful to the growing fetus. An interesting finding was that in exercised mares, the fetus was longer, which is similar to studies in women who exercise in early pregnancy as their fetuses have longer fetal crown to heel length.