Pregnancy Doesn’t Preclude Exercise in Horses

Pregnant mares can perform moderate exercise through at least the ninth month of pregnancy with no harmful effects on the fetus, according to research carried out at Rutgers University.

Unfit Standardbred mares were used in an inclined treadmill exercise program in which speed was increased at one-minute intervals. The mares were tested when they were nine months pregnant and again eight months later, after they had weaned their foals.

Heart rates and levels of plasma lactate rose in all mares as they exercised, but the rises were not as great in pregnant mares as in post-weaning mares. A possible explanation is that pregnant mares develop greater cardiovascular efficiency to meet the demands of gestation.

Plasma cortisol, a common indication of stress, rose during exercise in nonpregnant mares but not in pregnant ones. Before exercise, resting plasma cortisol levels were lower in pregnant than in nonpregnant mares. Fetal heart rates did not rise during the exercise periods, suggesting that foals were not stressed by the treadmill sessions.