Bigger isn’t always better—and that certainly appears to be the case for both broodmares and their foals. British researchers have confirmed that an overweight broodmare is more likely to give birth to a heavier foal, and that puts the foal at risk for health problems that could last a lifetime.

“Research has shown that heavier foals have an increased prevalence of conformational defects and nonseptic musculoskeletal disorders,” said Sarah Smith, MA, VetMB, MVetMed, Dipl. ACVIM, MRCVS, of Rossdales Equine Hospital, in Newmarket, U.K.

Smith and her fellow researchers examined 66 Thoroughbred broodmares’ body condition scores and evaluated blood work starting at 40 days of pregnancy. They found that the greater the mare’s body condition score—essentially, the more obese she was—the greater her foal’s birth weight.

That contradicts previous study results that have shown that higher body condition scores didn’t affect foal birth weight, Smith said. However, there was a significant difference in her study: The obese mares were already overweight when they got pregnant. In the previous studies, the mares were managed experimentally during the pregnancy to become obese. Scientific research has recently suggested that the metabolic status in early gestation can play an important role in “programming” the developing fetus, the researchers noted.

“In other mammals, we have known for a long time that maternal under- or overnutrition can affect the offspring, and there is increasing evidence that this also occurs in horses,” Smith said. “The aim should be for mares in midrange body condition, avoiding either the lower or upper extremes which may cause problems for the fetus or foal.”

Larger foals might seem “healthier” because they’re more robust, but this is a misconception, Smith said. “This desire for ‘big, strong, healthy foals’ probably stems from the association of small foal size at birth with immaturity and subsequent problems in the immediate post-foaling period,” she said. “This has led to the assumption that if a small foal may have problems, a big foal must be better.”

And, the increased health risks larger foals face don’t appear to balance out with perceived better competitive results, she added. In racehorses, studies have shown that “higher birth weight does not confer a performance advantage, where there is no association between birth weight and racing performance,” she said. Researchers have not evaluated associations between birth weight and performance in other disciplines.

Smith recommended owners track broodmares’ body condition during pregnancy, which she cautioned can be tricky due to their growing foals, using the Henneke body condition scoring system.

The study, “The effect of mare obesity and endocrine function on foal birthweight in Thoroughbreds,” was published in the Equine Veterinary Journal