Size Matters: Month Foals Are Born Affects Size by Sarah Coleman|08.07.201708.05.2017|7:45am3:58pm Researchers have determined that foals born in the winter are smaller than those born in warmer months. It's believed that a seasonal reduction in energy metabolism in the mare affects fetus size. Interestingly, the birth weight was not affected, but there were other important differences. Foals born near January, when temperatures tend to be coldest in the Northern Hemisphere, were shorter at the withers and had shorter legs than those foals born closer to the spring window. Seasons greatly affect animals, including horses, regarding metabolic, behavior and reproductive activity, reports HorseTalk. Researchers from the University of Veterinary Medicine, Vienna, noted that the size differences in foals lasted up to 12 weeks after birth. In the winter, horses reduce their metabolic activity in an effort to reduce heat loss; during the last stages of pregnancy is also when the fetus should be growing rapidly. Foals born in the coldest months of the year have more-limited nutrients available to them. To test the hypothesis that foals born in winter are smaller, the researchers studied 27 broodmares and their foals at the Graf Lehndorff Institute, a joint research unit of the University of Veterinary Medicine, Vienna, and the Brandenburg State Stud at Neustadt, Germany. The mares and foals were placed in one of three groups based upon when they foaled. Group One consisted of mare that foaled between February and early March; Group Two was foaled between early March and early April; Group Three was foaled from mid-April to May. Each foal was weighed and had a variety of parameters applied to them to test their size between birth and 12 weeks old. Also, the size and weight of the placenta were recorded at foaling. It was determined that all foals born in February were smaller than those born later in the year. In addition, the placenta size and weight were smaller in winter-foaling mares, indicating a reduced nutrient transfer to the fetus. In the wild, foals are very rarely born in winter; as a horse's gestation period is 11 months, most foals are born when temperatures and the nutrient supply would favor their survival. It was noted that although winter-born foals may take up to 12 weeks to make up for their size deficit, that these foals can still be months ahead of their later-born counterparts with regard to health and growth. Read more at HorseTalk.