Season determines behaviour, metabolism and reproductive activity in many animal species, including horses. Even in domesticated horse breeds metabolic activity is reduced in winter. Although these effects are known since a few years, effects on pregnant mares and their foals have not been investigated so far. Researchers from Vetmeduni Vienna have now demonstrated that seasonal changes during winter have a strong influence on pregnancy and foetal development. Foals born early in the year are smaller than herd mates born at a later time and these differences persist to at least 12 weeks after birth. The study has recently been published in Theriogenology.