Borrowed from human medical terminology it refers to one third of the duration of gestation. In mares pregnancy duration averages 338 days (~11 months) with a range between 230 and 360 days. Colt foals tend to have a slightly longer gestation by a few days. So, with this in mind the first trimester in a pregnant mare lasts from service to 110 days, and the third or last trimester from around 225 days to term.
From a nutrition standpoint we regard the third or final trimester, or from the 8th to 11th month as a time when the ration may need to be adjusted upwards to cope with the increased nutrient demands of the rapidly growing foetus. It is also an important time to ensure that mineral and trace element intake is balanced and sufficient because this is when the foetus lays down stores to carry it through the first few weeks of independent life. Mare weight and condition at this time is also important as obese mares tend to have more trouble with difficult foaling.