Pregnancy testing is commonly performed at 14, 30 and 45 days post ovulation. For mares where an ovulation check has not been performed a rough guide for the first pregnancy test is 15-16 days after the last service.
At 14 days post ovulation the pregnancy(s) are visible as a small approximately 1 cm fluid filled sac (vesicle). At this stage the pregnancy is still moving around the uterus. If twin pregnancies are detected one of the pregnancies may be reduced with a high chance of the other pregnancy surviving. (Please note that twin pregnancy in mares has a high complication rate often leading to loss of one or both foals and possible foaling complications for the mare. If detected it is generally recommended that the pregnancy be reduced to a singleton if possible or if not possible that both pregnancies be aborted.)
At 30 days post ovulation the pregnancy(s) are visible as fluid filled sacs with an embryo and its heartbeat can also been seen. It is still possible to reduce twin pregnancies to a singleton but there is a higher chance of losing both the pregnancies in that attempt. If a problem is detected with the pregnancy it is possible to use medication to abort the pregnancy with a high chance of the mare coming back in season for service.
At 45 days post ovulation the pregnancy(s) are visible as fluid filled sacs with the embryo and its heartbeat. It is much more difficult to reduce twin pregnancies to a single pregnancy at this stage, and even if the pregnancy is aborted using medication the mare may not come back in season for several months.