The hormone progesterone is commonly used in pregnant mares as some believe it helps to maintain pregnancy and prevent abortion. In reality, most of the mares supplemented with progesterone probably do not need it. Dr. Peter Daels indicates that progesterone is the only hormone that needs to be replaced to maintain pregnancy. You can even remove the ovaries in mares during the first 3 months and they will still remain pregnant as long as they are supplemented with progesterone. For the first 100 days of pregnancy, all of the progesterone is produced by a structure on the ovary. After 100 days, the placenta produces enough progesterone to maintain pregnancy. Use of progesterone after 100 days is not needed in most cases and since it costs about $3 per day, it is quite expensive.
Another reason to not use progesterone unless necessary is that a mare will not come back into heat if she aborts. For this reason, it is important to check mares regularly to make sure they are still pregnant when on Regu-Mate. Although there are several products with progesterone in them, the only ones shown to work in mares are Regu-Mate and progesterone injections. In early pregnancy, blood tests can determine if a mare needs to be supplemented. However, in later pregnancy, the progesterone produced by the placenta does not show up on blood tests. It is believed by many that mares that become ill during later pregnancy may benefit from Regu-Mate. Since blood tests can not determine if it is needed or not, it may be important to use Regu-Mate in pregnant mares while they are ill to help maintain pregnancy.