Veterinary Corner from the Vet Corner Archives Part of Horse Previews Magazine website.
Mares are also sneaky.
They often watch you suffer endless, sleepless nights in an attempt to attend the blessed event, and then go ahead and foal when you leave the barn for 10 minutes to get a cup of coffee. For this reason, all foalings should be attended. Maiden mares may show none of these signs, however, whereas some mares may show all of these signs for many days before foaling. Because outward signs can be unreliable, two test procedures have been developed to assist in the prediction process - measuring calcium levels in udder secretions and changes in rectal temperature. Always do this in the evening.