Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
Laboratory methods for early diagnosis of pregnancy have found their greatest application in the human where clinical symptoms of the condition are not manifest until approximately 6 weeks following conception. With domestic animals, the mare stands alone as the animal to which laboratory tests may be applied. Diagnosis of pregnancy by rectal palpation in the mare is desired over laboratory methods because it is faster, more accurate and can be applied earlier in the gestation period. However, laboratory tests are employed on those mares whose delicate rectal mucous membranes may not tolerate manipulation. In highly bred mares with nervous temperaments it may be impossible to perform rectal palpation due to incooperation and possibility of abortion.
Reese, R. L.
"Laboratory Diagnosis of Pregnancy,"
Iowa State University Veterinarian: Vol. 8
, Article 7.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol8/iss1/7