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
A portable, compact electronic device has been developed to detect pregnancy in the sow. Accuracy has been reported to be 90-95 percent in the 30 to 80 day post conception sow or gilt. The instrument is able to detect fluid in the gravid uterus with ultra-sound waves. Diagnosis is rapid and can be made on the farm.
Berthelsen, John and Stevermer, Emmett
"A New Method of Early Detection of Pregnancy in the Sow,"
Iowa State University Veterinarian: Vol. 35
, Article 6.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol35/iss3/6