Pregnancy Diagnosis in Sows and Gilts

Gordon King, Animal Science, University of Guelph

Physical Methods. Domesticated animals rarely exhibit any signs of sexual behavior one cycle interval after a fertile mating. Thus, for animals mated on known dates, demonstration of sexual receptivity when the next estrus period is due indicates no conception from the previous service. In contrast, the absence of sexual behavior is strong evidence for pregnancy. Sows should remain in the mating area with daily boar contact for more than three weeks after service. Once they pass three weeks without estrus signs they can be presumed pregnant and moved to the gestating area. It is unfortunate that many producers and even some of their professional advisers have apparently forgotten that female domesticated mammals have this inherent non-pregnancy indicator.

Rectal palpation is a useful procedure for pregnancy diagnosis in sows but cannot be used in gilts because of the smaller body size. European and Asian veterinarians perform rectal examinations on sows more often than their counterparts in America or Australia.

A number of ultrasound units are commercially available for pregnancy diagnosis. The most sophisticated and accurate are real-time ultrasound models complete with rectal probes for large or surface probes for smaller animals. A number of simpler ultrasound devices using either the Doppler or amplitude-depth principle are also available. Although these cannot be used as early in gestation or with the same accuracy as the real-time units, many are reliable for diagnosing pregnancy when used by experienced operators. The costs are substantially lower than real-time units and they are routinely used in some pig units.

Vaginal biopsies can be collected from sows for 30 to 90 days after mating and submitted to laboratories for microscopic examination. In pregnant animals the layers of stratified squamous epithilial cells lining the vaginal wall are reduced to only two or three, in contrast to the many layers found in cycling females. The method is accurate but requires special instruments for collection and subsequent shipment of specimens to the laboratory where they must be processed and examined by experienced personnel. Thus, although the test can be conducted for relatively low cost, it is not commercially available in most countries.

Chemical Methods. The first chemical test for confirmation of pregnancy was probably the Cuboni test introduced in 1934. In this procedure, sow urine is mixed with benzine and sulphuric acid that reacts with estrogens. The development of a fluorescent like sheen indicates substantial estrogen concentration in the sample and presumably pregnancy. The method could be used during or after the fifth week of gestation.

The development of immunological methods for determination of hormones in biological fluids provides the basis for a number of tests. Protein substances such as human or equine chorionic gonadotrophin can be readily detected by immunological methods but none have yet been identified in pigs. The steroid hormone, estrone sulphate increases in concentrations during porcine pregnancy. Plasma values are elevated substantially during the third week and significant amounts are excreted in urine and feces. A detection of substantial estrone sulphate in plasma, urine or feces, four to six weeks after mating, is a positive diagnosis of pregnancy in sows. This test was available commercially in UK but, as it involved the expense of sending samples to a central laboratory, has been discontinued.

The most recent innovation in methodology for detection of hormones involves the use of enzymes in place of radioisotopes. With these methods there is no need for the elaborate safety precautions to protect users from radiation so tests could be performed on farms.. Commercial kits for progesterone and estrone sulphate are or have been available using enzyme procedure. Some of these rely on simple color change and matching to standard images that indicate some or no hormone in the sample. The operational steps and measurements are simple, so complete tests can be conducted in stables or farm kitchens.

Although measurement of steroid hormones like progesterone in urine or feces at specific times after mating is useful in determining pregnancy status, negative results have much greater accuracy than positive ones. Ideally, pregnancy tests should be based on detection of materials that are produced only by viable conceptuses. The substantial concentration of estrone, which is present as the steroid conjugates in maternal fluid, is one such compound. Others, like early pregnancy or platelet activating factors could, with suitable improvement in detection methodology, be developed into very early pregnancy tests.

Livestock producers must recognize that there is one inherent problem that will continue to be associated with even the most reliable pregnancy test. No matter how accurate the procedure, the results are only valid for the time when the sample was collected or the physical examination conducted. There can be no guarantee that the embryonic or fetal piglets present at the time of testing will continue to develop normally throughout the remainder of gestation.

Return to Reproductive Management of Pigs.