Introduction

Early and accurate identification of pregnant and nonpregnant sows and gilts improves reproductive efficiency in commercial swine farms. Detection of returns to estrus after mating, ultrasound devices and other methods have been used for pregnancy diagnosis. Presently, there is not an ideal pregnancy detection technique that is commercially available. The sensitivity (ability to detect pregnant animals and represents the proportion of pregnant animals that test positively), specificity (ability to detect nonpregnant animals and represents the proportion of nonpregnant animals that test negatively) and positive predictive value (proportion of pregnant animals among those that test positively) are used to assess accuracy.

Detection of Estrus

Observation of the sow for failure to return to estrus after mating is the most common pregnancy detection method. This technique is based on the premise that pregnant sows rarely exhibit estrus during gestation, and that nonpregnant sows will return to estrus within 17 to 24 days after breeding. The producers' ability to detect signs of estrus is improved if the sow's behavior is observed in the presence of a boar. Estrous detection can be used as a means of pregnancy evaluation if gestation facilities are designed to allow daily fenceline contact between boars and sows, or if the boar and sow can be placed in the same pen each day.

Few studies have investigated the applicability of estrus detection for diagnosis of pregnancy. Accuracies of 39% to 98% were reported for the detection of return to estrus as a pregnancy diagnostic technique. It was concluded that daily estrus detection throughout gestation provided the best indicator of farrowing rate. False positive tests are obtained when sows become persistently anestrus due to cystic ovarian degeneration or to inactive, acyclic ovaries or become pseudopregnant. When the design of gestation facilities does not allow daily boar exposure to the bred sow, the likelihood of detecting returns to estrus is dramatically reduced. Because of the requirement for special facilities and increased labor, estrous detection often is not a favored method of pregnancy diagnosis.

Hormone Concentrations

Serum concentrations of prostaglandin-F2 (PGF), progesterone and estrone sulfate have been used as indicators of pregnancy. These hormone concentrations are dynamic and considerable knowledge regarding endocrine changes in pregnant and nonpregnant sows is required prior to using these techniques for pregnancy diagnosis. Presently, determination of serum progesterone concentrations is the only test with any commercial application.

The progesterone pregnancy test has been found to have an overall accuracy of >88%. It has >97% sensitivity, but has been found to have a specificity of 60 to 90%. Falsepositive results are common when nonconceiving sows and gilts have delayed or irregular returns to estrus, and when nonpregnant sows and gilt are anestrus due to cystic ovarian disease. In contrast, falsenegative results are rare with this technique which is consistent with the postulate that progesterone is required for pregnancy maintenance in swine. The method's limitations include the necessity of collecting blood and, until recently, the need of a laboratory for analysis. The advent of commercially available enzymelinked immunosorbent assays to measure blood concentrations of progesterone in swine makes the test more practical. These kits have not been well evaluated for use in swine.

The overall accuracy of the estrone sulfate test in the evaluation of pregnancy status has been found to range from 82 to 100%. As with other early tests of pregnancy, animals may be correctly diagnosed as pregnant, but fail to farrow if the fetuses die after the test has been conducted. Quantitative commercial assay kits for the determination of estrone sulfate concentrations in serum from swine are not available. The need to collect blood (or urine) samples limits the practical application of this technique for pregnancy diagnosis in swine.

Physical Methods

Rectal palpation: It was demonstrated that pregnancy diagnosis by rectal palpation of the sow was practical and reasonably accurate. The disadvantages of the technique were that the pelvic canal and rectum were often too small for the procedure to be used on low parity sows. False negative results, presumably due to errors in palpation technique or palpation too early, were more common than false positive diagnoses. Despite the potential application of this technique, it has not gained popularity in North America.

Other physical methods of pregnancy diagnosis include radiography, laparoscopy and vaginal biopsy. These methods are not practical nor feasible in commercial swine production.

Ultrasound Techniques

Mechanical ultrasound devices commonly are utilized because they are easy to use, are commercially available, and perceived as being accurate. Three types of ultrasound equipment are available for pregnancy diagnosis in swine.

Doppler Ultrasound: The Doppler ultrasound instruments utilize the transmission to and reflection of ultrasound beams from moving objects such as the fetal heart and pulsating umbilical vessels or uterine arteries. Blood flow to the uterine artery in the pregnant sow and gilt is detected as a regular 50 to 100 beats/minute while blood flow in the umbilical arteries is detected at 150 to 250 beats/minute.

Two types of transducer probes currently are available for use with the Doppler instrumentsan abdominal probe and a rectal probe. The abdominal probe is positioned on the flank of the animal, lateral to the nipples, and aimed at the sow's pelvis area. The ultrasound waves are emitted and received by transducers and are converted to an audible signal. The rectal probe functions similarly, with the obvious exception of the positioning of the transducer. There were no differences between the accuracies of the rectal and abdominal probes.

Both false positive and false negative diagnoses can be obtained when using either the abdominal or rectal probes. There was increased likelihood of false positive diagnoses if examinations were done when sows and gilts were in proestrus or estrus, or if sows and gilts had active endometritis. False negative diagnoses were obtained if examinations were conducted prior to approximately 30 days, if examinations were conducted in a noisy environment, or if feces became packed around the rectal probe.

AmplitudeDepth (Amode or pulse echo) ultrasound: Amplitudedepth machines utilize ultrasonic waves to detect the fluidfilled uterus. A transducer is placed against the flank and oriented toward the uterus. Since the contents of the gravid uterus differ in acoustic impedance from that of adjacent tissues, some of the emitted ultrasonic energy is reflected to the transducer and is converted to an audible signal, a deflection on an oscilloscope screen, or illumination of a light (diode) or series of lights.

Pregnancies were not confirmed prior to 20 days, but from approximately 30 days until 75 days after breeding, the overall accuracy was commonly >90%. The percentage of false negative and uncertain determinations increased from 75 days until farrowing. These changes in accuracy parallel alterations in volume of allantoic fluids and fetal growth. Our previous studies demonstrated that some models of amplitude depth instruments were more severely affected by inaccuracies. Errors in the placement of the transducer resulted in the detection of a fluidfilled urinary bladder, which yielded a falsepositive diagnosis. Falsepositive results were obtained when sows were affected with endometrial edema from zearalenone toxicosis, pyometra, or when the litter died and was neither aborted nor resorbed. Falsenegative results also were obtained when the test was made before 28 days of gestation or after day 80.

Real-time Ultrasound Scanning: Portable real-time ultrasound scanners were used to evaluate the reproductive tracts of mares, heifers, bitches and for pregnancy diagnosis in sows and gilts. The use and potential accuracy of real-time ultrasound for pregnancy diagnosis in female pigs is described elsewhere in these proceedings.

Besides pregnancy detection, there are other potential applications of real-time ultrasound. Pseudopregnant sows and gilts with uteri containing mummified fetuses were differentiated from pregnant sows. Sows experiencing difficult and/or prolonged farrowing can be examined for piglets retained in the uterus. In addition, sows and gilts with endometritis often were identified and distinguished from females in later stages of pregnancy; however, they are difficult to distinguish from sows that were at 18 to 21 days of gestation.

Summary

The advantages of accurate methods for early pregnancy diagnosis in swine include early detection of conception failure, forecasting production levels and early identification of nonpregnant animals which facilitates culling, treatment or rebreeding. Presently, the detection of nonconceiving sows that return to estrus and amplitude-depth ultrasound are the most widely used techniques for pregnancy diagnosis. Despite routine use of these traditional methods, it is common to note that many sows either fail to farrow after being considered pregnant or return to estrus at irregular times during a presumed pregnancy. It is evident that current techniques in pregnancy diagnosis are inadequate for modern swine production. Despite the obvious need for improved pregnancy diagnosis abilities for swine producers, little progress or changes in techniques has occurred in the last ten years.

Take-Home Message

Assess your pregnancy detection program - do you need to change?

Select the technique or program that reduces nonproductive sow days.