Brad Belstra

North Carolina State University

Department of Animal Science

Raleigh, NC  27695-2761


In 1953, Dr. Carl Hellmuth Hertz and Dr. Inge Edler (Swedish medical professors) sought the help of Werner von Siemens (founder of the Siemens company) to develop a special motion scanner to study the human heart based on his existing ultrasound technology.  Since then, the capabilities of ultrasonic imaging equipment have increased substantially and ultrasonography has proven to be a revolutionary tool with countless applications.  Doppler and A-mode (amplitude depth) ultrasound technology has been applied to swine research and production for the last 20 to 25 years.  The limited accuracy inherent to the operation of such devices however led to the use of B-mode (brightness modality), real-time ultrasound (RTU), which has become affordable enough for regular breeding herd use during the last 5 years.


Basic Physics of Ultrasonic Imaging


Ultrasound machines generate low-intensity, high-frequency sound waves through the electrical stimulation of piezoelectric crystals in the transducer (probe).  These waves do not travel through the air so the probe face must be ‘coupled’ to the tissue with a gel or an oil.  Hair, dirt, feces and other debris can also cause interference.  The number of crystals or elements in the probe plays a large role in determining the image quality of the machine.  Consequently the probe will typically represent 1/3 of the cost of the machine. 


Sound waves are both transmitted and received by the probe as they reflect or ‘echo’ back off the tissues and objects they encounter.  Soft tissues, especially fluid-filled objects, absorb many of the sound waves and echo few back (displayed as black) while hard tissues absorb few sound waves and echo many back (displayed as white).  Thus, the varying ‘echogenicity’ of the tissues examined is interpreted by the machine and displayed on the monitor as 32 or 64 shades of gray (black to white) which represent a thin (e.g., 2 mm), 2-dimensional slice of the tissues.  The image on the monitor is ‘real-time’ because the transmission and reception of ultrasound waves is constant and the display is updated rapidly at many frames/second. 


Some probes are linear and the crystals are arranged in a linear-array producing a square or rectangular image while other probes are sector scanning and the crystals are arranged on a spinning platform producing a pie-shaped image.  Linear probes are ideal for examining a large (i.e., wide) ultrasonic window while sector probes generally allow one to examine smaller areas with greater detail.  Linear probes are generally ideal for backfat/loin muscle measurements and transrectal reproductive imaging whereas sector probes are generally ideal for transabdominal reproductive imaging such as in pregnancy diagnosis.  The resolving power of a probe depends on its frequency (measured in Mega Hertz, MHz).  Low frequency probes provide deeper tissue penetration but less image resolution than high frequency probes (Table 1.). 

Table 1.  Transducer frequency versus penetration depth and image resolution.


Transducer Frequency, MHz

Penetration Depth, Inches

Image Resolution


0 – 7.9



0 – 4.7



0 – 2.8



Pregnancy Diagnosis


Almond and Dial (1987) defined several criteria that an ideal pregnancy test should meet and neither daily detection of estrus with a boar nor RTU meet all of them (Table 2.). 


Table 2.  Comparison of which ideal pregnancy test requirements that detection of estrus with a boar and real-time ultrasound scanning meet.


Ideal Preg. Test Requirement *

Detection of Estrus

Real-Time Ultrasound

High Accuracy, > 95%

Yes/No, can be 98% but may be much lower if not done consistently and correctly

Yes, can be 93 to 98% but accuracy is reduced before 22 d postmating

Low false-positives (diagnosis of non-pregnant as pregnant), < 5%

Yes/No, can be common, missed sows or anestrus sows due to inactive or cystic ovaries, pseudopregnancy, etc.

Yes, fairly rare as long as the bladder or uterine fluid from an infection is not mistaken as a pregnancy

Low false-negatives (diagnosis of pregnant as non-pregnant), < 5%

Yes, fairly rare, pregnant sows usually do not express estrus

Yes, but can be common if sows are checked to early postmating


Yes/No, handling boars can be a hazard, especially if they must be moved in and out of sow pens

Yes, should be less of an issue but depends on sow housing; still hard work


Yes, but takes a technician more time and practice to become highly accurate

Yes, shorter learning curve, either pregnant or not and recheck questionable sows


Yes, boar cost and maintenance is low

No, machine cost and maintenance is high

Low labor input

No, repeated heat checks of the same sows requires more labor; dependent on facility design

Yes, supposedly 50-100 sows/hr is possible; only check sows 1 to 3 times max.

Rapid results

Yes/No, back pressure tests can be preformed relatively rapidly depending on facility design

Yes, very rapid but dependent on sow housing, machine mobility, technician skill etc.

Results before non-pregnant sows return to estrus, < 21 d

No, return to estrus is the signal for a non-pregnant sow

No, can be preformed as early as 22 d but sows may have returned to estrus already

Applicable to sows a week or more apart in gestation

Yes/No, usually focused on specific breed groups of sows during regular return periods

Yes, as long as all sows are ³ 22 d postmating and breed dates are known

* Ideal pregnancy test requirements from Almond and Dial, 1987.

With the exceptions of being inexpensive and capable of detecting non-pregnant sows before they return to estrus (< 22 d postmating), RTU meets all of the other criteria of an ideal pregnancy test.  In addition, several companies have developed portable RTU machines designed specifically for swine pregnancy detection.  However, this does not mean that all herds should invest in their own RTU machine and stop checking mated sows for return to estrus. 


Detection of estrus during head-to-head boar exposure, especially between 17 to 25 days postmating, has traditionally been used to find non-pregnant sows.  One of the primary advantages of daily detection of estrus of mated sows is that when sows are found in estrus they can be mated again immediately and non-productive days (NPDs) can be minimized.  Embryonic vesicles (fluid-filled membranes) can be visualized as early as 14 to 15 days of gestation in the sow with advanced (expensive) RTU equipment.  However, at present, the earliest that acceptable accuracy can be achieved with the portable on-farm units is about 22 days postmating (Armstrong et al., 1997).  Some non-pregnant sows will have already returned to estrus by this time and thorough, consistent, detection of estrus is necessary to find and rebreed them.  If an improved (and affordable) RTU machine or some other ideal pregnancy test could be developed to accurately diagnose pregnancy at 16 to 18 days postmating, just before most sows would return to estrus, it would have substantial economic benefits.


Pregnant sows can also become open at any time after 21 days due to prenatal mortality or abortion and these sows can accumulate a large number of NPDs if the pregnancy check program depends solely on 1 or 2 ultrasound examinations.  The use of RTU in combination with accurate detection of estrus allows the shortcomings of each technique to be countered by the strengths of the other (Table 2.).  Unfortunately, RTU machines are still relatively expensive ($6,000 to 9,000) and several factors should be considered to determine how profitable the addition of an RTU unit to a herd’s pregnancy detection program could be:


·        Herd size, number of sows mated/week (RTU machines have been shared across several herds).

·        Herd conception rate

·        Number of NPDs and their source

·        Cost of a NPD

·        Labor availability (one more thing to do each day)


A target of 30 to 50 NPDs is not unattainable.  Median herd NPDs for the U.S. is currently at 77 days with the upper 10% of the farms in the database at 53 days (PigCHAMP, 2000).  The number of NPDs varies widely between herds and it is highly correlated with sow productivity.  The addition of RTU pregnancy diagnosis can reduce NPDs depending on their source.  Additional information on RTU pregnancy detection and the economics of NPDs can be found in the reproduction section of the Extension Swine Husbandry webpage:


The potential advantages of RTU pregnancy detection that have been suggested by herd managers and veterinarians who have implemented it are:


·        Reduce non-productive sow days

·        More accurate and rapid culling decisions

·        Reduce (but not eliminate) heat check labor and may motivate employees to heat check more thoroughly

·        Monitor effect of breeding technician, insemination procedure, etc. on RTU conception rate

·        Use RTU conception rate to set breeding targets

·        Increase confidence in sow movements and farrowing rate projections

·        Monitor effect of season, herd health status etc. on fall out rate (farrowing rate minus RTU conception rate for a breed group)


If you are considering the purchase of a real-time ultrasound machine for pregnancy detection you can view and request information through the following companies’ websites.          Ultra Scan                  Tringa                   Real McCoy                     SonoSite (mainly human applications but may attempt to market a unit for swine pregnancy detection?)


Other Reproductive Applications


A large number of other swine management applications for ultrasonography could become possible in the future, but they typically require more advanced (expensive) RTU equipment, more advanced knowledge of sow reproductive physiology and ultrasonography, and more labor.


Single examinations of the ovaries or uterus can be used to diagnosis reproductive dysfunction such as cystic ovarian follicles or metritis (pyometria) in the uterus.  The ovaries of anestrus weaned sows can be examined to determine if they lack follicular growth or have developed cystic follicles and what treatments might be appropriate.  The uterus of postpartum sows can be examined to determine if any retained pigs or placenta are present.  Swine veterinarians may someday perform such examinations with their own RTU equipment on farms with reproductive problems.  In effect, such procedures would be like a slaughter check without the need to actually slaughter the sows. 


Repeated examinations of the ovaries or uterus over time can be used to study dynamic processes such as follicular growth and ovulation, postpartum uterine involution and embryonic development/mortality.  Such studies are only feasible for research purposes due to the extensive labor and technical skills involved.  More than one research group has attempted to use 2 or 3 repeated postweaning scans of the ovaries to predict the time of ovulation in sows.  It is unlikely that such attempts will succeed since there appears to be little correlation between ovulation and the size or rate of growth of preovulatory follicles.  Useful information has however been obtained from the study of factors that influence the duration of estrus and its relationship with ovulation as determined by ultrasonography (Soede et al, 1994).


Current Research


            We are currently completing two ongoing ultrasound studies at NC State.  The first involves a single transabdominal examination of the ovary(ies) of cull sows just prior to slaughter by two technicians working independently.  A third party collects the reproductive tracts.  When approximately 50 randomly cycling sows and 50 sows weaned at a known date have been examined we will combine the data from all three parties and determine what sort of errors are made by technicians when they have no knowledge or some knowledge of the sows reproductive history.  This study will determine the capabilities of transabdominal examination of sow ovaries.  The second study is on the effect of herd, season, lactation length, weaning-to-estrus interval, sow genotype and parity on duration of estrus and time of ovulation.  This involves the repeated detection of estrus and transabdominal scanning of the ovaries of weaned sows every 6 hours.  Approximately 360 sows on two different farms are being studied to determine which factors explain differences in duration of estrus and time of ovulation within and between herds.  The goal is to provide information on which to base insemination schedules.


Take-Home Message


Use of real-time ultrasonography to detect pregnancy may be a feasible means to decrease non-productive days and increase litters/sow/year on your farm.  Use of this same technology to investigate function and dysfunction of the uterus and ovaries is limited by the cost of the more expensive equipment required and the lack of technicians with the sow reproductive physiology and ultrasonography knowledge required to perform the scans and interpret the images.  At present, only animal scientists and veterinarians use real-time ultrasonography to monitor sow reproductive functions due to the labor and technical skill involved.  Information from such studies will however continue to provide producers with knowledge that can be applied to many aspects of swine management at the farm level.




Almond, G.W. and G.D. Dial. 1987. Pregnancy diagnosis in swine: Principles, applications, and accuracy of available techniques. J. Amer. Vet. Med. Assoc. 191:858-870.


Armstrong, J.D., K.D. Zering, S.L. White, W.L. Flowers, T.O. Woodard, M.B. McCaw and G.W. Almond. 1997. Use of real-time ultrasound for pregnancy diagnosis in swine. Amer. Assoc. of Swine Practitioners. pp. 195-202.


PigCHAMP. 2000. Global Benchmarking in Swine Herds. PigCHAMPÒ, Inc. p 7.


Soede, N.M., C.C.H. Wetzels and B. Kemp. 1994. Ultrasonography of pig ovaries: Benefits in research and on farms. Reproduction in Domestic Animals 29:366-370.