CHAPTER 39 Pregnancy Diagnosis
Since the beginning of civilization, animal owners have been interested in determining whether or not conception has taken place, and various clinical signs and superstitions have been used for millennia to diagnose pregnancy.1 Over time, a number of more accurate methods for determination of pregnancy in cows and other female domestic animals have been developed, including observation, physical examination, chemical tests, and use of electronic instruments.
The purpose of examining cows for pregnancy is not to detect those that are pregnant, but to detect those that are not pregnant so that they can be inseminated again or culled from the herd. For profitable production, dairy cows should calve for the first time at approximately 24 months of age and should deliver subsequent calves at intervals of approximately 13 to 13.5 months. Thus, dairy cows should conceive within approximately 4 months or less of calving. Cows that are found to be not pregnant can be treated for any abnormalities of the reproductive tract discovered during the examination, observed closely for signs of spontaneous estrus, or treated with prostaglandin F2α (PGF2α) to predictably shorten the time until return to estrus.
An ideal test would accurately detect pregnancy before the first expected estrus after insemination (about 21 days) so that the cow could be reinseminated without further loss of time. Unfortunately, no tests are currently available that are practical to use and allow detection of pregnancy in cows before the first expected estrus. Most methods for pregnancy diagnosis are capable of detecting pregnancy between 25 and 40 days after conception, and most authors have traditionally recommended that nonpregnant cows be identified before the second expected estrus after insemination.2 In a recent prospective study, however, cows that were diagnosed as pregnant by palpation between 30 and 36 days after breeding were found to have a 2-week longer calving interval than that observed in cows examined for pregnancy later.3
The frequency of embryonic death in cattle is high during the first months of pregnancy, perhaps because of loss of abnormal embryos or failure of maternal recognition of pregnancy.4 Thus, cows that are diagnosed as pregnant soon after insemination are more likely to suffer embryonic death and return to estrus, to be found not pregnant at a subsequent examination, or to fail to deliver a calf at the expected time than are cows in which pregnancy is diagnosed later. Owners frequently misinterpret this course of events and conclude that the test for pregnancy was inaccurate (i.e., the cow was not pregnant at the time of examination) or that the test affected the embryo and directly or indirectly resulted in termination of pregnancy. After approximately 60 days, fetal death rates are low, and in most, but not all, cows that are found to be pregnant after this time, the pregnancy proceeds to term with birth of a calf.
Beef cows usually are examined for pregnancy when their calves are weaned at 6 to 7 months of age, although in intensively managed herds, individual cows can be examined earlier. In most management systems, nonpregnant cows are culled from the herd to save the cost of maintaining nonproductive animals, but in herds that have spring and autumn calving seasons, cows sometimes are moved to the other group and given a second opportunity, although this decision may be difficult to justify economically. Although the cost of maintaining a nonproductive beef cow is variously estimated to be between $250 and $400 per year, a survey of herd owners in 18 states revealed that only 17.7% examine cows and that 15.9% examine heifers for pregnancy.5
As described later on, palpation of reproductive structures per rectum (probably to be replaced by ultrasound examination in the foreseeable future) has been the customary method of pregnancy diagnosis in cattle. Some cattle owners, however, may rely on the history, or on clinical signs that can be observed, for a presumptive diagnosis of pregnancy.
A history of cohabitation with a bull, the observation of mating, or artificial insemination may be considered by some owners to be sufficient evidence that a cow has become pregnant. Although fertilization rates in cows typically are high, however, only approximately 50% of inseminations result in a detectable pregnancy. Conversely, unobserved, unplanned, or unrecorded matings are not uncommon, and cows frequently are presented with a complaint of infertility, only to be found pregnant when examined. Thus, history is not a reliable indicator of pregnancy status and sometimes may be deceptive.
Bovine embryos signal their presence around 15 to 17 days after ovulation: The corpus luteum is maintained, and the maternal estrous cycle is suspended. Thus, failure of a cow to return to estrus at approximately 18 to 24 days after mating suggests that conception has occurred. In fact, the most common cause of failure of cows to have normal estrous cycles is pregnancy.6
In beef herds in which natural service is used, perceptive managers may observe that a greater-than-expected number of cows return to estrus after mating. This situation suggests an infertile bull, the presence of a venereal infection, or some other cause of infertility, and an opportunity exists to take corrective action before the breeding season ends. Conversely, undernutrition is a common cause of anestrus in lactating beef cows, and the observation that few cows return to estrus after the first few weeks of the breeding season may mislead less astute managers to believe that cows have become pregnant when they are in fact not cycling.
Failure of a dairy cow to be detected in estrus at approximately 3 weeks after insemination is viewed as a favorable event by managers, and anestrous cows are assumed to be pregnant. Unfortunately, although the efficiency of estrus detection varies among observers and among dairy farms, it generally is lower than desirable, so this approach is not sufficiently accurate to be of use in diagnosis of pregnancy. Most dairy reproductive herd health programs are based on the practice of examining cows that have not been observed in estrus after insemination as indicated by some other method (most commonly rectal palpation) that more accurately differentiates between pregnant and nonpregnant cows.
A few pregnant cows show mild to conspicuous signs of estrus and may be mistakenly thought to be nonpregnant. Artificial insemination of pregnant cows may result in abortion if the insemination instrument is passed completely through the cervical canal and the fetal membranes are disrupted.7
A sanguineous vaginal discharge is common in cows 24 to 48 hours after estrus and is the result of hemorrhage from the caruncular capillaries due to the rapid decline in estrogen concentration that follows ovulation. Some owners subscribe to the notion that metestrus bleeding is a sign of conception failure; however, metestrus bleeding is observed in nearly all cows and heifers and is not an indication of whether conception occurred. If metestrus bleeding is observed in a cow that was not seen in estrus a few days previously, it is implied that estrus was unobserved and that the animal is not pregnant.
Palpation of the reproductive tract through the rectal wall (rectal palpation) has been used for diagnosis of pregnancy since the early 1900s and has been the customary method used in cattle for more than 50 years.8–11 Depending on the skill of the examiner and the age and size of the dam, rectal palpation is useful to diagnose pregnancy as early as day 30 and thereafter until term. Although a number of changes occur in the size, texture, location, and content of the uterus during pregnancy, four positive signs of pregnancy that are detectable by rectal palpation are recognized:
The examiner can detect the chorioallantois within the lumen of the pregnant uterus by compressing the uterine horn between the thumb and forefingers, lifting the uterus, and then allowing the horn to slowly “slip” from the grasp. If the cow is pregnant, the chorioallantois can be felt to slip through the fingers just in front of the uterine wall. This membrane slip can be felt in the pregnant uterine horn as early as 30 days of pregnancy and can be reliably detected by day 35. The fetal membrane slip can be detected in the nongravid horn by approximately day 70 of pregnancy. During early pregnancy, the fetal membranes are thin, and a delicate touch and some experience are required to recognize this sign of pregnancy.
The amnion contains the developing conceptus and the amniotic fluid and is palpable as early as 28 days after conception in heifers and by 32 to 35 days in pluriparous cows. The vesicle is recognized as a nearly spherical, turgid, fluid-filled structure that is approximately 1 cm in diameter at 28 days and increases in size as pregnancy advances. The amniotic vesicle is detected by encircling the uterine horn with the examiner’s thumb on one side and the fingers on the other. The vesicle is freefloating within the uterus but most commonly is found at the cranial edge of the intercornual ligament. The amniotic vesicle becomes progressively less turgid and is difficult to recognize by about day 65 of gestation. At that time, the vesicle softens and the fetus becomes palpable. In a bovine conceptus, the heart is external until approximately day 42; therefore, caution must be exercised in attempting to detect early pregnancies, and undue pressure must not be applied to the amniotic vesicle because rupture of the embryonic heart or other fragile organs may result. Intentional rupture of the amniotic vesicle has been used as a method to provoke abortion in cattle.12
In ruminants, cotyledons of the fetal placenta produce villi that project into crypts of maternal caruncles to form placentomes. Seventy-five to 120 maternal caruncles arranged in two dorsal and two ventral rows are present in the uterus of cows. Placentomes begin to form early in gestation and are of sufficient size to be palpable by 75 to 80 days. The size of placentomes varies with the stage of gestation and their location in the uterus. Placentomes are progressively larger near the middle of the gravid horn and are smaller at the cervical and ovarian poles. They are most consistent in size immediately cranial to the cervix and are palpated at that location to estimate the stage of pregnancy. Placentomes are identified by grasping a longitudinal fold of the uterine wall and rolling it between the thumb and fingers. In more advanced pregnancies, the examiner can palpate placentomes by passing a flattened hand over the uterine wall.
Placentomes remain palpable for a variable time after death of the conceptus, and detection of their presence may, in a few instances, result in a false positive diagnosis of pregnancy. A false positive diagnosis of pregnancy also can occur when an ovary is mistaken for a placentome consistent in size with a 120- to 180-day pregnancy. The examiner can easily avoid this mistake by identifying at least three placentomes before declaring the animal pregnant (there are only two ovaries).
A false negative diagnosis is possible during the fifth to eighth month of pregnancy, when the uterus is completely descended and may be out of reach, especially in large, deep-bodied cows, or to examiners with short arms. This mistake can be avoided if the examiner searches for other signs of pregnancy before declaring the cow pregnant, or positively identifies a normal, nongravid uterus before declaring the animal nonpregnant.
The fetus becomes palpable at approximately 65 days, when the amniotic membrane loses its turgidity, and remains theoretically palpable for the balance of gestation. In the early stages of gestation, the fetus can be grasped directly. Later, the fetus is detected by ballottement: The examiner sets the fetal fluids in motion by rocking the hand against the uterine wall and recognizes the fetus as it rebounds against the hand. The fetus is easily palpable as a freefloating, firm object within the fluid-filled uterus during the first 4 months of gestation. As pregnancy advances, increased weight of the fetus and fluid pulls the uterus ventrally and cranially until the fetus comes to rest on the abdominal floor during the fifth and sixth months. Continued growth of the fetus positions it closer to the maternal pelvis during the last trimester (period of ascent), so that palpation of the fetus is facilitated. Roberts11 has estimated that it is possible to palpate the fetus in greater than 95% of cows during the third and fourth months of gestation, in 40% to 70% of cows during the fifth and sixth months, in 80% at 7 months, and in greater than 95% during the eighth and ninth months.
False negative diagnoses are most likely if palpation of the fetus is relied on for diagnosis of pregnancy between the fifth and seventh months. Mistakes can be avoided if the cow is examined for other signs of pregnancy or if a nongravid uterus is identified before the animal is declared not pregnant.
False positive diagnosis is possible if another structure is mistaken for a fetus. Some of the normal maternal structures that may be incorrectly perceived as a fetus are the dorsal sac of the rumen and the left kidney. Among the abnormal structures that have been mistaken for a portion of the fetus are ovarian and uterine tumors, enlarged lymph nodes, adhesions and connective tissue secondary to previous surgical or obstetric trauma, and necrosis of abdominal fat. Mummified fetuses can be mistaken for a normal pregnancy if an examination is perfunctory. Although a mummified fetus can approximate the size of a viable fetus, mummies are recognized by a lack of uterine fluid and absence of fetal membranes and placentomes.
Examinations for pregnancy should always begin with palpation of both uterine horns to detect differences in their size. In very early pregnancies, the horns may be nearly identical in size, but as gestation advances, the pregnant horn enlarges. Asymmetry often is first detectable in the cranial portion of the gravid horn. If a difference in the diameter of the uterine horns is detected, the larger horn can be examined for presence of an amniotic vesicle or fetal membrane slip. Conditions other than pregnancy that potentially may cause enlargement of one uterine horn are pyometra, mucometra, and delayed postpartum uterine involution. In none of these conditions, however, is an amniotic vesicle or fetal membrane slip detectable.
As the gravid uterine horn fills with fluid, the wall thins and assumes a characteristic texture of resilience and fluctuance. With some experience, the texture of the uterine contents and the uterine wall typically associated with pregnancy will give the examiner an early indication that the cow is pregnant. This impression is confirmed by detection of at least one of the four positive signs of pregnancy.