Bovine ultrasonographers are advised to perform a similar analysis of their skills to identify their competence at various stages after insemination. The time it takes to perform examinations and the number of rechecks required at various stages should also be taken into consideration. Note the large difference in the amount of fluid and size of the embryo between 26 days (Figure 36.1a) and 28 days (Figure 36.1b). Note that the 26-day embryo is difficult to visualize against the uterine wall. The 28-day embryo, on the other hand, is 1 cm in length and easily seen within the uterine fluid. Embryonic development occurs very rapidly so waiting just a couple of days can improve accuracy and speed of pregnancy examinations. It is of no value to the producer if early pregnancy examinations do not lead to immediate management decisions.

Ovarian structures are helpful aids in pregnancy diagnosis. It can hasten the examination to know if a corpus luteum (CL) is present and, if so, on which side. Two corpora lutea indicate the potential for twins. A pregnancy with the CL contralateral to the embryo or fetus is very high risk for pregnancy loss. A small or poor-quality CL may indicate low progesterone levels that could lead to pregnancy loss. A cavitary CL is of no concern unless the cavity is so large and the rim of luteal tissue so small that total luteal volume is low. In cases of poor-quality or contralateral corpora lutea, treatment with gonadotropin-releasing hormone (GnRH) may produce an accessory CL to improve progesterone levels. Examination of ovarian structures is discussed further in later sections of this chapter.

The producer should understand that early diagnosis of open cows is critical for quick rebreeding of those animals, but that pregnant cows diagnosed before day 60 should be checked again after placentation is complete (see section Embryonic and fetal viability). This is especially important for lactating dairy animals due to their higher risk of late embryonic and early fetal death.

Embryonic and fetal aging

Ultrasound is far superior to manual palpation for staging pregnancies up to about 120 days in gestation. With ultrasound, embryonic or fetal size is measured rather than the size of the amniotic sac or total amount of pregnancy fluid. Embryonic and fetal growth is very consistent regardless of ambient temperature or breed, whereas the amount of fluid can vary.

Two methods can be used to determine the gestational age of the embryo or fetus. First, direct measurements of the embryo or fetal structures can be taken and correlated to age using a gestational aging chart.^9,10 Trunk diameter is the widest part of the ribcage at the level of the umbilicus. Braincase diameter is the maximum diameter of the skull just caudal to the eyes. Head length is measured from the top of the cranium to the tip of the nose. Crown–rump length is measured from the tailhead to the crown of the skull. Of these measurements, only crown–rump length has a simple formula for a portion of gestation. Until approximately 50 days of gestation embryonic or fetal age can be calculated thus:

Length of theembryo/fetus in millimeters + 18 = embryonic or fetal age in days

For measurements of other structures, or for fetal ages greater than 50 days, one must use fetal aging charts or an ultrasound unit that is equipped with software to measure and calculate gestational age.

The second method is to visually evaluate embryonic/fetal development. Sandra Curran and colleagues have studied when various structures first appear (Table 36.1).⁷ These authors find that in field conditions these structures are most easily seen at the high end of Dr Curran’s range. Using embryonic/fetal size and knowledge of development, experienced ultrasonographers learn to “eyeball” age quite accurately without referring to charts or software.

After approximately 120 days of gestation fetal aging is less accurate. It is difficult to reach and measure fetal structures. Also, fetal size begins to vary among animals with the same breeding dates. Other measurements, such as fetal eye diameter, placentome size, or uterine artery diameter, are too variable among animals to provide accurate estimates of fetal age.

Embryonic and fetal viability

Lactating dairy cattle experience considerable pregnancy loss, particularly before 60 days of gestation when placentation is not yet complete. Losses in lactating dairy cattle are approximately 3.6 times greater than in pregnant heifers¹¹ and often are 10–12% from day of initial pregnancy diagnosis to 90 days.¹² Hypotheses for this high rate of loss include reduced progesterone levels due to high milk production,¹³ a history of postpartum disorders,¹⁴ insemination by certain bulls,¹⁵ twinning,¹⁶ and heat stress.¹⁷ Regardless of cause, it is critical for producers to understand pregnancy loss and the need to reconfirm pregnancies diagnosed prior to about 60 days of gestation.

Fetal viability is easily assessed with ultrasound. Beginning at about 24 days of gestation the fetal heartbeat can be seen.⁷ The heart rate should be at least 130 bpm in early gestation, increasing to about 190 bpm by 60 days.² It can be difficult to identify the beating heart if the cow or the fetus is moving. For this reason, it is advisable to recheck pregnancies when no heartbeat is seen but all else appears normal.

The amniotic and chorioallantoic fluids should be clear and anechoic until at least 70 days of gestation. If the fluid is cloudy before 70 days and there is no heartbeat, the embryo or fetus can safely be assumed to be dead. Separated chorioallantoic membranes may also be seen in cases of late embryonic or early fetal death, often appearing to be floating in the pregnancy fluids. Compare the normal 35-day pregnancy in Figure 36.2a with the dead twin pregnancy in Figure 36.2b. Note the clear fluid and normal fetal development in the live fetus versus the flocculent fluid and unformed fetuses in the dead twins. Movement, other than a beating heart, is difficult to detect before 50 days because the limbs are not well developed yet.

After 70 days the fluids of pregnancy may begin to appear cloudy in normal pregnancies, but by that stage fetal movement can easily be appreciated. Separation of the chorioallantoic membrane is difficult to appreciate at this stage due to elongation and folding of all the placental tissues. Dead fetuses often are very heterogeneous, with many areas of high echogenicity that do not correspond to normal bony anatomy. There may also be fluid in the peritoneal cavity. Compare the dead 65-day fetus in Figure 36.3a to the normal 60-day fetus in Figure 36.3b. Fetal mummies (Figure 36.4) are easily identified by echogenic bony remnants, nearly complete lack of intrauterine fluid, lack of fetal movement, and lack of heartbeat.

In the absence of prostaglandin treatment and CL regression, dead embryos and fetuses can persist for weeks.¹⁸ In these cases there are usually still palpable cardinal signs of pregnancy such as membrane slip or amniotic vesicle. With ultrasound, however, they can be quickly identified without additional examinations and treated with prostaglandin. In most cases expulsion of the dead embryo/fetus and uterine contents will be complete within 48 hours after prostaglandin administration.

In some cases, fetal distress can be identified before death. A heart rate of less than 130 bpm is cause for reevaluation at the next visit. Mild separation of chorioallantoic membranes or abnormal fluid in the presence of a live fetus is also cause for follow-up. A live fetus smaller than expected may be cause for concern, but is usually due to incorrect recording of breeding date. Twin fetuses are not smaller than singletons until late in gestation. Fetal anomalies are at higher risk of loss throughout gestation and, in many cases, it is desirable to abort these abnormal pregnancies intentionally (see section Fetal anomalies). As discussed in the section on pregnancy diagnosis, a contralateral or poor-quality CL is also reason to recheck the pregnancy at the next opportunity.

Twins

High twinning rates are often seen in dairy herds and, less commonly, in beef herds. The primary mechanisms for twinning appear to be correlated to high milk production in dairy cattle¹⁹ and to genetic propensity in certain strains of beef cattle.²⁰ In dairy cattle there also appears to be a higher risk of twinning in cows with a history of cystic ovarian disease.¹⁶ In any case, twinning can lead to more pregnancy loss,^16,21 more dystocia, more periparturient disease, reduced future reproductive performance,²² freemartinism, and a higher risk of early culling for the dam.²³

Despite these risks it is generally not recommended to abort twin pregnancies. The possibility of failing to produce another pregnancy or of producing another set of twins makes it preferable to monitor and manage the cow carrying twins. This can include additional examinations to identify pregnancy loss, drying the cow off early to compensate for probable early calving, feeding a higher energy ration to improve body condition, providing extra monitoring and assistance at calving, and being alert for periparturient metabolic disease. In order to provide these management techniques it is important to the producer to know which animals are carrying twins. Manual palpation is not an accurate method for diagnosis of twins.²⁴ However, a careful ultrasonographic examination can identify most cases.

Because over 95% of twins in cattle are dizygous,²⁵ it is helpful to examine the ovaries for two or, occasionally, more corpora lutea. Approximately 60% of pregnant cows with two corpora lutea will have twins.²⁶ When twins are suspected a thorough examination of the entire uterus should be performed. It is imperative to follow the tract in a systematic fashion to avoid inadvertently counting a single fetus twice. Triplets are rare, but should be considered when three or more corpora lutea are present (Figure 36.5).

During examination of the uterus a twin line may be seen (Figure 36.6). This line represents the confluence of the chorioallantoic membranes of each fetus. It can be distinguished from the amniotic vesicle because it moves away from the fetus rather than encircling it. It will also not be confused with the umbilicus, which is not readily visible prior to 45 days and which is much thicker after that. A twin line may be seen with either ipsilateral or contralateral twins. In some cases it is not seen due to the angle of examination. In later pregnancy the twin line is difficult to identify because all the placental membranes are elongated and folded within the uterus. Twin pregnancies also tend to have more fluid than single pregnancies and often extend beyond the pelvic brim sooner. However, the only cardinal sign of twin pregnancy is the presence of two fetuses.

Twin pregnancies, particularly if ipsilateral, have higher rates of loss (Figure 36.2b) throughout gestation, but especially in the first 60 days before placentation is complete. This loss may be 3.1–6.9 times higher than that for singleton pregnancies.^15,27 However, one unpublished study (A. Scheidegger, personal communication, 2005) indicates that loss of contralateral twins may be more likely in mid gestation. In either case, recheck examinations are warranted to identify cases of loss.

Spontaneous reduction of twin embryos has been documented,^16,21 most commonly in the late embryonic or early fetal stages. Depending on the study, 8–24% of twins reduced to a single fetus. The varying incidence is probably due at least partly to different days after insemination for initial and follow-up examinations. CL reduction often occurs in the ovary ipsilateral to the uterine horn suffering embryo reduction. Attempts to manually reduce twins in cattle have been, at best, only marginally successful.²⁸ In the future these results may be improved upon by ultrasound guided needle aspiration of earlier embryos as is done successfully in mares.

Fetal gender determination

Ultrasonographic gender determination of the bovine fetus was first described by Curran et al.²⁹ in 1989. It was found that the relative location of the genital tubercle, which will become the penis in the male and the clitoris in the female, could be used to determine fetal gender beginning at day 55 of gestation. By this stage the genital tubercle has reached its final location immediately behind the umbilicus in the male (Figure 36.3b and Figure 36.7) or immediately under the tail in the female (Figure 36.8). Until about 80 days of gestation the genital tubercle usually appears as a highly echogenic bilobed structure in both genders, but may appear mono- or tri-lobed.

After 70 days of gestation the skin of the prepuce or vulva begins to cover the genital tubercle.² At this point the adult term for the male or female external genitalia replaces the term “genital tubercle.” By about 80 days the skin of the prepuce or vulva has developed enough to reduce the bright bilobed appearance of the genital structure, as shown in Figure 36.9. Also by this time the scrotum and mammary glands can be detected with most field ultrasound units. The scrotum may appear as three bright lines representing the lateral walls and median raphe or it may appear as a soft tissue sac. Teats appear as bright dots and may resemble the scrotum when it presents as three bright lines. High-quality ultrasound units can detect teats on bull fetuses, especially after about 90 days. Figure 36.10 shows teats on the same 98-day bull fetus shown in Figure 36.9. For these reasons, it is not advisable to diagnose fetal gender based on presence of a scrotum or teats without visualizing the genital tubercle, penis, or vulvar structures.

After 90 days the uterus may be over the brim of the pelvis and difficult to reach for gender determination in some animals. In others it may be possible to reach the fetus at 120 days or even later; 60–80 days is the ideal time for gender determination. At this stage the fetus is easily reached, small enough to orient the location of structures, and the genital tubercle is not obscured by skin.

Knowledge of fetal anatomic landmarks helps to locate the genital tubercle. The large tortuous umbilicus is easily identified (Figure 36.7) and the area immediately behind it can be examined for presence of a male genital tubercle. The fluid-filled anechoic rumen and abomasum (Figure 36.9) are also in the region of the male genital tubercle. The “V” shape of the ribcage permits easy identification of cranial and caudal aspects of the fetus. The head is easily identified by its echogenic bony structure.

The most common error in fetal gender diagnosis occurs because the operator does not appreciate how much the genital structures protrude from the body wall. For example, in a precise cross-sectional examination of a female fetus only the mono-lobed tailhead and female genital tubercle will be seen because they protrude away from other pelvic and hindlimb structures. Similarly, in the case of a male in precise longitudinal plane, only the male genital structure and the umbilicus will be seen. Therefore it is important not to stop short when scanning the fetus for gender determination.

It is also easy to move too quickly when scanning for fetal gender. There is no gap between the umbilicus and male genital tubercle by 60 days of gestation. Likewise, there is very little space between the female genital tubercle and tailhead by that stage. Very slow, small movements are critical to avoid passing through these structures before the eye can appreciate them.

Other errors occur when another structure or artifact is mistaken for the genital tubercle. Legs sometimes occlude the umbilical area and the bones in cross-section can resemble the genital tubercle. Specular reflections in the umbilical cord can be mistaken for a male genital tubercle. Figure 36.11 shows the female genital tubercle to the right of the image, along with two less distinct bright lines in the abdominal region where the male genital tubercle would be found. These lines are a specular reflection artifact between the fluid vessels and the walls of the umbilicus (see section Artifacts for further explanation). A cross-section of the tail can be mistaken for a female genital tubercle. When in doubt it is wise to examine both the umbilical region and the perineal region to confirm gender.

Finally, one must develop a quick eye to accurately identify landmarks and the genital tubercle. Because the cow and the fetus are moving it is usually not possible to isolate the structures for prolonged viewing. Training videos such as Brad Stroud’s Bovine Fetal Sexing Unedited are valuable for training the eye to detect fetal gender quickly and accurately.³⁰

Normal uterine and ovarian structures throughout the estrous cycle

Estrus

Figure 36.12

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3. Strategies to Decrease Neonatal Calf Loss in Beef Herds
4. Evaluation of Breeding Soundness

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