PREGNANCY DIAGNOSIS IN MARE

RECTAL PALPATION

Rectal palpation is a reliable and cost-effective method of determining pregnancy in mares. However, it is not accurate at less than 30 days of gestation and is useless for the early diagnosis of twins.

The first sign of pregnancy in a mare is an increase in uterine and cervical tone. This begins as early as 14 days but can only be reliably distinguished at 21 days.

If there is an obvious uterine tone (the uterus feels like a firm tube) and a firm finger-like feeling to the cervix, the mare is probably pregnant. However, one should not make a final diagnosis of pregnancy at this time because pseudopregnancy can mimic this situation (see notes on infertility).

There is an absence of behavioral estrus and serum progesterone concentrations remain high.

The earliest time at which a definite diagnosis of pregnancy can be made by rectal palpation is about 30 to 35 days. At this time, the ventral bulge of the fetoplacental unit (FPU) can be felt at the site of implantation i.e. the junction of either uterine horn and the uterine body. The FPU is about the size of a hen’s egg.

At 42 days, the bulge of the FPU is distinct and approximately the size of a small orange as seen here.

By 55 to 60 days (approximately as shown in the image at upper right) the uterus starts to lose tone and the fetal bulges become less distinct. From that point onward, it is very difficult to diagnose twins by rectal palpation. At this time, the FPU starts to grow into the uterine body. At 50 days, the FPU is the size of a standard orange. There is still considerable uterine tone, and the margins of the fetal bulge are well delineated. If there are bicornuate twins, the two bulges can often be felt quite easily.

At 60 days, the bulge of the FPU is about the size of a smallish cantaloupe (the ‘2-month melon’) and is quite soft and fluctuant.

At 90 days the FPU is about the size of a football but is quite soft. The uterus is usually dropping over the pelvic brim.

From 100 days onward, the fetus itself may be felt by internal ballottement. If the uterus is tapped lightly, the fetus can be felt like a hard floating body which bumps up against one’s hand. External ballottement is seldom possible in mares because of their abdominal muscle tone.

At 120 days, the uterus is well into the abdominal cavity and the cranial margin can barely be felt by rectal palpation. The fetus can still be balloted fairly easily.

At 5.5 to 6 months, the pregnant uterus descends into the lower abdomen and by the sixth month, the fetus can be impossible to ballot. However, by the seventh to eighth month, the fetus has usually grown enough so that it is again within reach of the palpator. Occasionally, even a 7 month-old fetus cannot be palpated because of depth of the abdomen in some mares.

From 7 to 8 months until term, pregnancy is determined by direct palpation of the fetus itself and an estimation of duration of pregnancy based on a mental comparison of the size of a neonate compared with the palpable potions of the conceptus. The cranium, fetlock joint and intermandibular space are convenient points to palpate for this comparison.

Mares should not be diagnosed as nonpregnant unless both uterine horns and the bifurcation have been carefully examined from the ovarian end of one horn to the ovarian end of the other. One must always be certain that the organ being examined is the uterus. If there is any doubt about the diagnosis, re-examination at a later date should be recommended.

Remember that mares should always be restrained as adequately as possible for rectal palpation. If the mare should move suddenly while one is palpating and a hand should penetrate the rectum, the operator will be held liable for the death of the animal. One is on safer legal grounds when the animal is restrained in stocks with a handler at the head of the mare. The safety of the operator can also be assured through the use of stocks. Persuade horse owners to construct stocks for general restraint for veterinary purposes.

HORMONAL TESTS FOR PREGNENCY

Several tests for equine chorionic gonadotropin (eCG=PMSG) have been developed since the original hemagglutination inhibition test (Mare Immunopregnancy or MIP test). These tests can be used between 40 and 120 days of gestation but are most accurate between 50 and 90 days when the endometrial cups are fully functional. False-positive results most occur when there has been embryonic or fetal death after the formation of the endometrial cups.

Starting from about 60 days of gestation, assays for serum/plasma estrone sulfate (ES) can be used to establish the presence of pregnancy. Because of many uncertain results in samples taken before 90 days of gestation we usually suggest that sampling should only begin after 90 days of gestation. In that regard, it is convenient to remember that estrone sulfate concentrations are predictably diagnostic after 100 days, best at 200, and become unreliable again after 300 days.

ES concentrations less than 1.0 ng/ml indicate non-pregnancy and those between 1 and 4 ng/ml are in an uncertain zone. Re-sampling 2-to-3 weeks later is necessary.

The concentrations of ES in urine (indexed against creatinine) milk and feces may also be useful for pregnancy diagnosis. These can be used in wild equids.

REAL-TIME ULTRASONOGRAPHY

Transrectal real-time ultrasound has become the state-of-the-art method for diagnosing pregnancy in mares. The spherical fetoplacental unit of equids lends itself to early detection by ultrasonography and pregnancy detection can be close to 100% accurate as early as 12 days of gestation. However, because of the possibility of missing twins at this time (and also because of early embryonic death, routine examinations are scheduled later than this for routine stud work; usually 14 to 15 days after an un-ovulated follicle was last seen.

The conceptus is spherical and quite mobile until about 16 days after ovulation. At that time, it usually becomes fixed in the caudal portion of the uterine horn near the bifurcation of the uterine body and horn.

After day 16, the conceptus starts to become irregular in shape as the sodium pump which keeps it tense before this time, tends to slow down.

Pregnancies of various ages; after an un-ovulated follicle was last seen.
14.15 days	18.19 days
24 days. Note normal thickening of the uterine wall.	27 days. A heartbeat is obvious. At 28.29 days, Y.S. and All. are the same size.

The embryo inside the vesicle is first detected ultrasonically on days 20 to 24 and is usually but not always, observed on the ventral aspect of the vesicle.

A heartbeat can be detected in the embryo by days 22 to 25.

By approximately day 28 to 30, with the allantois ventral to the embryo and the yolk sac dorsal to it the embryo is suspended approximately in the center of the vesicle. After this, the embryo lies in the dorsal portion of the vesicle until about day 45 then, because of its weight, it gravitates toward the ventral floor of the FPU where it lies in dorsal recumbency until advanced gestation.

FETAL SEXING

Fetal sexing is not as common in horses as cattle but it is occasionally done.

The gender of the fetus is based on the location of the genital tubercle (forerunner of the penis or clitoris) near the umbilicus (male) or near the tail (female) between days 60 and 80 of gestation. At that stage of gestation the genital tubercle appears as a bi-lobed, hyper-echogenic structure on ultrasound.

Fetuses up to 100 days of gestation have been sexed but as the fetus grows larger, it gradually moves too far away for adequate viewing. Fetal gender has also been determined accurately at 5 and 6 months of gestation in pony and pony x horse mares using a 3.5 MHz rectal transducer.

Good quality ultrasound equipment and subdued external lighting are required for fetal sexing. For further information, see: Ultrasonic Imaging and Animal Reproduction: Book 2, Horses by Ginther.

EXTERNAL SIGNS OF PREGNANCY

Some abdominal enlargement may be seen after the ninth month of gestation but in general, abdominal enlargement is not a reliable indicator of pregnancy.

In the last trimester, the udder becomes enlarged and in the last 7 to 10 days of gestation, a serum-like secretion can be expressed from the teats. The udder size also increases dramatically in the last 4-to-5 days before foaling. When the udder secretion turns a smokey-white or white color (due to an increasing casein content) parturition is imminent. It is not recommended that one milk out this secretion because it is rich in antibody content but a few squirts taken for examination just prior to parturition is acceptable. It is essential that the teats be cleaned before drawing this milk to prevent mastitis. Changes in milk chemistry are discussed elsewhere.

A clear, serum-like secretion sometimes accumulates on the tips of the nipples towards the end of gestation. This dries out and is known as ‘waxing’. Waxing may be seen in some mares up to 10 days prior to parturition but in the other mares, it may not occur at all.

In the last few days of gestation, the sacrosciatic ligaments relax and the vulva enlarges noticeably. In heavily muscled mares, relaxation of the sacrosciatic ligament may not be quite so obvious.