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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • Pregnancy—the condition post fertilization of an embryo or fetus developing and maturing in utero
  • Pregnancy diagnosis—determining pregnant state based on clinical signs and laboratory and physical findings, including TRP and US

Pathophysiology!!navigator!!

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Systems Affected!!navigator!!

  • Reproductive
  • Other systems may be affected in abnormal pregnancy

Genetics!!navigator!!

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Incidence/Prevalence!!navigator!!

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Signalment!!navigator!!

  • Nonspecific; puberty occurs between 12 and 24 months in females
  • Pregnancy may occur any time after puberty until advanced age in mares

Signs!!navigator!!

Historical Findings

Failure of a mare that has been bred to return to estrus 16–19 days post ovulation.

Physical Examination Findings

  • Early in pregnancy, little physical change may be noted
  • As pregnancy advances, most mares will develop recognizable abdominal distention and weight gain
  • In the final 2–4 weeks prior to parturition, most mares will have increased development of the mammary gland with secretion of fluid from the nipples ranging from thin and straw-colored to sticky and creamy

Causes!!navigator!!

Mating

Risk Factors!!navigator!!

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Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Other Causes of Failure to Cycle

  • Seasonal anestrus TRP and US—little ovarian activity; uterine and cervical tone is flaccid
  • Behavioral anestrus serial TRP and/or US—distinguish mares in estrus from those in diestrus or that are pregnant
  • Prolonged luteal life span—evidence of a CL by US examination of the ovary or progesterone assay. Responds to prostaglandin F2α treatment
  • Granulosa–theca cell tumor—abnormally enlarged, multicystic ovary and small contralateral ovary. Confirm with elevated serum inhibin and/or anti-Müllerian hormone concentrations
  • Chromosomal abnormalities (gonadal dysgenesis, testicular feminization)—confirm by karyotype determination

Other US Findings Resembling Early Pregnancy

  • Uterine/lymphatic cysts—US examination of the mare prior to breeding and pregnancy; record presence, number, size, shape of uterine cysts; a uterine map (horns and body) on each mare's record is easily referenced at early pregnancy examinations
    • This permanent record of cystic structures is beneficial in distinguishing uterine cysts from early embryonic vesicles
    • Update maps at the start of each breeding season, note changing appearance, number of cysts with increasing age

CBC/Biochemistry/Urinalysis!!navigator!!

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Other Laboratory Tests!!navigator!!

Progesterone Assay

  • ELISA and RIA for serum or milk progesterone concentrations
    • Elevated concentrations of progesterone at 18–21 days post ovulation imply that functional luteal tissue is present; a presumptive test for pregnancy
  • A useful adjunct to other methods of early diagnosis (i.e. TRP without US)
  • Confirmation of pregnancy, e.g. estrone sulfate or total estrogens, is advisable if early diagnosis was solely by a progesterone assay

eCG Assay

  • eCG is a hormone secreted by endometrial cups in the pregnant mare uterus
  • Endometrial cups form at ≈36–37 days of gestation, when fetal trophoblasts from the chorionic girdle actively invade the endometrial epithelium
    • The cups attain maximum size and hormone output at 55–70 days of gestation
    • Endometrial cups regress at 80–120 days of gestation; secretion of eCG ceases at that time
  • eCG is measured using an ELISA
  • False positives occur if fetal death occurs after formation of endometrial cups:
    • The cups can persist up to 3–4 months after a mare has lost a pregnancy, either a nonviable fetus in utero or fetal loss (early abortions often not noted)
  • False negatives occur in samples:
    • Before the endometrial cups form (<36 days of gestation)
    • After the regression of the cups (>120 days of gestation)

Estrogen Assay

  • Estrogens are secreted by the fetoplacental unit
  • Total estrogens or estrone sulfate (conjugated estrogen) can be measured from plasma or urine; diagnose pregnancy after day 60 using RIA
  • Estrone sulfate concentrations in milk are diagnostic for pregnancy after day 90 in the mare
  • Fetal death and/or compromise to the fetoplacental unit results in rapid decline in estrone sulfate concentration

Imaging!!navigator!!

Transrectal US

Pregnancy diagnosis can be determined as early as 9 days after ovulation, with a 5 MHz transducer and a high-quality US scanner.

Days 15–16 Post Ovulation

  • The optimal time to scan for early pregnancy. The embryonic vesicle is an anechoic, spherical yolk sac, averaging 6–20 mm in height
  • Detection of an embryonic vesicle is reliable during this period; twin embryonic vesicles can consistently be identified at this time
  • Early diagnosis of twins increases the success of twin reduction to a singleton; manual reduction (crush) technique

Days 18–24 Post Ovulation

  • The US appearance of the vesicle—more triangular by day 18 as the bilaminar wall of the embryonic vesicle becomes less turgid
  • The embryo proper often can be visualized by day 20–21; its heartbeat is visible as early as 24 days, should be evident with most US machines by 25 days
  • The allantoic sac is visible ventral to the embryo by day 24

Days 25–48 Post Ovulation

  • As the allantois develops and the yolk sac regresses, the embryo appears (during US examinations) to lift from the ventral aspect of the vesicle to a dorsal location
  • The embryo is visualized mid-vesicle by 28–30 days and is in the dorsal aspect of the vesicle by day 35
  • The umbilical cord forms and attaches at the dorsal aspect of the vesicle around day 40
  • As the cord elongates, with US, the fetus descends to a more ventral location in the vesicle of pregnancy, reaching near its ventral aspect by day 48

Fetal Sexing by US

Days 60–70 of Gestation

  • Determination of fetal gender is very useful in both horses and cattle, but high-resolution US equipment and experience are necessary for accurate identification of fetal gender
  • Fetal sex is determined by locating the position of the genital tubercle during its developmental migration
  • The genital tubercle is the precursor to the clitoris in females and the penis in males
  • The structure is located on the ventral midline and is imaged as a hyperechoic, bilobed structure that is approximately 2 mm in diameter
  • The tubercle migrates from between the rear legs caudally toward the tail in female fetuses and cranially toward the umbilicus in male fetuses. The location and orientation of the fetus must be determined—locate the mandible (points ventrally and caudally); the heart is imaged on the ventral midline of the thorax. Examine the fetus cranially to caudally, locate the abdominal attachment of the umbilicus. Immediately caudal to that attachment is the male genital tubercle. The female tubercle is best visualized at the caudal-most aspect of the fetus under the tail-head; its optimal image appears within a triangle formed by the tail-head and the distal tibias or hocks

Days 70–130 of Gestation

  • Days 70–75—the fetus can be visualized using transrectal US. There will be some variation depending on the mare's age and parity. At this time, the weight of the developing pregnancy pulls the uterus over the brim of the pelvis
  • At 95 days of gestation—the fetus may move more dorsally within the pregnant uterus and can be imaged to determine fetal gender
  • From 95 to 130 days of gestation—gender can be determined by locating external genital structures:
    • Female—mammary gland, teats, clitoris
    • Male—penis, prepuce, scrotum

Days 120–210 of Gestation

  • Combination of transrectal and transabdominal US; visualize fetal sex organs (up to 240 days)
  • A wider diagnostic window, more parameters to evaluate, more fully developed sex organs for easier diagnosis

Other Diagnostic Procedures!!navigator!!

Behavioral Assessment

If not pregnant—a mare teased to a stallion should begin to show signs of behavioral estrus 16–18 days after ovulation; a nonspecific indicator of the absence of pregnancy; serves only as an adjunct to more reliable means (TRP, US).

False Positive

  • Failure to show estrus even as she returns to heat (silent heat)
  • Pregnancy loss occurs after formation of endometrial cups
  • Prolonged luteal activity but not pregnant

False Negative

Mare continues to exhibit signs of behavioral estrus when pregnant.

Vaginal Speculum Examination

  • Under the influence of progesterone, the cervix is tightly closed, pale, and dry; not diagnostic for pregnancy as a functional CL in a cycling mare has the same effect on the cervix
  • Often used as an adjunct to TRP of the reproductive tract (nonspecific)

TRP of the Reproductive Tract

Days 15–18 Post Ovulation

  • Tubular tract becomes toned, and the “T” shape of the uterine bifurcation is often distinctly palpable. Palpation of a vesicle in the uterine horn reported as early as day 15; however, palpation of a true bulge at this stage is difficult in all but maiden mares with small uterine horns; possible to crush vesicle with harsh TRP
  • The cervix is generally tightly closed, narrow, and elongated
  • Both ovaries are active producing follicles during early pregnancy
  • False diagnosis of pregnancy based on TRP alone, may be due to early embryonic death or persistent/prolonged luteal activity

Days 25–30 of Gestation

  • Uterine tone is very distinct (elevated); the cervix is narrow and elongated
  • Follicular activity is present
  • A bulge (size—small hen's egg) at the caudoventral aspect of a uterine horn, adjacent to the uterine bifurcation
  • The uterine wall is slightly thinner over the fluid-filled, resilient vesicle

Days 35–40 of Gestation

  • Uterus still demonstrates increased tone; cervix is closed and elongated; ovaries active
  • A tennis ball-sized bulge noted at the base of the uterine horn on the side of pregnancy
  • Uterine tone begins to drop at/around the enlarging bulge as the uterine wall thins around the enlargement. Greatly increased uterine tone remains in the nonpregnant horn

Days 45–50 of Gestation

Palpable bulge increases to softball size.

Days 60–65 of Gestation

  • Vesicle begins to expand into the uterine body, and the palpable bulge resembles the shape of a child-sized football
  • Wall of the uterine horn is distinctly thinner at this stage and the pregnancy begins to lose some of its resiliency. Good uterine tone often maintained in the nongravid horn and at the tip of the gravid horn
  • The increasing size of the pregnancy begins to pull the uterus ventrally

Days 150–210 of Gestation

  • Uterine descent into the ventral abdomen is complete; ovaries often located near the midline
  • The fetus may consistently be ballotted within the fluid-filled uterus

To Term

  • Pregnancy continues to occupy more of the uterus and uterine tone diminishes. It expands dorsally and resembles the size of a basketball, eventually to a large, distended uterus. Near term the fetus can readily be palpated in the uterine body and its activity assessed, as well as fetal presentation and position.
  • The pregnant uterus can be confused with a full urinary bladder
  • To distinguish the 2, the fluid-filled uterus can be traced back to the closed cervix at the caudal aspect of the uterine body
  • Additionally, as the uterus continues to drop deeper into the abdomen, the ovaries are drawn ventrally and toward the midline
  • After 7 months, near term/late in gestation, the fetus is visible (US). The amnion (thin, floating membrane) and characterisitcs of both amniotic and allantoic fluids can be observed.

Pathologic Findings!!navigator!!

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Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

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Nursing Care!!navigator!!

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Activity!!navigator!!

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Diet!!navigator!!

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Client Education!!navigator!!

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Surgical Considerations!!navigator!!

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Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

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Contraindications!!navigator!!

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Precautions!!navigator!!

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Possible Interactions!!navigator!!

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Alternative Drugs!!navigator!!

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Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Pregnant mares are routinely examined in the last trimester of gestation to verify fetal viability. Can often determine presentation late in gestation (anterior, posterior, transverse)
  • The most common method of pregnancy diagnosis as this stage is TRP with ballottement of the fetus
  • Transabdominal US may be used to measure fetal parameters—heart rate, aortic diameter, activity, nature of fluid surrounding the fetus (amniotic, allantoic); combined thickness of uterus and placenta to monitor for placentitis

Prevention/Avoidance!!navigator!!

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Possible Complications!!navigator!!

Embryonic/fetal loss/abortion, twins, placentitis, ruptured prepubic tendon, abdominal wall herniation, hydrallantois, hydramnion, uterine torsion, uterine rupture, prolonged gestation, dystocia.

Expected Course and Prognosis!!navigator!!

A normal, viable fetus born at term gestation.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

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Age-Related Factors!!navigator!!

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Zoonotic Potential!!navigator!!

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Pregnancy/Fertility/Breeding!!navigator!!

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Abbreviations!!navigator!!

  • CL = corpus luteum
  • eCG = equine chorionic gonadotropin
  • ELISA = enzyme-linked immunosorbent assay
  • RIA = radioimmunoassay
  • TRP = transrectal palpation
  • US = ultrasonography, ultrasound

Suggested Reading ⬆ ⬇

Bucca S. Equine fetal gender determination from mid- to advanced-gestation by ultrasound. Theriogenology2005;64(3):568–571.

Ginther OJ. Reproductive Biology of the Mare, 2e. Cross Plains, WI: Equiservices, 1992.

McCue PM, McKinnon AO. Pregnancy examination. In: McKinnon AO, Squires EL, Vaala WE, Varner DD, eds. Equine Reproduction, 2e. Ames, IA: Wiley Blackwell, 2011:2245–2261.

Sitters S. Palpation of the pregnant mare per rectum. In: Dascanio J, McCue P, eds. Equine Reproductive Procedures. Ames, IA: Wiley Blackwell, 2014:185–187.

Author(s) ⬆

Author: Carla L. Carleton

Consulting Editor: Carla L. Carleton

Acknowledgment: The author/editor acknowledges the prior contribution of Margo L. Macpherson.