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Basics ⬇

Outline

BASICS

Definition!!navigator!!

The intrauterine production of 2 or more embryos/fetuses.

Pathophysiology!!navigator!!

  • The majority of multiple pregnancies are twins
    • Most twins are dizygotic; result from double ovulations
    • Early twin vesicles behave similarly to a singleton conceptus
    • The vesicles undergo TUM until 15 days of gestation; fixation occurs in 1 or both uterine horns at ≈ day 16 gestation
    • ≈75% of twin vesicles fix in the same horn (unicornual)
  • ≈75% of unicornual twin pregnancies <40 days of age undergo natural reduction to 1 embryo
    • The remaining embryo develops normally to term as a singleton
    • If natural reduction of 1 embryo fails to occur by day 40, there is a strong probability that twin development will continue, only to abort later in gestation
  • Bicornual twins do not undergo natural reduction; the twins usually develop through the last trimester of gestation, at which time abortion is common

Systems Affected!!navigator!!

Reproductive

Genetics!!navigator!!

Higher incidence of double ovulations in some horse breeds.

Incidence/Prevalence!!navigator!!

  • Occurs more frequently in older and barren mares, Thoroughbreds, draft breeds
  • Lower incidence in Arabians, Quarter Horses, ponies, primitive breeds

Signalment!!navigator!!

Mares that develop twin pregnancies tend to have twins in subsequent pregnancies.

Signs!!navigator!!

N/A

Causes!!navigator!!

  • Ovulation and fertilization of multiple ova, in most cases
  • Rare incidences of monozygotic multiple pregnancies have been reported

Risk Factors!!navigator!!

Breed, age, and reproductive status.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Differentiating Similar Signs

  • Uterine cysts can be confused with developing embryonic vesicles, causing an improper diagnosis of twin pregnancy
  • Recording location, size, and shape of lymphatic cysts prior to breeding (a cyst map) makes it easier to distinguish cysts from embryonic vesicles
  • If prior season records are unavailable, reexamine the mare in 2 days. The vesicle of pregnancy will demonstrate a noticeable increase in diameter by US; the uterine/lymphatic cyst will not increase in that period of time
  • A lymphatic cyst is also distinguished from an embryonic vesicle in which an embryo and an embryonic heartbeat will be observed at 24+ days of gestation

CBC/Biochemistry/Urinalysis!!navigator!!

N/A

Other Laboratory Tests!!navigator!!

N/A

Imaging!!navigator!!

Transrectal US

  • For prompt diagnosis and treatment of twin pregnancies
  • Twins may be detected as early as day 9 of gestation; however, twin pregnancies may differ in age by as much as 2 days. Because of the possible differences in age and size of early twins, US diagnosis, if a double ovulation is confirmed, is recommended between 14 and 15 days of gestation
  • The embryonic vesicles at this time are an anechoic, spherical yolk sac; average 14–20 mm in height
  • Twin vesicles are highly mobile for the first 15–16 days; they may be located adjacent to one another or in different locations within the uterus within a brief time frame
  • It is critical to scan both uterine horns and the uterine body to the cervix, sequentially, with at least 2 full sweeps across the tract

Transabdominal US

For diagnosing twin pregnancies at >75–100 days of gestation (generally by identifying 2 fetal heartbeats).

Other Diagnostic Procedures!!navigator!!

TRP of the Reproductive Tract

  • Pregnancy characteristics at days 25–30 by TRP—distinct uterine tone; a narrow, elongated cervix. A bulge the size of a small hen's egg at the caudoventral aspect of a uterine horn, adjacent to the uterine bifurcation
    • If twin vesicles are bicornual in location—may be possible to diagnose twin pregnancies by TRP
    • If twin vesicles are unicornual in location—it is impossible to diagnose twins by TRP alone. An experienced practitioner may note the bulge of pregnancy feels larger than anticipated

Pathologic Findings!!navigator!!

N/A

Treatment ⬆ ⬇

Outline

TREATMENT

!!navigator!!

Management of Twins Prior to Day 40 of Gestation

  • Twins detected during TUM of the conceptus (days 9–15) are best managed by crushing 1 embryonic vesicle. 1 vesicle may be manipulated transrectally to the tip of a uterine horn, pressure placed on the vesicle, and it is crushed. Alternatively, its location/distance from the bifurcation of the uterine horns is confirmed by US, trapping the vesicle at that location between 3 fingers on its ventral aspect and thumb on top, increasing pressure on the sphere until the crush is felt. The success of a crush should always be confirmed by a follow-up US examination; dispersed fluid will be evident
  • The remaining vesicle survives in approximately 90% of cases
  • The crush technique is useful for twins in the mobility phase and bilateral twins prior to 30 days of gestation
  • After 30 days of gestation, fluid released from the crushed vesicle tends to disrupt the remaining pregnancy. The increased pressure required to disrupt a larger vesicle may also result in prostaglandin release from the endometrium, leading to loss of the second vesicle
  • Unicornual twins are usually both destroyed when crushing of 1 is attempted

Management of Twins After Day 40 of Gestation

Craniocervical Dislocation

  • Technique of dislocating the first cervical vertebra from the cranium—involves a rocking motion applied to the head with pressure at the back of the skull to sever the spinal cord and disrupt the ligamentous attachments. Can be performed between 50 and 150 days of gestation; either transrectally, by laparotomy, or potentially via colpotomy
  • Management of twin pregnancies in the fetal period (≥day 40) is further complicated by the formation of endometrial cups
  • Pregnancy loss once endometrial cups have formed has been shown to cause irregular estrous cycles, and/or a delayed return to fertile cycles
  • Consequently, maintenance of a singleton pregnancy following a >40 day reduction procedure is critical to the reproductive success of the mare

Alternate Methods of Twin Management That Have Been Attempted

  • Dietary restriction—in 1 study, mares with twin pregnancies (diagnosed by TRP) were limited to poor quality grass hay early in gestation (day 21–49 of gestation)
    • 1 viable foal was delivered in 56% (23 of 41) of the cases examined, i.e. reduction had occurred
  • Surgical removal of 1 twin—was attempted in 7 unicornual and 8 bicornual twins at 41–62 days of gestation
    • None of the unicornual twins survived
    • 5 of 8 bicornual twin pregnancies were successfully reduced to a singleton
  • Intracardiac injection of potassium chloride (KCl)—using transcutaneous US, 1 fetal heart is located and fetal death is caused by intracardiac injection of KCl
    • The technique has been successful in approximately 50% of the cases attempted
    • This procedure is most useful during 115–130 days of gestation
  • Transvaginal allantocentesis—transvaginal US has been used to aid in identification of the allantoic sac of 1 twin fetus, a needle is passed into the allantois, allantoic fluid is aspirated
    • Fluid aspiration collapses the vesicle, leading to fetal death
    • To date, this technique has been successful in approximately 30% of attempted cases
    • Most applicable for bicornual twins

APPROPRIATE HEALTH CARE

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Nursing Care!!navigator!!

N/A

Activity!!navigator!!

N/A

Diet!!navigator!!

See Methods of Twin Management That Have Been Attempted.

Client Education!!navigator!!

  • Importance of TRP in the periovulatory period (before and after breeding and insemination), and noting >1 ovulation during an estrus
  • Emphasis of early (latest 16 days following ovulation), serial evaluations of a high-risk mare:
    • Multiple ovulations
    • Prior history of twinning
    • Breeds with higher incidence of twinning
    • Suspicious appearance of vesicle of pregnancy at initial examination warrants follow-up US within 2–4 days
  • Earlier reduction of twins (<16 days, prior to fixation) increases likelihood of success, continuation of remaining embryo/fetus to term

Medications ⬆ ⬇

Outline

MEDICATIONS

  • Flunixin meglumine 1 mg/kg IV is often administered at the time of attempted twin reduction to prevent prostaglandin release from the uterus and subsequent lysis of the corpus luteum
  • Exogenous progestins (altrenogest 1 mL/50 kg body weight (0.044 mg altrenogest/kg body weight) PO daily) at double the recommended dose (2 mL/50 kg) may be administered when twin reduction is attempted to maintain uterine and cervical tone following uterine manipulation; to counter the effects of possible fetal fluid release into the uterine lumen

Drug(s) of Choice!!navigator!!

N/A

Contraindications!!navigator!!

Twinning is undesirable in the mare and routinely ends in abortion.

Precautions!!navigator!!

N/A

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • After any method of twin reduction is performed, it is useful to monitor the progress and viability of the remaining embryo/fetus using US
  • It is critical to monitor the mare for embryo/fetal death if the mare is being treated with exogenous progestins
  • Once weekly US examinations are warranted for the first 3 weeks following the procedure
  • Less frequent examinations, i.e. once monthly, after the initial examinations are useful to monitor fetal progress
  • The mare can also be monitored for signs of abortion such as mammary development, vulvar discharge, or fetal expulsion

Prevention/Avoidance!!navigator!!

  • Serial, complete TRP, and maintenance of individual records for broodmares
  • Record sizes of all follicles >30 mm on both ovaries during estrus (average growth is 5–6 mm/day) to account for ovulation or regression
  • Double ovulation is the earliest indicator of mares at higher risk for developing twins
  • Early diagnosis of pregnancy in mares from families with a history of twinning, or one known to have twinned in a prior pregnancy (same season or prior years)
  • Earlier reduction is associated with greater success in achieving a singleton pregnancy

Possible Complications!!navigator!!

Embryonic or fetal loss, abortion, dystocia.

Expected Course and Prognosis!!navigator!!

Success Associated With Each Reduction Technique

  • 90% success with early crush of a bicornual twin
  • 50% with KCl intracardiac injection during days 115–130 of gestation
  • 20–30% with transvaginal aspiration of allantoic fluid from 1 of bicornual twins

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

N/A

Abbreviations!!navigator!!

  • TRP = transrectal palpation
  • TUM = transuterine migration
  • US = ultrasonography, ultrasound

Suggested Reading ⬆ ⬇

Ginther OJ. Ultrasonic Imaging and Reproductive Events in the Mare. Cross Plains, WI: Equiservices, 1986.

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

Putt E, Christensen BW, Wimmer A. Transrectal cranio-cervical dislocation of a twin fetus in a mare. Clin Theriogenol 2014;6(3):356.

Schnobrich MTR, Riddle WT, Stromberg AJ, LeBlanc MM. Factors affecting live foal rates of Thoroughbred mares that undergo manual twin elimination. Equine Vet J 2013;45(6):676–680.

Sitters S, Wolfsdorf K. Twin reduction: cranio-cervical dislocation. In: Dascanio JJ, McCue PM, eds. Equine Reproductive Procedures. Ames, IA: Wiley Blackwell, 2014:222–225.

Author(s) ⬆

Author: Carla L. Carleton

Consulting Editor: Carla L. Carleton

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