The diffuse microcotyledonary placentation of the mare makes it highly unlikely that a twin pregnancy will be carried to term. If the twin pregnancy is maintained until the latter part of gestation the placenta cannot meet the nutrient demands of the rapidly growing fetuses. Death of one or both fetuses is followed by abortion, with the characteristic avillous areas on the fetal membranes confirming the amount of placental disruption (). Twin abortions in the last few months of gestation are likely to cause a dystocia. The live birth of twin foals is extremely uncommon, and many of these neonates do not survive. The mares are prone to fetal membrane retention and may be difficult to rebreed. Thus it is not surprising that the equine breeding industry has always tried to avoid twin pregnancies. This chapter will review the management options that are currently available.
Monitoring Follicular Development And Ovulation
A high incidence of twin ovulations occurs in some breeds, such as Thoroughbreds and warmbloods, and mares that tend to double ovulate can be expected to do this repeatedly. Thus a mare with a tendency to double-ovulate should have this information noted prominently on her breeding record. Most twin pregnancies arise from such double ovulations. Owners need to appreciate that these double ovulations are generally asynchronous and may be separated by a couple of days. If a fertile stallion was used to breed the mare on the first ovulation, it is possible that viable sperm will still be present in the reproductive tract when the second oocyte arrives. This possibility must be remembered when scanning mares for pregnancy at 14 to 16 days. At that time, it is good practice to scan the ovaries for evidence of luteal tissue from a second ovulation.
In the past, one strategy that was employed when a veterinarian palpated two large (>30 mm) follicles was to wait to breed until the next cycle. This approach wasted valuable days in the breeding season, and many of these mares would repeat the same follicular process during the next cycle. An alternate approach was to hope that the second follicle would continue to develop for 10 to 12 hours after the first detected ovulation. Because the ovulated oocyte is unlikely to be viable at this time, a delayed breeding could be performed in anticipation of the second ovulation. Today the preferred strategy is to breed all eligible mares — irrespective of the number of preovulatory follicles. The widespread adoption of early ultrasonographic pregnancy examinations has permitted the focus to be placed on embryonic vesicle reduction once the presence of a twin pregnancy has been confirmed.
The increasing size of the embryonic vesicle, coupled with the increasing tone of the early pregnant uterus, tends to fix the conceptus at the base of one uterine horn by day 16. It is essential that the ultrasound scan of the uterus be thorough, with a complete examination of the length of both horns plus the uterine body as far back as the cervix. This is especially important before day 16 because the vesicle moves freely within the lumen of both horns and the uterine body. The advantage of these early scans is that if twin vesicles are detected it will be easier to manually separate them before day 16. Successful elimination of one vesicle is more likely at that time because the uterine walls are thin, and minimal pressure is required to crush a vesicle. A definite “pop” can be felt when the vesicle ruptures, but success should always be confirmed by ultrasound.
The downside to this approach is that an early embryonic vesicle can easily be confused with an endometrial cyst. The embryo itself does not become readily identifiable until the fourth week of pregnancy. Thus it is good practice to note the size and location of any cysts at the time the mare is being examined for breeding. However, it is not an uncommon occurrence that the veterinarian doing the early (14-16 days) pregnancy scan will be examining the mare for the first time. If no record of cyst size and location exists, it is virtually impossible to differentiate twin vesicles from a singleton and a cyst with a single examination. This is especially true because asynchronous ovulations are likely to result in considerable size discrepancy between the two vesicles. Under these circumstances it may be best to measure each suspect vesicle and note its location. A second scan in 1 to 2 days should note a size increase in any normally growing vesicle (~4 mm/day). Only then can a confident decision be made about attempting to “pinch” one of the growing vesicles. Unfortunately this delay may make separation of unilaterally fixed vesicles more difficult because of their ongoing growth and the increased uterine tone.
Manual reduction of bilaterally fixed vesicles requires less manipulation than with unilateral twins. It is a relatively easy procedure, and success rates exceeding 90% are not uncommon if the vesicle is crushed before day 16. If the vesicles are unilaterally fixed, the clinician should attempt to move the more proximal vesicle away towards the tip of the uterine horn. At this location the manual reduction procedure is less likely to disrupt the remaining vesicle. The vesicle can be crushed by pinching it between the thumb and fingers. Alternately, the vesicle is squeezed against the mare’s pelvis until it ruptures. If the twins can be separated before crushing, the success rate may be similar to that for reduction of bilateral twins. If the unilateral twins cannot be separated or are greater than 20 days’ gestation, the success rate is lower. The extra pressure used to eliminate a twin vesicle after fixation is the reason many clinicians will accompany reduction with antiinflammatory medications and progestin therapy. The likelihood of success improves with experience, and some clinicians develop a reputation for being especially adept at the procedure. Obviously the nature of the mare is an important factor, and those that strain excessively can make the procedure extremely difficult. If the unilateral vesicles are not detected until after day 20, manipulations can easily result in the disruption of both vesicles. The best option in these cases may be to wait and see whether natural reduction occurs.
Almost three quarters (70%) of twin embryonic vesicles become fixed unilaterally; only 30% of twin vesicles become fixed bilaterally. The advantage of this probability is that natural reduction to a single pregnancy is far more likely with unilaterally fixed vesicles. Over 80% of unilaterally fixed twins are likely to naturally reduce to a singleton, with over half of these occurring between days 16 and 20. On the other hand, the majority of bilaterally fixed vesicles will continue to develop. Late in the season these odds play an important part in any informed discussion about management options. Early in the season most veterinarians will opt to attempt reduction, knowing that if both vesicles are lost that it will still be possible to rebreed the mare. Close to the end of the season an unsuccessful attempt at reduction may preclude the mare from being rebred. If natural reduction does not occur by day 30, the advent of transvaginal reduction has opened a window for later attempts at reduction. If this fails, owners may opt to put the mare under lights and breed her early next season rather than be locked into a pattern of late foals.
Pregnancy Termination With Prostaglandin
If natural reduction does not occur, terminating the pregnancy with a prostaglandin injection is always possible. This will cause lysis of the corpora lutea that resulted from the double ovulation, and the precipitous decline in progesterone will bring the mare back into estrus. However, this treatment must be given before day 35. Once the endometrial cups form it may take repeated injections to terminate the pregnancy, and the mare is unlikely to return to estrus until the cups are sloughed. The endometrial cups originate from specialized fetal trophoblast cells. They secrete equine chorionic gonadotropin (eCG), a hormone that causes the development of accessory corpora lutea and thus augments the progesterone level in support of the early pregnancy.
Transvaginal Ultrasound-Guided Allantocentesis
Although the advent of transrectal ultrasonography has dramatically improved the ability of veterinarians to make an early diagnosis of twin pregnancies, diagnostic errors still occur. This could be due to an early pregnancy diagnosis when the second vesicle was too small to detect, incomplete examination of the entire uterus, poor image quality, or an inability of the clinician to differentiate two embryonic vesicles that are closely apposed to each other. If natural reduction does not occur or the diagnosis of twins is not confirmed until after 30 days, transvaginal aspiration of one vesicle is an option. The results are best if the procedure is performed before day 35. Although spontaneous reduction of twin pregnancies can occur even after day 40, the probability is low. Natural twin reduction is more likely to occur if an obvious size discrepancy is present between the two vesicles at this time.
If a transvaginal reduction is to be attempted, the mare should be treated with flunixin meglumine. Many clinicians will also administer oral altrenogest. Because sedation causes significant uterine relaxation, most clinicians use a lidocaine enema to reduce straining. The transvaginal aspiration technique employs a 5.0- or 7.5-MHz endovaginal curvilinear transducer. The transducer and casing should be cold-disinfected or sterilized before use. The assembled unit is then placed in a sterile transducer cover that has been filled with sterile lubricating gel. The transducer is advanced aseptically until it is seated lateral to the cervix. The clinician then grasps the pregnancy per rectum and advances a sterile 60-cm, 18-gauge spinal needle with an echogenic tip along the needle guide in the transducer casing. A dotted line on the ultrasound screen can be used to select a path for the needle entry into the embryonic vesicle. A sharp jab of the needle penetrates the vaginal wall, peritoneal lining, uterus, and ultimately the allantoic or yolk sac. A 60-ml syringe is attached to the needle, and the embryonic fluid aspirated. Aspiration should be stopped when danger of damaging the adjacent vesicle of unilateral twins arises. If a bilateral twin is being eliminated, the needle can be moved within the vesicle until all detectable fluid has been aspirated. The success rate is better for bilateral twin reductions. Death of the remaining twin is most likely to occur within 2 weeks of the procedure. Although reports are scarce, preliminary data suggest that experienced operators may achieve a live singleton birth in about one third of cases.
Transabdominal Ultrasound-Guided Fetal Cardiac Puncture
In advanced twin pregnancies, attempting reduction by a transabdominal approach is possible. Fetal intracardiac injection of potassium chloride is effective but requires accurate placement of the KC1 into the fetal heart. Best results are obtained when the pregnancy is between 115 and 130 days. At this stage experienced operators can achieve a 50% success rate. Procaine penicillin G can cause fetal death when injected into either the fetal thorax or abdomen, but the effect is not instantaneous. The advantage of the latter treatment is that it does not require precise placement of the injection into the fetal heart. Mares should be started on oral altrenogest, systemic antibiotics, and flunixin meglumine on the day of the procedure. The antibiotic coverage and antiinflammatory medication should be continued for 3 days.
A 3.0-MHz transducer can be used to image the 90- to 130-day fetus in the caudal abdomen, just cranial to the udder (Figure 5.9-2). Once the mare has been sedated, the uterus will relax, and the location of the fetuses will shift cranially. A sedative/analgesic combination that works well for this procedure is acepromazine (10 mg), xylazine (100 mg), and butorphanol (10 mg). The smallest and/or most easily accessible fetus is selected for reduction. The ventral abdomen should be surgically prepared, and local anesthetic infiltrated at the puncture site. Some clinicians are adept at a “free-hand” injection technique, whereby the fetus is injected by merely observing the ultrasound image. Others prefer to use an ultrasound transducer that is fitted with a biopsy guide. An 18-gauge, 6- to 8-inch spinal needle with stylet can be used for most fetal injections. The distance from the skin surface to the fetus determines the length of the needle that is required. Specialized needles with echogenic tips are available to provide better visualization via ultrasound. Once the location of the selected twin’s thorax is confirmed, the needle is introduced through the prepared skin, abdominal wall, and uterus. If procaine penicillin G is to be injected, the needle may puncture either the fetal thorax or abdomen. Up to 20 ml is typically injected into the fetus. Fetal death should be confirmed the following day.
Although the benefits of supplemental progestin therapy are debatable, many clinicians suggest that the mare be medicated for at least 2 weeks if the initial twin reduction has been successful. It is essential that fetal viability be checked regularly because supplemental progestin therapy may prevent elimination of the dead fetuses if both die. Most abortions will occur within 1 to 2 months after the reduction procedure. Survival of the remaining twin seems to depend somewhat on the amount of endometrial surface that was its domain before the reduction. If the operator is experienced in the technique, between 30% and 60% of cases can be expected to deliver a singleton foal, although the ultimate size and viability may be suboptimal. The eliminated twin in these cases can be seen as a mummified remnant contained within an invaginated pouch that protrudes into the allantoic space of the viable foal’s fetal membranes.