The development of twin-management techniques with the use of ultrasound has saved millions of dollars for horse owners and turned the incidence of abortions due to twinning from commonplace to a rare occurrence.

Avoidance of twinning can be attempted in several different ways. Careful examination of the ovaries to detect multiple follicles to try to breed between the ovulations, or not breeding at all with the presence of multiple follicles, lessens the incidence of twinning. Avoiding use of ovulation induction products (e.g., deslorelin, hCG), which may encourage multiple follicles to ovulate, would reduce the incidence of twinning. However, the presence of multiple follicles and multiple ovulations increase the chance of mares becoming pregnant; consequently, in the author’s opinion, it is better to increase the chance of becoming pregnant and deal with the occurrence of twins should they occur, rather than have a non-pregnant mare.

When twins do occur, there are many useful questions that should be considered when deciding to manually reduce a twin vesicle during early gestation (~14–16 days of gestation):

• Are there definitely multiple vesicles or is it a cyst and a singleton?
  
• Is the vesicle that I am leaving definitely a pregnancy?
  
• Will the vesicle move? (cysts are reluctant to move)
  
• Has the vesicle grown from the day before? (cysts rarely change size – pregnancies increase in size rapidly, ~ 3 mm per day, at this stage)
  
• Is the vesicle large enough to be easily pinched or is it so small it may be difficult to express?
  
• If I have difficulty, cannot express the twin and have to try again later, will I know which vesicle I manipulated?
  
• Should I let the vesicles grow a day? What day of pregnancy is it?
  
• Should I come back in a few hours or try to separate the unilateral twins now?
  
• Is there a good chance the twin will reduce on its own? Approximately 75% unilateral twins at 17 days reduce on their own. There is a greater chance twins will reduce on their own if they are of different sizes (personal experience)
  
• Is there a large size discrepancy between the vesicles? Is the size discrepancy related to ovulation dates?

All of these questions should be answered before manually reducing a twin.

Examination for pregnancy at the proper stage of gestation is mandatory to detect twins and have the opportunity to successfully express the selected twin vesicle. Some practitioners like to check 15–16 days from breeding to be sure they are still in the mobility phase. Some like to check 14–15 days from ovulation. It is sometimes difficult to tell exactly when ovulation occurs and accuracy of ovulation date is dependent upon when the mare is checked post breeding. If the mare is examined 2 days after breeding for ovulation then ovulation could have occurred anytime from when she was last checked before breeding to 2 days after breeding. This could be a 3-day span. Personal preference dictates when the examinations are done, but it is important not to wait longer than 16 days from conception. After that time, if the twins are unilateral, they could be very difficult to separate. The author uses 15–16 days from breeding because the pregnancy cannot be any older than 15 or 16 days.

At 14–15 days from conception the vesicles are in their mobility phase and can be found anywhere in the uterus. A thorough examination of the mare’s uterus is mandatory, regardless of the number of ovulations thought to have occurred. It is important to examine the most distal tip of each horn and near the cervix. If the examination occurs after 16 days from conception and the twins are unilateral, it is often difficult to separate the vesicles due to increased tone of the uterus resulting from the progesterone influence from the corpus luteum and fragility of the vesicles. Thus, the examination is best done during the mobility phase, before 17–18 days post conception.

During the mobility phase the twins can be separated, even if they are unilateral and in close proximity. Sometimes two yolk sac membranes will come together forming a very faint line indicating twins (Figure 17.1). This can be difficult to see. If the twins are touching closely it is sometimes best to leave and return in 4–5 hours (Figure 17.2). If they are in the mobility phase they frequently will separate on their own when given time to move (Figure 17.3). If they continue to be in close proximity or re-examination is not possible, 10 mg IV of acepromazine maleate (Vedco, Inc., Saint Joseph, MO, USA) relaxes the uterus. Allow about 5 minutes after acepromazine administration before attempting to separate unilateral twins transrectally. The twins can then be separated manually much more easily with less risk of unintentionally injuring either vesicle. The trick is to only injure the vesicle you are trying to rupture and not both. A twin vesicle is more easily expressed in a maiden mare than in a post-foaling mare with a larger uterus because the wall of the maiden mare’s uterus is thinner.

If the mare is straining or resisting your examination, it is important to use a rectal spasmolytic such as N-butylscopolammonium bromide (Buscopan Injectable Solution, 60 mg IV, Boehringer Ingelheim Vetmedica, Saint Joseph, MO, USA) or propantheline (30 mg IV), to allow ease in handling the uterus. It is difficult to use delicate manipulation when the mare is straining against you. Sometimes gentle nudging of both vesicles will slightly separate them enough to allow placement of the ultrasound transducer between them to gently spread them apart. Slow and gentle pushing of the transducer into the area between the vesicles causes the vesicles to separate. For good results it is best to manipulate the uterus and vesicles as little as possible. With proper technique the percentage of normal single pregnancies at 60 days from twins that have been pinched at ~ 15 days is equal to a group of mares pregnant with a single pregnancy at ~ 15 days (personal experience).

If there are cysts present in the uterus, it is critical to be certain that the vesicle being left is a true pregnancy. A cyst can often be identified by having a visible wall that can be readily seen (Figures 17.4A and 17.4B). Cysts usually have an irregular shape and thickened wall, while the vesicle of a 15-day pregnancy should be spherical or slightly oval in shape with a wall that is difficult to visualize (Figures 17.5 and 17.6). Occasionally cysts appear just like a pregnancy (Figure 17.4C). Pregnancies at 18 + days can be slightly irregular in shape (Figure 17.7). A cyst cannot move much in comparison to a vesicle that can move from one end of the uterus to the other before 17 days. A cyst does not grow in size when examined the next day, while a pregnancy grows very noticeably at the 14–15-day stage. The size of the vesicle should be the proper size for the number of days of the pregnancy.

When encountering two vesicles of different sizes it is a good idea to try to reduce the smaller twin if possible (Figure 17.5). Sometimes a vesicle is smaller than expected because it is from a later ovulation, but it is possible that it is smaller because it is defective for some reason. If a vesicle is smaller than 15 mm, it is frequently evasive and difficult to express (Figure 17.6). It is preferable to let a vesicle grow 1 day to become larger than 15 mm to facilitate expressing it and to avoid extensive manipulation of the uterus. Letting the vesicle grow 1 day is another good way to differentiate a cyst from a pregnancy; the pregnancy should increase noticeably in size daily while the cyst will remain the same size. Monitoring the vesicles with measurements and saved ultrasonographic images is useful to determine size changes.

When a vesicle is reduced it can be pinched between the thumb and forefingers or it can be expressed by pressure between the transducer and the pelvis. There should be a very slight fremitus (vibration) felt as the vesicle ruptures and a small amount of free fluid is visible in the uterus at the spot of the rupture (Figure 17.8). Frequently, the fluid from the ruptured vesicle migrates toward, and can be seen around, the pregnancy that is still present (Figure 17.9). If a twin is near the cervix, it can be expressed by forming a cup with four fingertips around the vesicle and dragging the hand posteriorly, toward the cervix, pressing down on the floor of the pelvis until the vesicle ruptures. The ability to express a vesicle is difficult at first, but becomes much easier with experience. If you are expressing a twin in the horns of the uterus while one twin is in the body or near the cervix, be careful that the forearm does not put pressure on and accidentally rupture a twin near the cervix. If the twin is in the horn of the uterus it usually can be pushed to the tip of the horn where it can be easily trapped and expressed.

After the vesicle is reduced, in the author’s opinion, it is worthwhile to treat the mare with flunixin meglumine (1.1 mg/kg IV) once daily for 3 days. This has an anti-prostaglandin effect in case there is a prostaglandin release from the manipulation of the uterus or in response to fluid from the pinched vesicle. It is important to remember to minimize manipulation of the remaining vesicle whenever possible. Some clinicians recommend altrenogest (0.044 mg/kg PO SID) in case there is compromise of the corpus luteal tissue supporting the pregnancy.

Occasionally, more than two pregnancies are present (Figure 17.11). The procedure is the same as for twin reduction; however, some clinicians like to reduce one vesicle and come back the next day to reduce one more. This prevents a large amount of fluid from the rupture of two vesicles being released into the uterus at one time.

It is possible when examining a mare during the mobility phase at 15 days post ovulation that the mare could have a 14–15-day pregnancy and a 9–10-day pregnancy. Missing the 9–10-day pregnancy would be easy to do because of its small size. A transrectal scan 4–5 days later at 19–20 days from breeding should alleviate that problem, because the 9–10-day pregnancy would have grown and be easy to see at that time.

The next exam should be at 28–30 days of gestation, again looking for twins that could have been missed. If twins are found at this stage and are bilateral they can be expressed with about a 50–60% success rate (personal experience). If the twins found at this examination are unilateral, it is very difficult to express one without injuring the other (Figure 17.10). In this case, it is best to follow the twins until about 33–34 days of gestation to see if they reduce to a singleton on their own. Finding two viable heartbeats is the best indicator of twins persisting. At ~ 35 days, the endometrial cups are forming, which may prevent the mare from returning to estrus if pregnancy loss were to occur. The endometrial cups produce pregnant mare serum gonadotropin (PMSG; equine chorionic gonadotrophin), causing secondary corpora lutea development for maintenance of the pregnancy. Prior to endometrial cup formation, and taking into consideration if there is time to rebreed in the breeding season, the mare can be given prostaglandin to return to estrus and rebreed. Most mares do well becoming pregnant again from these subsequent breedings. If it is too late in the breeding season to rebreed the mare, it is best to see if the twin reduces on its own. The author has had poor results from manually expressing twins past 40 days, or trans-vaginal twin aspiration and transabdominal twin injection.

If twins are found after fixation there are several procedures that can be attempted past 40 days from conception. One technique involves traumatizing usually the smaller of the two fetuses, through manipulation and percussion using manual compression or the ultrasound probe. A success rate approaching 50% may be attained [1]. Multiple attempts may be necessary to complete reduction and it is often difficult to assure manipulation of the same fetus at each attempt.

Another technique is transabdominal injection of KCl into the heart of the fetus, using the assistance of an ultrasound biopsy guide connected to the transducer [2]. This may result in a success rate near 40% [3]. Injections of 10–20 cc of procaine penicillin G into the thorax or abdomen of the fetus resulted in a 50–60% success rate [1]. The optimal time for this procedure is 115–130 days of gestation. No difference in survival rate were seen when NSAIDs, antibiotics, and altrenogest are used post procedure [1].

If twins persist beyond 60 days, the author recommends a procedure associated with a approximate 50–60% success rate [4]: manual cranio-cervical dislocation of the fetal head to terminate one fetus in an attempt to preserve the other. This can be performed transrectally after 60–70 days up to about 110 days of gestation. This involves grasping the head of the fetus and breaking down the tissue at the base of the posterior skull to sever it from the spinal cord. This does not immediately result in death of the twin but it will usually die in a few weeks. The untouched twin will be born normally about 50–60% of the time, while 40–50% of mares will abort both fetuses. This technique can be very difficult to perform transrectally, consequently a sterile surgical procedure was developed via a sterile surgical flank incision and grasping the head of the fetus through the uterine wall and dislocating it. The head is dislocated by manual manipulation at the base of the fetal skull with the thumb. The uterine wall stays intact. An advantage of this procedure is that a fetal sex determination can be performed and when there is a filly and a colt, the owner can select which sex to keep. A fetal sex determination is performed with transrectal ultrasonography during the surgery by another practitioner (Video 54), and the proper fetus is identified for elimination to the surgeon going in through the flank.