Twin Pregnancies

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   Dr Mina C.G. Davies Morel, University of Wales, Aberystwyth

   Incidence of twin pregnancies

   Twinning is an increasing problem in stud management, especially in intensively bred
   horses such as Thoroughbreds, Arabs and Warmbloods. Twin pregnancies result from the
   fertilisation of two ova. The incidence of twin ovulations, and, therefore, twin pregnancies,
   varies significantly with breed and age of mare. It has been reported that only 1% of
   oestrous cycles in pony mares result in a multiple ovulation (MO) whereas in
   Thoroughbred, Arab and Warmblood horses up to 40% of cycles demonstrate MO. The
   incidence of MO also increases with age. All these ovulations have the potential to be
   fertilised and result in pregnancy. Mortality of some of these twin embryos does naturally
   occur, as will be discussed later, but as most mares are scanned very early in pregnancy
   (Day14-20 p.c.) the incidence of twin pregnancies is very similar to, and follows the same
   pattern as, multiple ovulation. As all these pregnancies have the chance to develop further
   and it is not possible at this stage to accurately identify those that will naturally die, all twin
   pregnancies must be considered to be of concern. Additionally it is increasingly evident
   that twinning is a heritable trait.

                                                                      Why are twin pregnancies a
                                                                      problem in the mare?

                                                                  The mare is classified as
                                                                  monocotous, her uterus only
                                                                  being able to adequately
                                                                  support a singleton. The
                                                                  placenta for that pregnancy
                                                                  needs to attach to the entire
                                                                  surface of the uterus in order
                                                                  to satisfy the demands of the
                                                                  foetus, especially in the last
                                                                  trimester. The development
                                                                  of a twin pregnancy results
                                                                  in the surface of the uterus
   available to each placenta being reduced, so compromising foetal development and
   ultimately survival. Hence twin pregnancies rarely survive to term, most commonly
   resulting in abortion in mid to late pregnancy (9-10months). They are in fact the most
   common cause of non-infectious abortion, accounting for 20-30% of all occurrences. Only


   9% of twin pregnancies survive to term: 64.5% of which result in 2 dead foals, 21% in one
   live foal and 14.5% in 2 live foals.

   Twin pregnancies, like multiple ovulations can be classified as unilateral, both in the same
   horn, or bilateral one in each. 70% of pregnancies are reported to be unilateral and 30%
                                               bilateral. With regard to the possible prognosis
                                               of a twin pregnancy they can be further
                                               classified into: i) equal division of the uterus
                                               (50%:50%); ii) minor unequal division of the
                                               uterus (40%:60%); iii) or major unequal
                                               division of the uterus (20%:80%) (Figure 1).

                                                i) Equal division of the uterus (50%:50%)

                                                If the division of uterine surface area available
                                                to the placenta of each twin is equal, then the
                                                pregnancy is bilateral and both twins have an
   equal chance of survival. However, survival is in no way guaranteed and due to placental
   restriction their birth weights of any surviving twins will be reduced. Such foals often die at
   or soon after birth and if they do survive rarely achieve their expected mature size or
   performance or race potential.

   ii) Unequal division of the uterus (60%:40%)

   If the division of the uterus is unequal, but only a minor inequality, the pregnancy will be
   bilateral and it is likely that it will develop normally initially. Problems will become evident
   when placental restriction affects the smaller twin, often in the last trimester. The smaller
   twin will die and cause the whole pregnancy to abort.

   iii) Unequal division of the uterus (20%:80%).

   If the division is unequal, with a major inequality, the pregnancy will be unilateral and one
   twin will predominate over the smaller twin, which is confined to the upper part of the
   uterine horn. The smaller twin is unable to develop to any extent and, therefore, dies early
   on. In such a scenario it is possible that the smaller twin will become mummified and so not
   cause abortion of the whole pregnancy. The larger twin will continue to develop normally.
   However, if the smaller twin develops a placenta to any extent and survives to the stage
   when placental size has been fixed (approximately Days100-150) then after its death,
   though part of the uterine surface will have been vacated, the placenta of the surviving twin
   will be unable to take advantage of them as full placental size has already been determined.
   In such a case a singleton will be born but have a smaller birth weight than might be

   expected. As the mummified foetus may not be noticed in the placental tissue the reason
   for a low birth weight may not be immediately apparent.

   B) Equal split (50%:50%).                     C) Unequal split (60%:40%).

   D) Unequal split (80%:20%).

   Figure 1 Placental configurations in the equine singleton and twin pregnancies

                A) Singleton.

                 B) Equal split (50%:50%).
                 C) Unequal split (60%:40%).
                 D) Unequal split (80%:20%).

   What would happen naturally to twin pregnancies in the mare?

   Naturally mares that conceive twins would rarely produce a live foal. The continuance of
   the trait for twinning within the population would, therefore, be limited. This is largely
   what has occurred in the more native type pony breeds and why their twinning rates are so
   low. However, modern techniques such as scanning allow us to manage mares that
   conceive twins, as discussed below, in such a way that they can produce offspring but this
   results in a perpetuation of the twinning trait within the population. Hence intensively
   managed mares have the highest twinning rates. If a twin pregnancy in any breed in
   monitored a significant natural reduction (loss of one embryo) does occur in early
   pregnancy. In this reduction the position of the pregnancies is important as 85% of
   unilateral pregnancies reduce naturally at around Day18, but very limited reduction of
   bilateral pregnancies occurs.

   Managing the mare that conceives twins?

   As the conception of twins invariably does not result in viable offspring it is advantageous
   to identify and manage twin pregnancies as early as possible, especially if recovering is
   planned. The advent of ultrasonic scanning has significantly helped this aim.

   There are 4 main management practices used to reduce the incidence of twins: i) monitor
   ovulation; ii) wait and see; iii) manually reduce; or iv) treat with PGF 2.

   i) Monitor ovulation.

   Historically, the incidence of twinning was reduced by monitoring ovarian activity using
   rectal palpation and withholding covering from mares with more than 1 large follicle. The
   mare would then be covered on the next natural or the next artificially advanced oestrus.
   This successfully reduced twinning rates within a population but with it conception rates
   declined and the time interval between parturition and successful covering increased. In
   order for these drawbacks to be addressed, identification and treatment of actual twin
   pregnancies rather than potential twin pregnancies is required.

   ii) Wait and see.


   As up to 85% of unilateral twin pregnancies are reported to reduce by Day 18 one option is
   to monitor the pregnancy and observe if natural reduction occurs. If not induced abortion
   at a later stage may be advocated. The advent of scanning now allows such monitoring to
   occur easily.

   iii) Manually reduce

   An alternative to natural reduction is to manually reduce. Manual reduction of twins to a
   single has been reported to be up to 96% successful between days 13 and 16. Manual
   reduction involves the manual squeezing of the smallest embryo, identified by ultrasound,
   either between the thumb and forefinger or by using the scanner probe to push the
                                                 conceptus against the uterine wall and pelvis
                                                 until the vesicle ruptures. This is best done
                                                 prior to fixation (Day18) so is normally
                                                 carried out at initial scanning Days11-12.
                                                 After fixation the manual reduction of
                                                 bilateral twins can still be very successful but
                                                 reduction of unilateral twins runs a higher
                                                 risk of loosing the whole pregnancy. Other
                                                 methods of manual reduction at later stage of
                                                 pregnancy (after Day 40) have been reported
                                                 such as ultrasound guided allantocentesis
                                                 and transabdominal foetal cardiac puncture,
                                                 but have not proved as sucsuccessful as early
                                                 manual reduction.

                                                 Figure 2. The appearance of twins on an early
   ultrasonic scanning picture. Note the two dark spheres indicating two embryonic vesicles.

   iv) Treat with PGF2

   An alternative to manual reduction of one twin is to artificially induce abortion of the
   whole pregnancy and recover the mare at the next advanced oestrus. Abortion and
   subsequent return to oestrus and ovulation can be induced using a single injection of
   PGF2, multiple injections may be required later on in pregnancy. Abortion can be induced
   prior to the next expected oestrus, ie before Day21 of pregnancy. This is often done at the

   time of first scanning which may be as early as Day11. In which case the mare’s return to
   pregnancy post abortion may be delayed by only 15 days or so. Alternatively, the
   pregnancy may be allowed to progress longer in the hope that natural reduction may occur
   before PGF2 is required. However, if a rapid return to oestrus is required then PGF 2 must
   be administered prior to the development of the endometrial cups at Day40.

   The high incidence of twin pregnancies is of considerable concern. The advent of modern
   techniques such as ultrasonic scanning and hormone therapy allows us to effectively
   manage mares with twin conceptuses. However, the industry must be aware that in
   allowing mares that habitually throw twin pregnancies to reproduce, the trait for twinning
   is being perpetuated within the equine population and with it the need for human

   *If you are interested in helping current research into the neurochemistry of equine
   stereotypy, veterinarians or horse-owners in the unfortunate position of having to put
   down either stereotypy or non-stereotypy animals can donate brain material by contacting
   The University of Wales Aberystwyth on 01970 621690 or by e-mailing