Whilst the thought of a mare giving birth to twins may sound heart-warming, the reality is far from it, with only the occasional mare giving birth to two healthy foals. With this in mind, it is usual for vets to scan the mare 14 to 16 days after ovulation to identify whether she is pregnant with twins. Here Ed Lyall BvetMed CertEM MRCVS of the Sussex Equine Hospital explains the process used for identifying twins and what can be done if a mare is found to be carrying a twin pregnancy.
It is standard for mares to be examined by ultrasound for pregnancy between 14 and 16 days from ovulation after either natural service or being mated by artificial insemination. At this stage we commonly find that mares are in foal carrying twin pregnancies. A twin pregnancy occurs due to the mare ovulating twice rather than due to splitting of the embryo after conception, as occurs in many other species.
Splitting of an embryo results in identical twins, which is not seen in horses. Multiple ovulations result in multiple eggs produced by the mare that can be fertilized by the stallion’s semen, the usual result is twins, however occasionally we find triplets. There are several hundred million sperm cells in an ejaculate used for mating, as such there are plenty of sperm cells to fertilize multiple eggs.
Mares are rarely able to carry twins, as their uterus can not provide the nutrients to rear two foals. if a mare does become pregnant with twins there are several likely outcomes:
1. One of the embryos is reabsorbed and the other remains resulting in a single pregnancy at full term.
2. Both embryos die and are reabsorbed.
3. One foetus dies later in the pregnancy and becomes mummified, resulting in a single pregnancy at term.
4. Both foetus die and are aborted usually later in the pregnancy.
5. Both foetus survive in utero and are born at term.
Twins put mares at significant risk, a mare that aborts in a paddock unattended may be missed as having slipped the foals, she may have a retained placenta that potentially can be life threatening. There may be significant uterine scarring after placental retention and future infertility problems. The actual foaling, if the mare does go to term with live twins, can be very difficult as the foals can easily become stuck during the birth. This can be very difficult to deal with at a yard, the mare would then have to travel in labour to a clinic with suitable facilities to deal with such a dystocia.
Mares in foal with twins that do go to term usually produce small, ill thrifty foals that are weak and require significant veterinary attention to get them to survive. Often these foals then go on to have developmental orthopaedic abnormalities such as osteochondrosis desicans (OCD) later in life.
It is very uncommon for a mare to give birth to a pair of normal strong foals without any problem. The bigger the mare, the better the chances. There does seem to be a size related relationship between the incidence of twins in mares, it is very unusual for pony mares to go in foal with twins, however in some seasons I can scan as many as 20% of warmbloods in foal with twins.
There are advantages
One interesting observation is that a mare that ovulates twice on a cycle resulting in twins will often do this repeatedly and I have seen daughters of twinning mares do this too. Twins can be an advantage when we are doing embryo transfer, as for a single cycle we may be able to flush two embryos, from one dose of semen. If we suspect a mare may be going to have twins we set two recipient mares up to synchronise with the twinning donor mare.
With the modern management of broodmares, the incidence of twins being aborted or born is very low now. The control of twins starts early on in the mare’s cycle, around the time of service we look with an ultrasound scanner at the ovaries primarily to get the timing of mating accurate, but also to see when and how many times the mare has ovulated. During the covering phase of the cycle we also map out where the mare has endometrial cysts in her uterus. We then accurately scan the mare 14 days after ovulation as this is the best time to identify pregnancies in the uterus. My routine is to then scan the mare at 16 days, as this will pick up a second pregnancy from an asynchronous ovulation (ovulations several days apart resulting in pregnancies several days different in age – sperm cells can survive in the oviduct for several days). It is then standard to scan the mare again at around 28 days from ovulation.
Ultrasound can be used to detect an embryo in the uterus from around 11 days, so the routine described above would pick up twins. Mares scanned later in gestation are harder to clearly identify twins as the structures within the uterus such as the membranes and the umbilical cord become complicated.
As vets, we make every effort to make sure a mare is not in foal with twins, however occasionally twin embryos can be missed. There are several reasons for this happening:
1. The mare is presented at the wrong stage in her cycle by the owner for an accurate scan to be carried out.
2. Endometrial cysts in the uterus confuse the scanning procedure as they have similar appearances to embryos.
3. The mare is scanned in suboptimum conditions, for example where the light is too bright to see the ultrasound scanner screen or the mare is not restrained properly and she moves around too much.
4. There are asynchronous ovulations, meaning that an embryo may be several days younger than another detected on scan and is not yet visible.
When twins are identified on the 14 or 16 day pregnancy scan we can carefully crush one of the embryos with the ultrasound scanner. Up to about 17 days from ovulation the embryos have not bonded or implanted on the wall of the uterus, as such they can be moved around in the uterus with gentle pressure from the ultrasound scanner probe. Invariably twins will end up sitting next to each other, it is not wise to attempt to crush one in this state, so we move the one we want to crush, usually the smaller of the two, away to the end of the uterine horn and apply pressure until it’s membranes pop. I medicate the mare with an anti-inflammatory at this stage and then scan her again two days later to make sure the single embryo is retained.
Occasionally a mare will lose the second embryo following manual reduction in this way. Often this is due to inability to separate the embryos, probably due to being scanned too late after implantation. This is why accurate timing of scanning is important, the best window for twin reduction is between
14 and 16 days after ovulation.
If twins are identified in a mare at 28 days it is still possible to pinch the membranes of one of them, but the success is much lower at this stage. If the twin has not been reduced to a single by 35 days we usually administer a prostaglandin injection and abort both embryos as after this stage the mare’s uterus produces endometrial cups and then will not come back into season for over three months, meaning she can not be rebred.
If twins are identified after 35 days there are several different techniques to inject substances by ultrasound guidance into one of the foetus. The success rate with these techniques is very low.