21. Twin gestation
Horses rarely deliver twins: one half percent of all horse deliveries described in literature concerns twins. Still, when scanning breeding mares for an early gestation diagnosis within three weeks following conception, you do regularly come across twins. What happens during the following weeks inside the mare's uterus becomes evident by repeated ultrasound scanning.
The functioning of a scanner is based on echography: ultrasound is emitted through a probe and the echoes of the ultrasound are subsequently picked up. This inaudible sound is reflected by the various types of tissue in the body in varying intensities. The echoes result in a shadow image on the monitor. The technique is safe for both the unborn embryo as well as for the mother animal. The very first ultrasound scanners to be used in veterinary practices were geared to applications used in hospitals. By way of illustration: the directions for use for a device that we purchased in 1987 stated: ‘The duration of the pregnancy can be determined by measuring the distance between the crown and the tail bone’. That is no horse talk.
The child in the uterus of the mother-to-be is visible on the scanner when the probe is held against the belly. This is not possible in mares: the distance between the abdominal wall and the uterus is too great to be able to make a gestation diagnosis in that way. In order to get close enough to the uterus, the probe must be inserted in the mare’s rectum. The uterus is located in the pelvis directly beneath the rectum. If the mare is with young, then you can observe a small amniotic sac on the eighteenth day following the natural cover or artificial insemination. It is visible on screen as a small black ball around a centimetre in diameter. What you actually see is the fluid inside the sac. The embryo is still microscopically small and is therefore not visible on the monitor.
During the first season that I worked with the scanner, I examined the gestation of about fifty mares. I came across no less than six with twins. That is twelve percent! Whereas twins in mares were known to occur in one half percent of the cases. This was twenty-four times that number! Evidently, I was not using the device correctly. Therefore, I examined the six mares a second time three weeks later. Before the introduction of the scanner, I would examine mares for pregnancy by feeling the uterus through the rectum at six weeks of pregnancy. So I knew what to expect during the follow-up examination with the scanner. One of the mares was found not to be with young anymore: both amniotic sacs had disappeared. She became in heat shortly afterwards and was serviced again. The other five mares were found to still have two amniotic sacs. They had both meanwhile grown, but they had not both grown consistently. On screen, I could distinguish an embryo in the larger sac; but the image of the smaller sac was ‘messy’. I decided to wait and see. In the spring to come, all six mares gave birth to a single foal. The ‘messy’ contents of the smaller amniotic sacs had been embryo’s that had died. An early embryonal death of this kind is common in mares.
Horse breeders are not pleased with the prospect of twins: they prefer one good quality foal to two poor quality foals. And twins don't yield two good quality foals. When the scanner became generally established in veterinary horse practice for gestation examinations, twins were found so often that this resulted in the launch of a new type of treatment. It involved ‘crushing’. It soon became popular in the horse breeding field. Via the rectum, through the wall of the uterus, the smaller of the two amniotic sacs was pressed against the pelvis rim of the mare and crushed. However, it was often found that the other embryo soon died off as well. To avoid this as much as possible, the crushing should occur as early on as possible, preferably at two
weeks’ gestation. A follow-up treatment with an anti-inflammatory agent would then diminish the irritation of the mare’s uterus as a result of the crushing. And extra pregnancy hormone was administered to prevent the rejection of the remaining embryo.
It is 1988 and mare Paulette is fourteen years old. She had been with twins on two previous occasions. Both times ended in disappointment. The owner has heard that it is possible to determine whether a mare is bearing twins early on in the gestation using a scanner and he requests such an examination. We get to work in the stable. Paulette is given a twitch on her upper lip and the owner holds it. The scanner is standing a bit to one side of the mare on a garden chair. It looks like a television. An electric cord connects it to the power socket in the feed-alley. The probe is connected to the second cord; it looks like a black cigar. It is made of hard plastic that can resist horse manure. I hold it in my right hand as I stand behind the mare for the rectal examination. I am wearing a long plastic glove on my arm.
Two amniotic sacs
With my arm in the rectum and the probe in my hand, it takes a minute to find the underlying structures. I see the outlines of the uterus on the screen in the garden chair. That is the left horn and there … yes! A small black ball appears on the screen. I then lead my hand from the left ovary across the uterus to the right side and the other ovary. A second small black ball appears on screen. I give a start, but I say nothing: I have to be sure first. But the owner can tell by my expression that something is wrong. I again lead the probe across the uterus, this time from right to left. And sure enough: there is a small black sac in both the right and left uterine tube. The black ball on the right is somewhat larger than the one on the left, but only marginally. It is a huge disappointment for the owner: twins again! When had the mare been serviced? Everything has been recorded on the calendar on the stable door: trimming, servicing, worming and vaccination. Today marks day twenty after servicing. Now what?
mare and owner
Wait and see
I have always refused to use the crushing technique. First of all, crushing an embryo goes against the grain with me and secondly, it is not good for the uterus. But chiefly: it is almost never necessary as nature itself provides an elegant solution in the form of the premature death of one of the embryos. After all, the six mares with two small amniotic sacs in their uterus all delivered just one healthy foal. I propose that we repeat the examination of Paulette in three weeks’ time. The owner feels a glimmer of hope: could the veterinary surgeon be wrong?
The small ball in the left horn of the uterus is found to have grown considerably three weeks later: now the embryo is clearly visible on screen. But the black ball on the right has not grown at all. There is no embryo to be seen: it looks exactly like it did three weeks before. And so the right image concerns a cyst: a cavity in the wall of the uterus that is filled with fluid. It is a benign disorder that mainly occurs in older mares. This cyst is round and is indistinguishable from an amnion sac in an early stage of gestation. According to the guide-lines for crushing, I should have crushed the smaller of the two, the one in the left uterus horn, straight away during the initial examination at twenty days of pregnancy. That would have ended the gestation of the mare, as that was the amnion sac. The cyst on the right would have been all that remained.
How it ends
Paulette delivered a beautiful foal in the spring of the next year. She remained fertile until late in her life. But she developed more and more cysts in her uterus. As a result, I could not make a proper diagnosis of her gestation until after the fifth week, when the embryo itself was visible. Her last foal was delivered at the age of twenty-three. It was a mare foal and she was named ‘Paulette deux’. As far as the naming of foals goes, the studbook dictates the first letter of the name of every foal to be born in a certain year. It follows the alphabet, but the letters Q, X and Y are not included. It is too difficult to think of a lot of different names that start with one of those letters. And so you are back to A after 23 years. In the case of
Paulette, she came full circle with the birth of her last foal.
© Leo Rogier Verberne