There are a number of things that can go wrong during the last phase of pregnancy. Some of these problems can be resolved or dealt with rather easily, and others are more serious -- and can be life threatening to the cow or fetus.
EXCESS FLUID AROUND THE FETUS -- On occasion, either the amnion sac surrounding the fetus or the allontioc sac (the outer "water bag") may produce too much fluid. Extra fluid in the amnion sac is more rare (a condition called hydramnios); it occurs mainly in Dexter cattle that have "bulldog: calves--a hereditary condition which may produce extra fluid as early as the 3rd of 4rth month of gestation. More commonly, excess fluid is only produced in the outer water sac. This condition, called hydroallantois, is seen in the last trimester. The fetus is often quite small for its stage of gestation and there is a sudden increase of fluid, which becomes noticeable by about six to seven months when the cow develops a huge belly. The later this happens, the better chance she'll survive until the end of pregnancy.
The large volume of fluid puts pressure on her lungs and gut; she doesn't have room to breathe and can't eat very much — losing weight. She may become so weak and/or impaired by her large belly that she has trouble getting up and down. If she survives til term she usually needs help to calve because she is weak, her uterus may not contract properly (being so distended) and the cervix may not dilate fully.
Your vet may advise terminating the pregnancy early, before she becomes that compromised, or in some cases she should be humanely slaughtered. If she is near term, you may choose to have your vet do a C-section, but it must be done carefully, draining off the extra fluid slowly. If it rushes out too fast, the cow may go into shock. After the calf is removed (either by traction through the birth canal after excess fluid is drained away, or by C-section), the uterus should be checked to make sure there's not a second calf; twins are common in cases of hydroallantois.
UTERINE TORSION -- Sometimes the pregnant uterus rotates, putting a twist in the end closest to the cervix. This occurs most often at calving (during early labor) but can happen in late gestation. The lower, larger part of the uterus is supported by the floor of the abdomen, but this makes it more prone to swinging or shifting as the cow gets up and down (as she is kneeling on her front legs and standing with her hind legs). While the uterus is thus suspended in the abdominal cavity, any sudden shift due to the cow slipping or falling, or being knocked by another cow, or the fetus making a quick movement, may cause the uterus to swing and turn over. Cows that fight a lot may experience a torsion.
This problem is rare in heifers, seen more frequently in cows that have had calves before--especially cows with a deep, spacious abdomen. Cows with twins seldom develop torsions, since two calves more completely fill the abdominal space and create a broader base for the uterus as it rests on the floor of the abdomen.
Some torsions occur in late gestation and may be present for days or weeks without any signs, only becoming obvious when the cow goes into labor and can't deliver the calf. If a torsion is minor (the uterus turned only 45 to 90 degrees) it may correct itself during labor. Torsions greater than 180 degrees will completely obstruct the birth canal (the twist closes it off) and the calf can't be born unless the torsion is corrected. Torsions greater than 180 degrees occuring before calving may create obstruction of blood vessels, interfering with blood supply to the fetus. If it dies, the cow may show signs of abdominal pain and go into shock and die, or the fetus may mummify and the cow shows few symptoms.
ABDOMINAL HERNIA -- On rare occasion a cow in late pregnancy may suffer rupture in the lower portion of the abdominal wall, due to a severe blow to the belly, or weakness in muscles of the abdomen. This allows the uterus to drop through, with nothing between it and the outside world but the cow's skin and some swelling.
The problem may first be noticed as a swelling about the size of a football (mostly fluid), then it becomes larger, drooping down to hock level. The heavy uterus and fetus within it have dropped completely out of the abdomen. The cow should be put in a safe place where she won't be jostled by other cows, and closely watched. She'll usually be able to have a live calf, but may need help to calve. At that point your vet can examine her and determine whether the hernia can be repaired after removal of the calf or if the cow should be butchered.
SWELLING UNDER THE BELLY -- Some cows, especially heifers, develop a lot of swelling under the belly as they approach term, and at first this may be mistaken for the beginning of a hernia. Swelling is due to pressure on the mammary veins by the distended uterus. It may become so large that it extends forward almost to the elbow, and droops quite low under the belly. This is usually nothing to worry about, however, and will resolve after she calves.
VAGINAL PROLAPSE -- Sometimes in late pregnancy a cow will prolapse her vagina. Some cows have a structural weakness; the vaginal tissue is not as well anchored as it should be. This can be an inherited problem, passed from mother to daughter or from sire to daughter if the bull's mother had this weakness. Too much fat in the connective tissue around the vagina may predispose a cow to this problem; it happens more often in fat cows.
The pressure and weight of the large uterus in late pregnancy causes the vaginal tissue to protrude from the vulva when the cow is lying down, especially if her hind end is slightly downhill. Weight of the calf (and gravity) puts pressure on the rear parts of the reproductive tract and it bulges through the vulva. The problem occurs most often during the last two months of gestation. If the cow is still several weeks away from calving when she starts prolapsing, the worse it will get before she calves. Advancing pregnancy puts more pressure on the vaginal tissue.
At first the bulge may be the size of an orange or grapefruit -- a pink ball sticking out of the vulva. It will usually go back in when the cow gets up and the pressure is relieved. But if she starts prolapsing every time she lies down, and strains while lying there, she forces more of the tissue out. A heavily pregnant cow often strains when passing manure while lying down, or even just from the presense and irritation of a mild prolapse, making a small problem into a bigger one. The exposed tissue becomes dirty, since manure flows over it when she defecates. This causes irritation and more straining. The ball of tissue becomes so large (up to volleyball size) that it can't go back in even when the cow gets up and walks around.
Unless it's pushed back soon after it comes out, the prolapsed tissue becomes very dirty and dried out, and may become damaged an infected. Blood circulation to these tissues is impaired, so they become swollen and vulnerable to injury and infection. The longer the inverted tissue is left outside the body, the more swelling occurs (and drying of the membranes) and the harder it is to replace it. Swelling also restricts the passage from the bladder; the cow may not be able to urinate until the prolapse is resolved. She may strain while trying to urinate, aggravating the problem. If she goes into labor while the prolapse is out, the bulge of tissue will impede the birth of her calf. Thus it is imperative to replace the prolapsed tissues if they don't automatically go back inside when the cow gets up.
Some cows have a mild prolapse (when lying down) every time they approach calving, but it never becomes a problem. Others get worse every year. If a cow has a bigger problem the next year, and especially if you have to put it back in and stitch it, you should cull her from the herd unless you want the time commitment of putting it back in and stitching her every year (and then having to watch her closely until she calves, so you can take the stitches out when she goes into labor). Never keep a heifer nor a bull from any cow that prolapses, and if you are unlucky enough to buy a bull that sires daughters with this problem, don't keep any more heifers from him.
To replace a prolapse, restrain the cow, wash up the prolapsed tissue and push it back in. Then take at least three stitches across the vulva with umbilical tape (strong cotton strips), using a curved surgical needle. Emergency stitching can be done, however, with disinfected baling twine poked through holes in the skin with a clean, sharp pocket knife. Umbilical tape is best, however, because it's less apt to pull out than regular suture thread and less abrasive than baling twine.
HARDWARE -- Cows that ingest sharp objects with their feed (nails, pieces of barbed wire or baling wire that got baled up in hay, for instance) are at risk for hardware disease, since the action of the digestive tract tends to push a sharp foreign body into or through the gut lining. Cows become more at risk during late pregnancy, because the increasing size of the uterus puts more pressure on the digestive tract. A foreign object that may have been sitting harmless in the stomach, or only partially embedded in the lining, may get pushed clear through when the gut becomes short on space due to the enlarging uterus. Cows in late gestation tend to have a higher incidence of problems, so if a heavily pregnant cow becomes dull, goes off feed, or shows other signs of illness or peritonitis, hardware should be one of the possibilities to be considered; have your vet examine her.
INABILITY TO GET UP -- On rare occasion a heavily pregnant cow may become weak due to mineral deficiencies or some other dietary problem that interferes with proper metabolism and muscle function. Grass tetany sometimes occurs when pregnant cows are on green pasture (especially cereal grasses such as wheat pastures) in fall or early spring, due to deficiencies in the fast growing forage. In these instances the cow usually recovers if given calcium or magnesium. If a cow is down and unable to get up, consult your vet immediately, for proper diagnosis and treatment.
There are many things that can go wrong during pregnancy, but if you are watching the cows and discover a problem early, you generally have a good chance to do something about it--and are more likely to save the cow and/or calf.