Once a cow is bred, she should calve about 283 days later. However, sometimes the pregnancy is lost.
Late pregnancy loss  is usually visible; you find the aborted fetus or the cow with placental membranes hanging from the vulva. “But with early pregnancy loss, you don’t have a clue,” says Ahmed Tibary, DVM and Washington State University (WSU) professor of theriogenology.
Tibary says pregnancy loss should be considered a possibility whenever there’s a longer-than-average calving season, or higher-than-usual number of late-calving cows. Cows that settled late may have bred early but lost their pregnancies and rebred on a later cycle.
Ram Kasimanickam, a DVM in WSU’s Department of Veterinary Clinical Sciences, says there are three categories of pregnancy loss – early embryonic death, abortion and stillbirth.
The conceptus is an embryo during the first 42 days; after that, all major organs and body systems have formed and it becomes a fetus. If loss occurs before 42 days, it’s termed early embryonic death. After that, it is considered an abortion.
“Abortion involves expulsion of a dead fetus, or a living one incapable of maintaining life outside the uterus. Stillbirth is a full-term calf that’s dead at birth,” Kasimanickam explains.
Many things can terminate pregnancy. “Most of the time we suspect infectious causes like trichomoniasis or campylobacteriosis (vibrio), but there may be non-infectious causes,” Tibary says. These could include genetic abnormalities, malnutrition, stress or poisons.
Early pregnancy loss
In a well-managed herd with no health or nutritional problems, cows should breed quickly, Tibary says. Many producers allow only two cycles for heifers and three for cows; all should be bred and settle in that length of time.
“Early loss generally doesn’t affect length of time in which the cow returns to estrus. She comes back in heat on schedule, just as though she wasn’t bred,” he explains.
When a cow is cycling and bred to a fertile bull, the chances of fertilization are well above 90%. “Most lost embryos degenerate and die before the eighth day – before or just as they come down through the fallopian tube into the uterus. There’s also substantial loss observed before day 14, which won’t affect the cow’s return to heat in her next cycle,” Tibary says.
The causes of early loss include alterations in the egg or in the fertilizing semen. “Researchers are looking at possible defects in semen and the effect of genetics on quality of the embryo. In some situations, the bull/cow combination (genetically) may lead to increased early embryonic loss,” he says.
Some studies have shown that the quality of the cycle just previous to breeding affects the quality of the egg, as well as the embryo’s chances to survive to term, Tibary adds. Postpartum cows that haven’t yet cycled normally before they’re put with a bull – and bred on their first heat after calving – may have less chance for normal pregnancy.
Some cows have a short cycle in their first heat after calving. They may breed, suffer embryonic loss, return to heat 8-10 days later, and settle when bred on that heat.
“This is due to the quality of the corpus luteum that develops on the ovary. These cows may not have enough progesterone to maintain pregnancy. They come back in heat and cycle more normally after that,” he explains.
This is one reason not to breed heifers before they’re sufficiently developed and have normal cyclicity. “They’ll be more fertile, and more able to maintain pregnancy, once they’ve established normal cycles,” he says.
Tibary says synchronization schemes for artificial insemination (AI) should be carefully followed to quantify early pregnancy losses.
“Observe the rate of return to estrus; do an early pregnancy diagnosis, then do a confirmation check later. More than 65% of all losses happen before the embryo attaches to the uterine lining. We just see cows coming back into heat, so we don’t know whether it was the fault of the cow or the bull,” he says.
A smaller proportion – 5-8% of losses – of pregnancies that continue beyond 21 days are lost in the first 42 days – as the conceptus transitions from embryo to fetus, Tibary says. These are called late embryo/early fetal losses.
Rapid weight loss in the cow, or other stress such as transportation in early pregnancy, can lead to losses. There are also risks with toxic plants. Endophyte fungus in tall fescue, for instance, is often responsible for early pregnancy loss, in addition to hindering the animal’s ability to thermo-regulate. In some situations, feed may contain mycotoxins, mold, or estrogen-like substances that may interfere with pregnancy.
“The biggest factors include a sudden change in nutrition, and environmental stresses such as severe cold or heat . Stressful conditions may cause hormonal disturbances. Moving cattle, particularly in the first 42 days of pregnancy, can be a factor, especially if it involves a lot of stress,” Tibary explains. A long truck haul, gathering cattle swiftly out of a range pasture to get them away from an approaching fire, or cattle being continually harassed by wolves, are examples of stress that may lead to pregnancy loss.
A common environmental factor in early pregnancy loss is a high heat index (combined heat and humidity). Losses from heat stress  occur in the first week or two after breeding, but heat and humidity can also have a long-term effect on egg quality and, thus, embryo quality.
“Changes in the eggs can last a long time. Even when temperature goes back to normal, the cow still has poor-quality eggs in her ovaries,” Tibary explains. It might take several cycles/breedings before she can carry a pregnancy.
Mid to late-term abortion
“Some toxicants affect the fetus – things like lupine, hemlock, locoweed, pine needles, ergot alkaloids, certain molds, etc. For instance, pine needle abortion causes fetal loss in late pregnancy when cows consume needles of Ponderosa pine during the third trimester,” Kasimanickam says.
The abortion or premature parturition can occur three days to 2-3 weeks after exposure. “The greatest risk is when cows consume pine needles for three days or more, and have a high level of toxicant, which constricts the vascular network in the placenta,” he says. Blood circulation to the fetus can be reduced to 35-50%, with the restricted supply of oxygen and nutrients killing the fetus. The toxicant also increases uterine contractions that lead to expulsion of the fetus.
“Environmental stress can be a factor in how much toxicant a cow consumes; if she stays under a pine tree for shelter during stormy weather, she may eat a lot of pine needles. It’s best to keep cattle out of areas where there are Ponderosa pine trees, at least during the last trimester of pregnancy,” Kasimanickam explains.
Human error, such as giving a pregnant cow drugs that might cause abortion, can be another factor. Treating a cow with dexamethasone or some other steroidal anti-inflammatory, for instance, can cause pregnancy loss, especially in the last trimester.
Accidental administration of drugs such as prostaglandin F2a (Lutalyse®) or dexamethasone (cortisol) may cause abortion. “If a cow needs treatment with anti-inflammatories, producers should consult with a veterinarian about choice of drugs, dose and frequency,” Kasimanickam says.
“Any product, including vaccines, should be carefully studied before administration,” Tibary adds. “Read the label and check with your veterinarian to see if it’s contraindicated in a certain stage of pregnancy.”
Sidebar: Shipping stress
Transport stress can be a factor in lost pregnancies, especially in the first month after being bred. Shipping cattle at 1-4 days after artificial insemination (AI) is best, as the embryo is still in the oviduct and less likely to be affected by uterine changes, says Ram Kasimanickam, a DVM in Washington State University’s Department of Veterinary Clinical Sciences. It’s also safer after day 42 -- when the pregnancy is well established, the placenta is fully attached, and the fetus is less susceptible to any changes that might result from stress, Kasimanickam adds.
“Shipping at seven weeks after AI is usually considered safe, but pregnancy loss due to shipping has been reported up to 60 days after AI. Care should be taken to reduce stress when animals are transported; never overcrowd trucks or trailers, and handle cattle as gently and calmly as possible,” he advises.
Sidebar: Nutritional stress
Nutrition – especially trace mineral deficiencies – has been linked to increased embryo loss in a herd. And poor preparation of heifers for breeding, or cows losing a lot of body condition just before or immediately after being bred, can result in nutritional stress.
“This change affects hormonal profiles. Nutritional stress can cause early or late pregnancy loss. Specific nutritional deficiency (such as certain trace minerals) can be a factor, even in a herd fed adequate energy and protein. The most important trace minerals affecting reproduction are selenium, copper and manganese,” says Ahmed Tibary, DVM and Washington State University professor of theriogenology.
Sidebar: Diagnosing a problem
“We generally discover problems at pregnancy diagnosis , and try to resolve it for the following year,” says Ahmed Tibary, DVM and Washington State University professor of theriogenology. The overall pregnancy rate might be adequate, but if cows are calving late, something must have happened during the first cycle, he says.
When there’s a poor pregnancy rate or spread-out calving season, cows should be closely monitored  the next breeding season to confirm that breeding activity is occurring or if females are returning to estrus after being bred.
“In one herd, we checked cows halfway through the breeding season to see how many bred in the first cycle. After a 21-day breeding period, we determined pregnancy rate 30 days later by either ultrasound or a blood test to check for pregnancy-specific protein B (PSPB),” Tibary says.
“The blood test works well on heifers, but cows must be more than 90 days post-calving when tested. If they’re bred and tested too soon after calving, there may be some PSPB still in their bloodstream from the previous pregnancy,” he says.
To determine the cause, Tibary looks at the cows, all herd records and historical data to illuminate changes in nutrition or other factors.
“The challenge in investigating poor calving rates is that we’re usually looking back and trying to figure out what might have happened months ago. Were the cows not cycling soon enough (nutritional problem), or did they get bred and lose the pregnancy? If that’s the case, we want to figure out if it was infectious, or due to heat/humidity or some other stress,” he says.
Without information on the cows’ body condition at that time, or nutritional level or mineral status, it’s hard to determine what happened, he says. You could compare pregnancy status to previous breeding/calving seasons, which also necessitates good records.
“You can also look at groups of cows – whether you have the same pregnancy rate/loss in heifers, first-calf cows, or older cows that have had two or more calves,” Tibary says.
Heather Smith Thomas is a rancher and freelance writer based in Salmon, ID.