Pregnancy success in beef cattle depends on much more than fertilization. In many cases, the embryo is alive and the cow is diagnosed pregnant around days 28-35, but the pregnancy is still not fully secure. One of the most important biological events during this period is early placentation, which is the process by which the developing calf begins to build a functional connection with the uterus. 

When that process does not progress normally, pregnancy loss occurs during what is often called late embryonic or early fetal mortality, generally referring to losses after maternal recognition of pregnancy and through the early fetal period.

In cattle, the placenta is different from that of humans and other species. Rather than forming one large attachment area, cattle develop many smaller attachment sites called placentomes. Each placentome is formed when the fetal side of the placenta, called a cotyledon, connects with a maternal caruncle in the uterus. These placentomes become the major exchange points for oxygen, nutrients and waste products between the cow and the developing calf. Cattle typically have many placentomes, rather than one single placental disk, which is why healthy early placental development is so important for supporting fetal growth.

The timeline is rapid. After fertilization, the early embryo reaches the uterus and eventually hatches from its protective covering around days 9-10.

For a few days it remains small and round, but by about day 16 it begins to elongate dramatically. This elongation is not just a change in shape; it allows the developing conceptus to signal its presence to the cow and prepare for attachment. By about days 19-22, the fetal membranes are closely associated with the uterine lining, and the earliest stages of attachment are underway.

Maintaining pregnancy

During this early window, progesterone is one of the key hormones supporting pregnancy. Producers often think of progesterone as the hormone that “keeps the cow pregnant,” and that is accurate; but its role is broader. Progesterone helps prepare the uterine environment by stimulating secretions that nourish the developing embryo. These secretions, often called histotrophs, include nutrients, growth factors, sugars, lipids, amino acids and other compounds needed before the placenta is fully functional.

If the uterine environment is not properly prepared, the embryo may fail to elongate, attach or develop a normal placenta.

A second important event is the development of specialized placental cells known as trophoblast giant cells. These cells arise from the fetal side of the placenta and are especially important because they help remodel the uterine lining and release pregnancy-associated glycoproteins, commonly called PAGs. These PAGs eventually enter the cow’s bloodstream and are used in many blood-based pregnancy tests. However, PAGs are more than just a pregnancy-test signal. They also give us insight into how well the early placenta is developing.

Research has shown cows with greater circulating concentrations of PAG around Day 28 are more likely to maintain pregnancy than cows with lower PAG concentrations. Lower PAG does not always mean the embryo is already dead, but it can indicate placental development is weaker or delayed. In practical terms, PAGs may help identify pregnancies at greater risk for loss before that loss is obvious by ultrasound or by the cow returning to heat.

Another group of compounds involved in this process is prostaglandins. Most producers are familiar with prostaglandin F2α because it is used to regress the corpus luteum, especially during an estrous synchronization protocol. Because of that, prostaglandins are often thought of only as “pregnancy-ending” compounds. The biology is more complicated. During early pregnancy, the conceptus and uterus also produce prostaglandins that appear to help regulate attachment, cell growth, tissue remodeling and blood flow at the developing maternal-fetal interface. 

In other words, the same family of compounds that can be used to synchronize cows also appears to have normal roles in building the placenta when present at the right time, place and amount.

The period from approximately days 21-45 of gestation is especially important. This is when the pregnancy moves from recognition and early attachment toward formation of functional placentomes. It is also a period when late embryonic losses can occur and may be difficult to explain from a management standpoint. The cow may have conceived, the embryo may have signaled pregnancy and an early pregnancy diagnosis may have been positive, yet placental development may not be strong enough to carry the pregnancy forward.

Future productivity

For producers, the economic effects of these losses are real. Recent work in beef cows has shown cows experiencing late embryonic or early fetal mortality can have reduced final pregnancy rates, lower calving rates, lower weaning rates, later calving in the next season and lighter calves at weaning. That means a pregnancy lost after the first diagnosis may affect not only the current calf crop, but also the cow’s future productivity. 

Many factors can contribute to pregnancy loss during this stage. These include oocyte quality, semen and embryo quality, heat stress, disease, nutrition, body condition, uterine health, hormone patterns, and the cow’s ability to support placental development. 

Editor’s note: Cliff Lamb is the animal science department head and a professor at Texas A&M University in College Station, Texas.