Venous Thrombosis During Pregnancy

What is Venous Thrombosis?

Thrombosis is the medical word for a blood clot that occurs within a blood vessel preventing blood from moving through the vessel. Clotting can occur in arteries, the blood vessels that carry blood away from the heart to nourish body tissues. Clots can also develop in veins, which are the blood vessels that carry blood back to the heart. Usually, arterial thrombosis results from a disease process in the wall lining the inside of the arteries, while venous thrombosis is caused by problems with what’s called the coagulation system. This system consists of various proteins called clotting factors (chemical agents that promote clotting), anti-coagulation factors (agents that slow down blood the clotting process), and platelets (thrombocytes), which are special cells that carry some of the clotting agents and also become part or the clot (thrombus).

Venous Thrombosis During Pregnancy

Most pregnant women are healthy and young, so usually blood vessel wall disease is not an issue. However, pregnancy alters the coagulation system. Thus, venous thrombosis is a risk and most cases of thrombosis in pregnancy are in veins, not arteries. In anatomy, veins are categorized as superficial (close to the surface of the body), deep (deep in the limps), or central (deep in the trunk). The principal problems are deep venous thrombosis (DVT) and central vein thrombosis (CVT). Both DVT and CVT can obstruct blood flow severely. Additionally, the clot, or a piece of the clot, can break off and become a traveling clot, called an embolism. Emboli are extremely dangerous, because they can get stuck in different locations throughout the body. An embolism getting stuck in the blood vessels of the lungs, called a pulmonary embolism, is an example of such a danger. Since virtually all of the blood passes through the lungs, a pulmonary embolism is potentially fatal if it is not recognized and treated early. Usually, the prospect of a pulmonary embolism is the biggest concern in women who suffer a DVT or a CVT, because an embolism generated in a deep or central vein will be carried through the right side of the heart to the lungs, where the embolism will get trapped. In some people, however, blood and anything carried in the blood can pass from the right side of the heart directly to the left side without going through the lungs. This can be a result of a failure of what is called the foramen ovale –an opening between the right and left atria (upper heart chambers) during fetal life– to close after birth. Known as a patent foremen ovale, the condition can be present all the time, but in many cases the foramen ovale opens only in a particular circumstance when the pressure on the right side of the heart increases substantially compared with the pressure on the left side. In any event, an embolism that bypasses the lungs and goes directly to the left side of the heart can get into the arteries that supply the heart itself (coronary embolism), or into the brain (cerebral embolism). As with a pulmonary embolism, the situation is extremely dangerous.

In addition to increasing tendency for clot formation, pregnancy also raises the pressure inside the pelvis. This can cause the right iliac artery to push a deep vein called the left iliac vein against the lower spine. The combination of blood flow obstruction and increased clotting tendency can cause a CVT.

Clots can form throughout pregnancy and the post-partum period, because pregnancy alters the coagulation system in a way that speeds up clotting. This helps to compensate for blood loss during delivery, but the flip side is that the speedier clotting can be harmful in some women. The thrombosis risk is especially high at particular times. In the late first trimester, risk is elevated, for instance, possibly due to the coagulation system shifting toward increased clotting to protect against bleeding in placental blood vessels, thereby lowering the risk of spontaneous abortion (miscarriage). From the middle of pregnancy onward, the tendency for clotting decreases slightly, but then it rises up to a very high peak just after delivery. The risk remains elevated for several weeks after delivery. Consequently, more than half of pregnancy-related DVTs occur during this period. Although DVTs outside of pregnancy tend to be in the legs, during pregnancy and in the post-partum period, these clots tend to be in the pelvis.

Diagnosis of Venous Thrombosis

Doctors can use a range of procedures for diagnosing venous thrombosis. One diagnostic technique for diagnosing DVT is called compression ultrasound (CUS).  It is painless and noninvasive. For diagnosing CVT, the best technique is called time-of-flight magnetic resonance venography (TOF-MRV). Doctors must observe pregnant women who may have venous thrombosis in order to look for signs of pulmonary embolism and other dangerous complication. A major sign of pulmonary embolism is difficulty with breathing. One way to check for pulmonary embolism by way of a ventilation/perfusion (V/Q) scan. This procedure analyzes the flow of air using a radioactive substance. Another method for diagnosing pulmonary embolism is called computer tomographic pulmonary angiography (CT-PA). Some chest specialists opt for using simple flat X-ray scans instead of CT-PA in order to reduce amount of radiation to which the fetus is exposed, but this is a highly controversial choice, because CT-PA is a much better test for detecting pulmonary embolism, a condition that can kill the woman and her fetus, if not detected and treated immediately.

Treatment of Venous Thrombosis

To treat venous thrombosis, the strategy is to dissolve the clot, and also to lower the tendency for the woman’s blood to clot by modifying the coagulation system. Physicians do this with an agent called heparin, which binds to an anti-clotting factor called anti-thrombin. In binding, heparin helps anti-thrombin work better so it takes longer for clots to form, leading clot to break up. Different types of heparin are available. For pregnant women, the choice is called low molecular weight heparin (LMWH). When no LMWH is available, your doctor would chose a type of heparin called unfractionated heparin (UFH).

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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