Why Does Your Blood Clot More Easily During Pregnancy?

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Here on The Pulse, we have explored how various aspects of the body change during pregnancy. One example is red blood cells, which become less concentrated as pregnancy advances. This is not because there are fewer of them, but because the volume of blood increases substantially and red blood cell production cannot keep up with this. Despite being more diluted, blood is more prone to coagulation, the formation of blood clots, also called thrombosis. Hypercoagulability is most pronounced toward the end of pregnancy and for a few weeks after you deliver.

Since your blood is more susceptible to forming blood clots during this time than when you’re not pregnant, doctors advise pregnant women to avoid sitting down for long periods, or doing other things that cause blood deep veins to to slow down or stop moving. Known as stasis, the slowing of blood happens on account of things that you cannot control, such as the growing womb putting pressure on the deep veins in your pelvis, but also is affected by factors that you can control, namely how much you move your legs. Getting up from your chair and moving around for a few minutes a couple of times per hour can help prevent stasis or at least minimize it. Stasis and hypercoagulability are two prongs of what doctors call the Virchow triad. This is named for 19th century physician Rudolf Virchow, who realized that having more than one of these things at the same time makes thrombosis more likely than having just one. The third prong in Virchow’s triad, damage to the inner lining of blood vessels or the inner lining of something else containing blood, like the heart, is not usually an issue in healthy pregnant women. But stasis together with hypercoagulability of blood makes the deep veins a place where clots can form easily. This leads to deep venous thrombosis (DVT) in a limb, most often a leg, and it leads to central venous thrombosis, which is like a DVT, but in the pelvis.

Along with causing local problems where they are located, the bigger danger of such clots is that that can embolize, meaning that pieces of them can break off and travel to other parts of the body. Known as an embolus, such a traveling clot can get trapped somewhere, grow into a bigger clot, and obstruct blood flow in that remote place. This is called an embolism and it can happen throughout the body, but the main place where it can happen from a DVT is in the lungs. This is called a pulmonary embolism (PE) and it can be life threatening. Together, the conditions of blood clotting in deep veins and emboli that can result are called venous thromboembolic embolism.

By why is the blood of pregnant women hypercoagulable in the first place? The reasons are complex, but it comes down to the various proteins that participate in the coagulation progress. The concentrations of some of these clotting factors rises in the blood during pregnancy. One such protein that rises is factor I, also called fibrinogen, which transforms into fibrin, which connects platelets together in a blood clot. Another such protein that rises during pregnancy is factor VII, whose job is to trigger a cascade of various other factors becoming active, leading to simulation of coagulation. Factor VIII, which also rises in pregnancy, facilitates the activation of another factor that promotes coagulation. People who suffer from hemophilia A have a deficiency of factor VIII, so pregnancy is sort of the opposite of hemophilia A when it comes to factor VIII.

Finally, another protein whose concentration goes up during pregnancy is von Willebrand factor. Present in endothelial cells that line the inside of blood vessels and in platelets (clotting cells), von Willebrand factor functions to enable platelets to stick to one another and to the walls of blood vessels when there is a wound. This works because wounding damages blood vessels, causing the von Willebrand factor to leak from the endothelium cells and reach platelets circulating in the blood. Also, von Willebrand factor has another job, which is to carry clotting factor VIII, thus protecting it from substances in the blood that would otherwise degrade it. During pregnancy, the concentration of von Willebrand factor rises making it easier for platelets to stick to one another and to blood vessel walls, and since von Willebrand factor protects factor VIII, the concentration of factor VIII also rises, making the coagulation pathway more prone to be triggered, adding further to the clotting tendency.

But now let’s imaging the opposite situation, namely, when von Willebrand factor is deficient, or when something is wrong with it. Known as von Willebrand disease, this is the most common reason for blood not being able to clot well enough and it manifests in different forms. The most common type of von Willebrand disease, type 1, is simply a deficiency of von Willebrand factor, resulting from one of the two copies of the gene for this protein being abnormal. There are other types of von Willebrand disease and discussing it can get complicated, but type 1 is the most common type, accounting for 60-80 percent of cases. Since this type of von Willebrand disease results from a problem with just one of the two copies of the gene, received from either of your parents, the condition is said to be inherited with dominant genetics. This is in contrast with recessive diseases, in which each parent supplies a bad gene. An example of a recessive disease is actually type 3 von Willebrand disease. This is when both copies of the von Willebrand gene, one from each parent, are bad.

Von Willebrand disease manifests with certain bleeding abnormalities, one common one being nose bleeds, another one being heavy, excessively long-lasting menstruation, which is called menorrhagia. Since women with type 1 von Willebrand disease have one normal gene copy for the protein, their concentration of normally functioning von Willebrand factor rises during pregnancy, just like the von Willebrand factor levels of women without von Willebrand disease. Consequently, women with this condition tend to see improvement when they are pregnant. Of course, in pregnancy, they won’t see improvement with menorrhagia since menstruation ceases altogether during pregnancy. But those whose von Willebrand disease typically gives them nosebleeds, bleeding when they brush their teeth, or bleeding with minor cuts or minor surgical procedures do improve as pregnancy advances. The same is generally not true for women with type 3 von Willebrand disease, because they lack a good gene to make the protein.

As for type 2 von Willebrand disease, this is where things get complex whether you are pregnant or not, because type 2 means that you make von Willebrand factor, but it functions abnormally. As you can imaging, there are different ways that it can function abnormally. It may leave platelets not sticky enough, so clotting is slow, or it may make platelets more sticky than they’re supposed to be, causing enormous clots initially, which then consume platelets from the blood leaving the person prone to hemorrhage. These and other abnormally functioning von Willebrand situations all constitute different subcategories of type 2 von Willebrand disease and whether they improve, worsen, or stay the same during pregnancy depends on the particular subtype.

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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