Body Changes and Blood Clotting in Pregnancy

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In previous installments of this series, we have looked at how pregnancy changes the physiology of the respiratory system ,the circulatory system, the digestive system, and the urinary system. Today, we’ll discuss the physiology of blood clotting and what happens to it during pregnancy and the post-partum period.

The body constantly maintains what doctors and scientists call homeostasis. This is a phenomenon that applies to all organ systems, tissues, and cells, and it means that things are kept within certain ranges that represent a range of normal values. A normal heart rate in an adult, for example, ranges from 60 to 100 beats per minute. If some process drives the heart rate higher, the body will try to slow it down, although sometimes the heart rate increasing is the result of the body trying to keep something else normal, such as the blood pressure, or the amount of oxygen and glucose (blood sugar) reaching body tissues. Similar things happen with breathing, body temperature, the concentration of glucose in the blood, and the amount of water in the body, which in turn affects the concentration of various electrolytes, notably sodium, in the blood. Even body weight is subject to homeostasis, which you can think about as a kind of Goldilocks effect. Things can be too fast, too slow, or just right, too hot, too cold, or just right, pushing too hard or too softly, or just right, and many of the normal ranges —the just right zones— can shift in one direction or the other when a woman becomes pregnant and as pregnancy advances.

Blood clotting is one such example. As pregnancy advances, your blood has an increased tendency to form clots compared with its tendency to form clots when you are not pregnant, and the higher tendency to clot continues until a few weeks after delivery. The tendency of blood to clot relates to changes in the biology of the inner lining of blood vessels, red blood cells, and a plethora of chemicals known as clotting factors, which are mostly proteins, and also precursors to these clotting factors. If you live in the United States, or in New Zealand (two countries that allow advertisements on television for medications), you probably have seen numerous TV commercials for various medications that decrease the risk of forming clots, all of which work against the actions of particular clotting factors. Many of these television-advertised anti-clotting medications fall into a category of drugs that doctors call direct oral anti-coagulants (DOACs). This is an acronym that distinguishes them from an older, commonly used medication called warfarin, but, like warfarin, they all shift the homeostasis of blood clotting in the direction away from clotting and more toward bleeding.

Pregnancy has the opposite effect of taking warfarin or DOACs, in that it shifts the homeostasis of blood clotting more toward clotting and away from bleeding. If we use temperature as an analogy for clotting, this is like adjusting the thermostat slightly in one direction or the other. Everybody can clot, or bleed, so with pregnancy we are talking about getting a little better at clotting and a little worse at bleeding, but just because your blood can clot more easily does not mean that you will form blood clots, because other phenomena also affect the tendency to form clots. An increased ability of your blood to form clots is actually one of three phenomena that contribute to thrombosis (clotting). The other two phenomena are injury to, or abnormalities of, the inner lining of blood vessels, and stasis, the slowing, stopping, or pooling of blood. Together, these three phenomena are known in medicine as the triad of Virchow, or Virchow’s triad, named for the famous 19th century, German physician, Rudolf Virchow (1821-1902), who described these three phenomena and how they cause thromboses, blood clots.

The more of the three prongs of the Virchow triad that you have, the more likely you are to form thrombi —blood clots— in the wrong time and place. Normal, healthy pregnant women, generally don’t experience the blood vessel lining injury/disease part of Virchow’s triad. The blood vessel pathology prong of the triad is a big factor in common, life-threatening conditions like heart attacks and strokes. But the other two factors are very prominent in pregnancy in young, healthy women. Your blood clotting system is pushed in the direction of forming more clots, allowing less bleeding. Also, because the growing womb can compress deep veins in the pelvis, you can have stasis of blood. These two factors together put pregnant women, and women who have just given birth, at risk for deep venous thrombosis (DVT, a clot in a deep vein usually in the leg), or a pelvic venous thrombosis (a clot in a big vein the pelvis).

In addition to causing pain and swelling, a DVT puts you at risk for complications resulting from pieces of the thrombus (the clot) in the deep vein breaking off, thereby becoming emboli. Emboli (singular embolus) will travel in the blood stream until they arrive either some place where they are removed, or some place that is too narrow for them to pass through. Traveling within blood from a deep vein, an embolus will enter the heart through the right atrium, pass through the right ventricle, through the pulmonary artery, and then to increasingly narrower arteries of the lungs that lead to capillaries where oxygen and carbon dioxide are exchanged with the air. This provides an excellent opportunity for the lungs to filter out emboli, that is any emboli that are fairly small. In more than 25 percent of people however, there is a way for emboli potentially to detour from the right atrium directly to the left side of the heart. Mostly this happens through a patent foramen ovale (PFO), which is like a swinging door that can open at times when the pressure in your left atrium drops and/or the pressure in your right atrium rises. This happens during what doctors call a Valsalva maneuver, as when you cough, or forcefully increase the pressure in your torso, for instance when pushing during vaginal delivery. In people with a PFO, during moments when pressure changes allow blood to move from the right atrium to the left, any embolus traveling in that blood can then move into the heart’s left ventricle and from there into the arteries of the systemic circulation. This means that the embolus could get into major arteries, such as the coronary arteries that supply the heart, or the arteries that supply the brain. This can lead to the embolus getting stuck as soon as it reaches a point too narrow for it to pass through, so it causes a blood clot known as an embolism. If it happens in the brain, for instance, it’s a cerebral embolism, which causes a stroke.

In most people, however, including most pregnant women, the opening between the right and left atria (the foramen ovale) has sealed shut by about six months after birth. So, as long as there is no other connection between the right and left circulation (such connections are more rare than a PFO), any emboli from the body’s venous system will travel through the lungs, and, if they’re small enough, be filtered out. However, if an embolus is not small enough for the lungs to remove it, it will get stuck in the lungs, causing what’s called a pulmonary embolism (PE), the main complication that concerns doctors when a woman has a DVT. Usually, PE is easily treatable, as long as it is recognized and diagnosed early, in which case they will admit you to the hospital (a pregnant woman with PE will never be sent home) and give you medications that dissolve the clot and also medications that keep you from forming new clots in deep veins. The way to avoid a PE is to avoid getting a DVT in the first place and otherwise recognizing that something is wrong if you experience swelling or pain, so that doctors can check if you have a clot. As for preventing such DVTs and similar clots in the first place, you cannot do much about the pregnancy physiology pushing your blood clotting system more in the direction of clotting, but you can do something about the blood stasis prong of the Virchow triad; you can move around, doing light exercise with your legs, avoiding long distance air and car travel that keeps you seated. Also, you can avoid smoking. You should be avoiding tobacco anyway for multiple reasons during pregnancy, and when you’re not pregnant, but smoking is a notorious risk factor for DVT that, when combined with the other factors raising your clotting risk, results in a risk much higher than simply the sum of the two risks.

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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