How do blood clots occur in pregnancy?

Thank you so much for joining me on today’s episode of Simple Health Radio. One of our listeners named Aharon Franco sent in a question. Aharon is the administrator of the Phoenix Center. This is one of the premier skilled nursing facilities in Pennsylvania. You can learn more about their services by visiting

Aharon had a great question about blood clots. Let’s take a listen.

Let me thank Aharon for taking the time to record a very detailed question about a problem that affects many pregnant women.

There are different names for a blood clot depending on which part of the body is affected. If a blood clot is in a vein and it’s on the leg or the arm, we call it a venous thrombosis. If it’s in one of the deep veins, it’s called a DVT which means a deep vein thrombosis.

If the blood clot starts in one place and travels to another, we call that an embolism. PE stands for Pulmonary Embolism. Pulmonary means lung and embolism mean blood clot. That means the blood clot probably started in the leg traveled up through the veins went through the heart and then was pumped into an artery near the lungs. Wherever I got stuck in the location of the pulmonary embolism or PE.

Blood clots can develop any time there is a disruption in the normal blood circulation or if there are external factors that cause the blood to become thicker than usual. We call that a hypercoagulable state.

Coagulation is a process where the body wants to create a blood clot. For example, if you cut your finger there be some bleeding for a few minutes. Platelets will form and there are chemicals that are released which cause coagulation which is a good type of blood clot. Without the coagulation pathway, a simple cut would be life-threatening because people won’t stop bleeding.

There are some situations that are considered hypercoagulable.

For example, if someone just had major surgery such as hernia repair or appendicitis, that person will be at a higher risk of developing blood clots. First, those people are less mobile. After surgery, people don’t really want to walk or exercise much because it hurts and different parts of their body will be swollen. That alone causes the blood to become thicker and increases the risk of blood clots.

If a person has a broken ankle or a broken arm, they will usually wear a cast for 6 or 7 weeks. The part of the body that’s enclosed in the cast will have less range of motion and some muscle breakdown. This affects the blood circulation and again makes the blood thicker and more likely to have a blood clot.

When people travel long distances, there is a high risk of developing a blood clot. This is true of someone is on a very long car ride such as from New York to Colorado. It can also happen if someone flies such as from Los Angeles to Miami.

International travel has a very high risk of developing blood clots and airlines now is required to show videos to promote the health of the passengers and encourage them to walk whenever possible to reduce the risk of developing a blood clot.

Some people actually have problems with their blood due to family history or a genetic defect. Others may have some type of tumor, such as colon cancer or leukemia, which can increase the risk of deadly blood clots.

The category that we will focus on now is pregnancy. Pregnancy is a unique state of health. We know that women who are pregnant will have changes in their hormones. The hormonal changes are necessary to allow the developing baby to receive the proper vitamins, nutrients, and development through the placenta. However, some hormones can also increase the risk of blood clots. That’s why people who take testosterone or estrogen supplements are counseled regarding the risk of developing a life-threatening pulmonary embolism or PE.

In pregnancy, doctors like to be as specific as possible when talking about the gestational age. That means we don’t say someone is 6 months pregnant or 8 months pregnant. Rather, we identified in terms of weeks. So if someone is 12 weeks or less, that’s the 1st trimester. Between 12 and 24 weeks will be the 2nd trimester. From 25 weeks until 40 weeks or the time of delivery, that would be considered the 3rd trimester.

In the 1st trimester, the baby’s nervous system and brain are being actively developed. Whenever there’s a problem such as infection, medication side effects, or trauma, there is a high risk of miscarriage because the developing baby doesn’t have much reserve and many complications can arise that affect the chromosomes or organ development.

In the 2nd and 3rd trimester, the baby will continue to grow but will be less susceptible to side effects from certain medications. The risk is never 0, but it does decrease as the baby becomes stronger.

During pregnancy, the mother will also gain weight and become less mobile. It’s a lot easier for a woman to walk around when she’s 10 weeks pregnant compared to when she’s 35 weeks pregnant. Many women retain fluid in their legs and become very swollen. Other women can’t sleep in certain positions because it affects their breathing and the baby becomes uncomfortable.

Based on statistics, 1 out of 1600 pregnancies will result in some type of blood clot. Pulmonary embolism is the #6 cause of death in pregnant women in the United States. About 9% of all maternal deaths are due to blood clots. Black women have a 4x higher risk of dying compared to white women who are pregnant.

How do we diagnose a blood clot? In the ER, all of the nurses and doctors are trained to look for specific signs of any respiratory and distress. If a blood clot is affecting the lungs, we expect those people to have shortness of breath, chest pain, difficulty speaking in full sentences, increased heart rate, and possibly a drop in her blood pressure. Those people aren’t circulating blood the way they’re supposed to and the body is not able to compensate very well.

If the DVT is in the leg or the arm, an ultrasound is used to scan that region to look at blood flow. If blood flow is blocked, there is a high risk that a blood clot is sitting there. That blood clot can break off and travel up to the brain, heart, or lungs. If it ends up in the brain, that’s called a stroke. If it ends up in the heart that would be a type of heart attack. If it ends up in the lungs, that’s a pulmonary embolism which is a PE.

If the blood clot is already in the lungs, an ultrasound will not help you. A chest x-ray will also not show blood clots.

In the ER, there are 2 tests that can be performed. One option is called a VQ scan also called a ventilation and perfusion scan. This is the preferred test during pregnancy. The VQ scan looks at blood flow and air circulation in different areas of the lungs.

The other test is a CT scan of the lung, also called a CTA or CT pulmonary angiography. In this situation, the contrast dye is injected into a vein and the CT scanner is turned on. As the contrast travels throughout the lungs and into the arteries, the CT scan will create a map.

Any place that doesn’t light up with contrast is going to be a blockage. That means the blood clot is sitting at that location and nothing can circulate downstream. This could be in one lung or both or anywhere in between.

The smaller arteries are going to be the highest risk of developing a blood clot because it’s all based on size. For example, if the blood clot is 5 mm, it’s going to pass through all the vessels that are 10 mm 8 mm all the way down to 6 mm. But once it hits a blood vessel that’s only 4 mm across, it’s going to be stuck. Everything that comes after that vessel is going to lose blood supply. That includes muscles, organs, and tissues.

CT scans use contrast which is radioactive. If a woman is pregnant, we don’t want radiation near her baby. Radiation damages chromosomes and DNA. One CT scan has as much radiation as 150 chest x-rays. So that just gives you an idea of how much radiation was talking about.

The contrast also has to circulate out through the kidneys. That means not only could the contrast be toxic to the baby, but it can be toxic to the mother’s kidneys. That’s why we prefer the VQ scan as opposed to a CT scan if it’s available in that emergency department.

To summarize the 1st part of Aharon’s question, we prefer to use the VQ scan because there’s no risk to the mother or the baby. However, not every hospital ER has it, so a CT scan with contrast would be the next option. We have to counsel the mother and the family members about the risks and benefits of both tests and then decide at that point what would be best.

The 2nd part of Aharon’s question is even more important. What is the treatment option for a pregnant woman?

There are several options that we can use in men or nonpregnant women. The most common oral ones include warfarin, also called Coumadin. Unfortunately, warfarin cannot be used in pregnancy because it causes miscarriages.

The 2nd group is oral medications such as Xarelto and Eliquis. These medications have only been around for a few years. We don’t have enough data on them, so we cannot use them during pregnancy.

That leaves us with heparin and Lovenox. Lovenox is a type of heparin. Lovenox is also known as low molecular weight heparin.

Heparin has been around for many years. It works by inactivating thrombin, which prevents fibrin from turning into a blood clot. Trombone and Fibrin are part of the coagulation cascade that we talked about earlier. There are multiple steps that have to be performed in order to create a blood clot. Heparin does not come in a pill. It has to be given IV or injected under the skin.

The problem with heparin is that it causes a lot of excessive bleeding and there is a complication called HIT or heparin-induced thrombocytopenia. This affects almost 5% of people on heparin.

During HIT, the immune system unexpectedly creates antibodies against heparin, and it binds to a protein that we call platelet factor #4. This results in platelet activation all throughout the body. This causes thousands of tiny blood clots to develop. Unfortunately, all the platelets get consumed rapidly. The blood clots continue to spread into the arteries in the veins. These people need to be in the ICU and they have to receive other types of blood thinners for stabilization and to prevent death.

The newer form of heparin is called Lovenox or low molecular weight heparin. Lovenox is much safer than heparin and there is a less than 1% chance of developing HIT. The Lovenox is injected under the skin and it can be used safely at home. Many patients are sent home with teaching.

The Lovenox has to be continued every day for the rest of the pregnancy. The reason is that whatever caused the blood clot in the 1st place is probably related to the pregnancy and those risk factors are going to remain until the baby is born.

When a woman goes into labor, her doctor should stop the Lovenox. Otherwise, she’ll have excessive bleeding and that can cause postpartum hemorrhage.

After delivery, the doctor is most likely going to restart the Lovenox or use an oral medication such as warfarin for at least 6 weeks. The reason is that the woman is still good to have bleeding and she will naturally have some clotting either due to a vaginal delivery or from a C-section. After 6 weeks, if there are no problems in the patient’s risk factors have resolved, the doctor will probably stop all blood thinning medications.

So get going back to the original question, of VQ scan is preferable to diagnose a PE or a blood clot in the lungs if the woman is pregnant. A CT scan can be done, but the risk of contrast can cause harm to the baby.

Lovenox is preferable over heparin because it’s equally effective and has less risk of complications of extra bleeding or developing those micro blood clots.

Lovenox is recommended for use during the entire pregnancy and about 6 weeks after delivery to prevent the redeveloping of any blood clots.

So again, I want to thank Ahron for sending in a great question. You can visit our website and read the show notes about this episode. I also have links to a couple of other websites if you want to learn more about blood clots in pregnancy, pulmonary embolism, and some of the medications that are available.

If you have questions that you been thinking about, just record it on your phone or computer and email it to us. You can connect with us on social media and send your recording via email.

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Thank you so much for listening to this week’s episode.




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