Blood clots and pregnancy

What is a blood clot?

A blood clot (also called a thrombosis) is a mass or clump of blood that forms when blood changes from a liquid to a solid. The body normally makes blood clots to stop the bleeding after a scrape or cut. But sometimes blood clots can partly or completely block the flow of blood in a blood vessel, like a vein or artery.

Anyone can develop a blood clot, but you are at higher risk for a blood clot during pregnancy and up to 3 months after giving birth to your baby. Although birthing people with blood clotting conditions have healthy pregnancies, these conditions may cause problems for some. In severe cases, they can cause death for both the birthing person and and baby. The good news is that testing and treatment can help protect and save both you and your baby.

If you’re pregnant or trying to get pregnant and have had problems with blood clots in the past, tell your health care provider at your preconception checkup (checkup before pregnancy) or at your first prenatal care checkup.

If you or someone in your family has had problems with blood clots, talk to your provider.  Blood clots may run in your family.  You may also talk to your provider about getting a blood test to see if you have a thrombophilia. This is a health condition that increases your chances of making abnormal blood clots. Some pregnant people with thrombophilias need treatment with medicines called blood thinners. They stop clots from getting bigger and prevent new clots from forming.

Why are pregnant people at greater risk for blood clots?

Pregnant people are 5 times more likely to experience a blood clot compared to those who are not pregnant. This may be because:

• When you’re pregnant, your blood clots more easily to lessen blood loss during labor and delivery.
• In pregnant people, the blood may flow less to the legs later in pregnancy because the growing baby presses upon blood vessels around your pelvis. 
• When you’re pregnant, you may experience less movement or immobility (not moving a lot) like when you’re on bed rest or recovering from giving birth.

What are other reasons people may be at risk for having blood clots?

Certain things make you more likely than others to have a blood clot. These are called risk factors. Having a risk factor doesn’t mean for sure that you’ll have a blood clot. But it may increase your chances. Talk to your provider about what you can do to help reduce your risk.

Risk factors for blood clots include:

• Having certain health conditions, like a thrombophilia, high blood pressure, diabetes or being overweight or obese. A family history of blood clotting problems also increases your chances of blood clots.
• Taking certain medicines, like birth control pills or estrogen hormones. These medicines can increase the risk of clotting. If you’ve had problems with blood clots or thrombophilias or have a family history of these conditions, birth control pills may not be safe for you to use. Talk to your provider about other birth control options.
• Smoking. Smoking damages the lining of blood vessels, which can cause blood clots to form.
• Having surgery, like a Cesarean section (also called c-section). The American College of Obstetricians and Gynecologists (ACOG) recommends that doctors help prevent blood clots in women during a c-section. This may include using compression devices that put pressure on your legs to help keep your blood flowing during the c-section.
• Being dehydrated. Dehydration causes blood vessels to narrow and your blood to thicken, which makes you more likely to have blood clots.
• Not moving around much. This may be because you’re on bed rest during pregnancy or recovering from surgery or an accident. Being still for long periods of time can lead to poor blood flow, which makes you more likely to have blood clots. Even sitting for long periods of time, like when traveling by car or plane for 4 hours or more, can increase your chances of having a blood clot.
• Having a baby. You’re more likely to have a blood clot in the first 6 weeks after birth than people who haven’t given birth recently.

What problems can blood clots cause during pregnancy?

If you have a blood clot or a kind of thrombophilia called antiphospholipid syndrome (also called APS), you may be more likely to have complications that can affect your health and your baby’s health, including:

Blood clots in the placenta. A blood clot in the placenta can stop blood flow to your baby and harm your baby.

Heart attack. This usually happens when a blood clot blocks blood and oxygen flow to the heart. A heart attack can lead to lasting heart damage or death.

Intauterine growth restriction (also called IUGR). This is when your baby grows poorly in the womb.

Miscarriage.

Placental insufficiency. This is when the placenta doesn’t work as well as it should so your baby gets less food and oxygen.

Preeclampsia. 

Preterm birth. 

Pulmonary embolism (also called PE). An embolism is a blood clot that moves from where it formed to another place in the body. When the clot moves to a lung, it’s a PE. PE can cause low oxygen levels in your blood and damage your body organs. It’s an emergency and a leading cause of death during pregnancy. Signs and symptoms of PE may include:

• Trouble breathing
• Fast or irregular heartbeat
• Chest pain
• Fainting
• Coughing up blood

Stillbirth. 

Stroke. This happens when a blood clot forms in a blood vessel that brings blood to the brain, or when a blood vessel in the brain bursts open.

Thrombosis. This happens when a blood clot forms in a blood vessel and blocks blood flow. It most often happens in the deep veins of the legs but can be in other places of the body:

• Cerebral vein thrombosis (also called CVT). This happens when a blood clot forms in a vein in the brain. CVT can lead to stroke. Signs and symptoms include headache, vision problems and seizures.   
• Deep vein thrombosis (also called DVT). This happens when a blood clot forms in a vein deep in the body, usually in the lower leg or thigh. Signs and symptoms may include warmth and tenderness over the vein and pain, swelling or skin redness in the affected area.

Venous thromboembolism (also called VTE). This happens when a blood clot breaks off and travels through blood to vital organs, like the brain, lungs or heart. This condition includes DVT and PE. VTEs that block blood vessels in the brain or heart can cause stroke or heart attack.

How are these conditions treated?

Your provider may use tests to find out if you have a blot clot or clotting conditions. These can include ultrasound or magnetic resonance imaging (also called MRI) These tests are painless and safe for you and your baby.

If you’re pregnant and have a clotting condition, you may need to go for prenatal care checkups more often than a pregnant person who doesn’t have these blood clot conditions. At these visits, your provider checks your blood pressure and can use other tests, like blood tests, to monitor your health.

Your provider also checks your baby’s health in the womb using tests like:

• Ultrasound to check your baby’s growth and development. 
• Fetal heart rate monitoring (also called a nonstress test or NST) to check your baby’s heart rate in the womb.

During pregnancy, your provider may give you a blood thinner called heparin (low-molecular weight heparin or unfractionated heparin). If you have APS, your provider may instruct you to take heparin along with low-dose aspirin. Your provider also may refer you to a hematologist. This is a doctor who treats blood conditions.

After you give birth, your provider may continue to treat you with heparin. Or you may be treated with a blood thinner called warfarin. Warfarin is safe to take after pregnancy, even if you’re breastfeeding. Warfarin is not safe to take during pregnancy because it may cause birth defects.

Don’t take combined hormonal methods of birth control during the first 21-42 days after delivery. The risk of DVT is highest in the first 21 days. 

What can I do to reduce my risk of blood clots?

• Know the signs and symptoms of a blood clot.  On an affected limb like a leg or arm, you may notice swelling, pain or tenderness that was not caused by an injury, warm skin when you touch it or redness and discoloration. Contact your provider if you experience any of these symptoms.
• Talk to your provider about your risk. If you or a family member like a parent, brother or sister have had blood clots before, tell your provider. 
• Move or stretch on long trips. If you sit for more than 4 hours on a trip, try to move your legs often. If you can walk around, you may do so. If you can’t, you may try seated leg stretches like extending your legs straight out and moving your ankles to move your toes toward and away from you. You may also pull your knee to your chest and hold it there with your hands for 15 seconds. 
• Follow other travel tips for reducing risk of blood clots. These include drinking lots of fluids like water, wearing loose-fitted clothing or wearing special stockings that compress your legs below the knee. Talk to your provider before trying these stockings.
• Follow your provider’s instructions during pregnancy and after giving birth. Your provider may give you medications like blood thinners or ask you to come in for additional prenatal care checkups.   

Last reviewed July 2024

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