Pregnancy increases your risk of developing a blood clot by about five times compared to when you’re not pregnant. This happens because your body deliberately shifts its blood chemistry toward faster clotting, a built-in safeguard against heavy bleeding during delivery. The trade-off is that between 0.5 and 2 out of every 1,000 pregnant women develop a dangerous clot in a vein, most commonly in the legs or lungs. Understanding what drives this risk can help you recognize the warning signs and know which factors put you at higher risk.
How Pregnancy Changes Your Blood
From early in the first trimester, your body starts producing more of the proteins responsible for forming clots. Three key players rise substantially: fibrinogen (the protein that forms the structural mesh of a clot), Factor VIII (which accelerates the clotting chain reaction), and von Willebrand factor (which helps platelets stick to damaged blood vessel walls). Together, these elevated proteins make your blood clot faster and more firmly than it normally would.
At the same time, one of your body’s natural brakes on clotting, protein S, drops. Protein S normally helps break down clots before they grow too large. Its levels fall early in pregnancy and stay low, which means your blood not only clots more easily but also has a harder time dissolving clots once they start forming. This combination of more gas and less brake is what doctors call a hypercoagulable state, and it intensifies as pregnancy progresses.
Your blood volume also increases by roughly 50% during pregnancy, and the growing uterus presses on the large veins in your pelvis. This slows blood flow in your legs, giving clots more opportunity to form. Slower flow, stickier blood, and less natural clot control create a perfect environment for a clot to develop, particularly in the deep veins of the legs.
Risk Factors That Raise the Odds
While every pregnant person has some increased clot risk, certain factors push that risk higher. Some are things you can influence, others are not.
- Obesity: A BMI over 30 is one of the strongest modifiable risk factors. Excess weight further compresses pelvic veins and promotes inflammation that activates clotting.
- Age over 35: Older maternal age is associated with higher clot risk, partly because vein walls become less elastic over time.
- Prolonged immobility: Bed rest, long flights, or extended hospital stays slow blood flow in the legs significantly.
- Personal or family history of clots: If you or a close relative has had a blood clot before, your baseline clotting tendency is already elevated.
- Inherited clotting disorders: Genetic conditions like Factor V Leiden or prothrombin gene mutations mean your blood was already prone to clotting before pregnancy amplified the effect.
- Carrying multiples: Twin or triplet pregnancies increase blood volume and uterine pressure even more than a singleton pregnancy.
How Delivery Method Affects Risk
A cesarean section raises clot risk roughly fourfold compared to vaginal delivery, according to a meta-analysis published in CHEST Journal. This increase holds for both planned and unplanned C-sections, but the risk is even higher with emergency procedures. The reasons are layered: surgery itself triggers a clotting response as the body works to seal surgical wounds, anesthesia can dilate veins and slow blood flow, and recovery from a C-section typically involves more time lying in bed.
Vaginal delivery still carries some clot risk, particularly if labor is prolonged or if significant tearing or blood loss occurs. But the difference between the two delivery methods is substantial enough that your care team may recommend preventive measures after a C-section, such as early walking, compression stockings, or in some cases a short course of blood thinners.
Medical Conditions That Add Risk
Certain health conditions layer additional clot risk on top of the changes pregnancy already causes. Preeclampsia, a pregnancy complication involving high blood pressure and organ stress, damages the lining of blood vessels and activates the clotting system. Women with preeclampsia face elevated clot risk both during pregnancy and in the weeks after delivery.
Autoimmune conditions also play a role. People with lupus experience blood clots, preeclampsia, and pregnancy loss at higher rates than the general pregnant population. Part of this is driven by antiphospholipid antibodies, proteins that some people with lupus carry in their blood. These antibodies interfere with normal clotting regulation and significantly raise the chance of clots forming in both arteries and veins. Antiphospholipid antibodies can also be present in people without lupus, and testing for them is common if you’ve had unexplained pregnancy losses or a prior clot.
The Postpartum Window
Many people assume the risk disappears once the baby is born, but the postpartum period is actually when clot risk peaks. According to the CDC, the elevated risk persists for up to three months after delivery. The first six weeks are the most dangerous stretch. Your clotting factors don’t drop back to normal immediately, and the physical toll of delivery, whether vaginal or surgical, triggers additional clotting activity as your body heals.
Postpartum immobility compounds the problem. Recovery from birth, especially after a C-section, often means less movement during the exact window when your blood is most prone to clotting. Dehydration from breastfeeding can thicken the blood further. This is why getting up and moving as soon as you’re able after delivery is one of the simplest and most effective things you can do to lower your risk.
Clot Symptoms vs. Normal Pregnancy Swelling
One of the trickiest aspects of blood clots in pregnancy is that the warning signs overlap with normal pregnancy discomfort. Swollen ankles and achy legs are common in the third trimester, which can make it easy to dismiss a clot as just another pregnancy symptom. The key differences come down to pattern and location.
A blood clot in the leg, called a deep vein thrombosis, typically causes swelling in only one leg rather than both. The affected leg may feel painful or tender without any injury to explain it, and the skin over the area may be warm to the touch, red, or noticeably discolored. Normal pregnancy swelling tends to affect both legs equally, worsen at the end of the day, and improve with elevation.
A clot that travels to the lungs, called a pulmonary embolism, causes sudden shortness of breath, chest pain that worsens with deep breathing, a rapid heartbeat, or coughing up blood. This is a medical emergency. If you experience any of these symptoms, especially combined with one-sided leg swelling, seek immediate care. Pulmonary embolism is one of the leading causes of maternal death, but it is highly treatable when caught early.
Reducing Your Risk
You can’t eliminate pregnancy’s effect on your clotting system, but you can minimize the additional factors that stack on top of it. Staying physically active throughout pregnancy, even with daily walks, keeps blood flowing through your legs and reduces the chance of a clot forming. Staying well hydrated helps maintain blood fluidity. If you’re on a long flight or car ride, getting up to move every hour or two makes a measurable difference.
If you have known risk factors like a prior clot, a clotting disorder, or a BMI over 30, your provider may recommend preventive blood thinners during pregnancy or the postpartum period. Compression stockings, which apply gentle pressure to your lower legs, are another low-risk tool that helps push blood back toward the heart and prevents it from pooling.
Knowing your personal risk profile matters. If you have a family history of clotting disorders, getting tested before or early in pregnancy gives your care team the information they need to plan ahead. Most pregnancy-related clots are preventable when the risk is identified early.