What Causes Blood Clots During Pregnancy?

Medically reviewed by Shala A. Salem, MD and Rifaat D. Salem, MD PHD of Pacific Reproductive Center

Quick Answer: Blood clots during pregnancy are primarily caused by natural physiological changes, including increased blood volume, elevated hormone levels that alter clotting factors, and reduced blood flow in the lower body as the growing uterus presses on pelvic veins. While these changes protect against heavy bleeding during delivery, pre-existing clotting disorders like antiphospholipid syndrome can significantly increase complication risks.

What are blood clots?

A blood clot, clinically referred to as a thrombus, is a gel-like clump of blood cells and proteins that forms inside a blood vessel.

During a normal pregnancy, your body undergoes massive changes to prepare for childbirth. Your total blood volume increases significantly, your blood pressure lowers, and your body naturally increases its level of clotting factors.

This natural shift is actually a protective mechanism designed to keep you from losing too much blood during labor and delivery. However, these same shifts can sometimes cause blood to collect and clot too easily.

Clots can develop in deep veins, arteries, or even within the uterus and umbilical cord, potentially disrupting healthy blood flow to your vital organs or your developing baby.

Causes of blood clots during pregnancy

Finding blood clots or spotting during pregnancy can feel incredibly alarming, but the underlying causes vary from minor bodily adjustments to serious medical conditions. Understanding why these clots form helps patients know when to rest and when to reach out for medical support.

The Centers for Disease Control and Prevention lists the following factors that can increase blood clot risk:

• History of blood clots
• Genetic blood clotting disorder
• Family history of blood clots
• Previous C-section delivery
• Extended immobility
• Pregnancy complications
• Pregnancy over the age of 34
• Diabetes
• Obesity
• Smoking

Non-Worrisome Causes of Clots

In the first trimester, passing tiny, dark red blood clots without any physical pain is often caused by normal changes. As the fertilized egg attaches itself to the rich uterine lining, implantation bleeding can occur, which sometimes contains small, harmless clots.

Additionally, a web of new blood vessels rapidly develops within the cervix during early pregnancy. Because these vessels are very delicate, routine activities like a pelvic exam or sexual intercourse can cause light bleeding and minor clotting.

Serious Causes Involving Pelvic Pain or Cramping

If you are passing blood clots that are accompanied by sharp pelvic pain, heavy bleeding, or severe abdominal cramping, you need to contact your doctor right away. These painful symptoms can point to the following serious pregnancy complications:

• Ectopic Pregnancy: This occurs when an embryo implants outside the uterus, most commonly inside a fallopian tube, requiring immediate emergency intervention.
• Miscarriage: Painful cramping paired with the passage of tissue and blood clots is a primary sign of early pregnancy loss.
• Subchorionic Hematoma: This happens when blood collects in the space between the uterine wall and the fetal amniotic sac. While some small hematomas resolve safely on their own, a large blood clot in the uterus can increase the risk of preterm delivery.
• Infection: Certain untreated urinary tract infections or sexually transmitted infections can inflame reproductive tissues, leading to abnormal bleeding and clotting.
• Molar Pregnancy: A rare genetic complication where a fluid-filled mass of cells grows inside the uterus instead of a healthy placenta.

Blood clot symptoms during pregnancy

Blood clot symptoms can vary greatly depending on where the blood clot is located in your body. It is vital to recognize the differences between a localized clot and a life-threatening medical emergency.

Deep Vein Thrombosis (DVT) Symptoms

Deep vein thrombosis occurs when a blood clot forms in one of the deep veins of your lower body, typically in a single leg. Watch for these distinct signs:

• Swelling and inflammation in the lower leg or ankle
• A noticeable feeling of warmth or heat in the calf muscle
• Tenderness, aching pain, or cramping that gets worse when walking
• Skin redness or visible, bulging surface veins

Pulmonary Embolism (PE) Symptoms

If a DVT clot breaks free, it can travel through your bloodstream to your lungs, causing a blocked artery known as a pulmonary embolism. A pulmonary embolism is a critical medical emergency that requires an immediate call to 911. Look out for these sudden symptoms:

• Sudden, sharp, or stabbing chest pain
• Unexplained shortness of breath or rapid breathing
• A sudden coughing fit, sometimes coughing up blood
• Dizziness, lightheadedness, or a total loss of consciousness (fainting)

Blood clotting disorders and fertility treatment

Unresolved blood clotting issues can directly impact your ability to conceive or carry a pregnancy to term.

Certain inherited or acquired conditions, such as antiphospholipid syndrome (APS), cause the body to form tiny clots within the delicate blood vessels of the developing placenta. This can cut off vital nutrients and oxygen to the embryo, leading to recurrent pregnancy loss or early implantation failure during IVF cycles.

High estrogen levels during ovarian stimulation can slightly alter how your blood clots. By identifying these risks early, fertility specialists can structure custom treatment plans and fertility medications that protect your health and minimize potential IVF risks.

Concerned about clotting disorders and your fertility treatment plan?

Our experienced medical team is here to help you navigate your options safely. Contact PRC for a consultation to discuss your unique path forward.

How are blood clotting disorders diagnosed and managed?

If you have experienced pregnancy complications or have a family history of clots, your fertility specialist can run specialized diagnostic testing before you begin an IVF cycle.

Doctors typically use targeted blood work to check for specific clotting factors, autoimmune proteins, or genetic mutations. If a clot is suspected in your legs or organs, non-invasive imaging tools like an ultrasound can quickly find the exact location of the blockage.

Managing these disorders involves personalizing your care plan to lower your risks without disrupting your fertility goals. Your medical team will monitor you closely through regular checkups to ensure you stay safe and healthy throughout your entire journey.

Frequently Asked Questions

Are blood clots during early pregnancy always a sign of miscarriage?

No, passing small clots in the first trimester is not always a sign of pregnancy loss. If the clotting is light and happens without any pelvic pain or severe cramping, it may be caused by normal implantation or minor cervical changes. However, you should always report any bleeding or clots to your healthcare provider to be safe.

What does a blood clot look like during pregnancy?

Vaginal blood clots passed during pregnancy usually look like dark red, jelly-like clumps. They can vary in size from tiny flecks to larger pieces. If a clot is accompanied by grayish tissue, heavy bright red bleeding, or severe pain, it requires immediate medical evaluation.

Can blood clotting disorders affect IVF success?

Yes, untreated blood clotting conditions can decrease IVF success rates. These disorders can cause tiny clots to block blood flow to the newly implanted embryo, which can lead to implantation failure or early pregnancy loss. Screening for these conditions before your cycle allows your medical team to build safe, protective therapies into your plan.

When should I go to the emergency room for a blood clot during pregnancy?

You should seek emergency care immediately if you have any signs of a pulmonary embolism. These warning signs include sudden, stabbing chest pain, unexpected shortness of breath, a severe coughing fit, or feeling dizzy and fainting. If you notice sudden, severe pain and major swelling in only one leg, you should also seek urgent medical attention.

How is antiphospholipid syndrome (APS) managed during pregnancy?

Specialists typically manage APS using a customized combination of low-dose aspirin and daily blood-thinner injections, such as heparin. This safe treatment stops micro-clots from forming in the placenta, keeping blood flowing smoothly to your baby and significantly lowering the risk of miscarriage.

Talk to a Fertility Specialist About Your Risks

Navigating reproductive health and understanding blood clotting risks can feel overwhelming, but you do not have to walk this path alone. It is completely normal to have questions about how your body will adapt during an IVF cycle or early pregnancy.

By identifying potential risks before your treatments begin, our team can build a safe, personalized strategy that protects your long-term wellness and your future baby.

If you have concerns about your history or want to learn more about our testing options, please contact Pacific Reproductive Center today to set up your initial consultation.