Pregnancy-Related Thrombosis (Blood Clots in Pregnancy)

Why pregnancy increases clot risk

Three changes combine in pregnancy:

• Blood clots more easily: levels of clotting factors rise and natural anticoagulants fall, a protection against bleeding at birth.
• Slower blood flow: hormones relax vein walls, and the growing uterus presses on pelvic veins, especially on the left side.
• Vessel injury: delivery, particularly cesarean section, can damage veins.

The risk continues for up to 12 weeks after birth and is highest in the first 6 weeks. Venous thromboembolism remains one of the leading causes of maternal death in high-income countries, which is why risk assessment and prompt diagnosis matter.

Risk factors

Before pregnancy

• previous blood clot
• inherited thrombophilia, such as factor V Leiden or prothrombin gene mutation
• antiphospholipid syndrome
• family history of clots
• age over 35
• obesity (BMI 30 or more)
• smoking
• varicose veins, lupus, sickle cell disease, heart or kidney disease

During pregnancy and delivery

• multiple pregnancy
• IVF pregnancy
• preeclampsia
• severe vomiting and dehydration
• bed rest or immobility, including long-distance travel
• cesarean section, especially emergency
• prolonged labor, heavy bleeding or blood transfusion
• infection after birth

Symptoms

Deep vein thrombosis

• pain, tenderness or cramping in the calf or thigh
• swelling of one leg, often the left in pregnancy
• warmth and redness
• lower abdominal, back or buttock pain with a pelvic vein clot

Normal pregnancy swelling usually affects both legs fairly equally and is not painful or red. One-sided swelling should always be checked.

Pulmonary embolism

• sudden breathlessness
• sharp chest pain, worse when breathing in
• coughing, sometimes with blood
• fast heart rate, dizziness or fainting

Some breathlessness is normal in pregnancy, but sudden or severe breathlessness, especially with chest pain, needs emergency care.

Diagnosis

• Compression ultrasound of the leg veins: the first test for DVT. MRI may be used for suspected pelvic vein clots.
• D-dimer blood tests are less useful in pregnancy, because levels rise normally.
• For suspected PE: chest X-ray, then either a ventilation-perfusion (V/Q) scan or CT pulmonary angiography. Both use radiation doses considered safe for the baby, and the risk of a missed PE is far greater.
• ECG and blood tests, including blood count and kidney function.

Treatment is often started while tests are pending if suspicion is high.

Treatment

• Low-molecular-weight heparin (LMWH), such as enoxaparin, given by injection under the skin, is the treatment of choice in pregnancy. It does not cross the placenta and is safe for the baby.
• Treatment duration: for the rest of pregnancy and at least 6 weeks after birth, with a minimum total of 3 months.
• Around delivery: injections are timed or paused before labor or cesarean to allow epidural anesthesia safely; your team will plan this.
• After birth: heparin or warfarin can be used, and both are compatible with breastfeeding. Direct oral anticoagulants are generally avoided in pregnancy and breastfeeding.
• Warfarin is avoided during pregnancy because it can harm the baby.
• Compression stockings may ease leg symptoms.
• Severe PE may need clot-dissolving medicines or procedures in hospital.

Prevention

Every pregnant woman should have a clot risk assessment early in pregnancy, on hospital admission and after delivery. Depending on risk:

• Preventive LMWH for women with previous clots, high-risk thrombophilia or several risk factors, during pregnancy and/or for 10 days to 6 weeks after birth.
• Stay active, avoid long periods of sitting, and walk around regularly on long journeys.
• Stay well hydrated, especially with vomiting.
• Get up and move soon after delivery, including after cesarean.
• Compression stockings for some women and on long flights.
• Stop smoking.

Future pregnancies and contraception

Women who have had a pregnancy-related clot usually need preventive heparin in future pregnancies. Estrogen-containing contraceptives are generally avoided after a clot; progestin-only methods and copper or hormonal IUDs are safer options. Discuss with your doctor.

Frequently asked questions

What are the signs of a blood clot during pregnancy?

Signs of a leg clot are pain, swelling, warmth or redness in one leg, often the calf or thigh, and more commonly the left. Signs of a clot in the lungs are sudden breathlessness, chest pain on breathing, coughing blood, a racing heart or fainting, which need emergency care.

When is the risk of blood clots highest?

The risk is highest in the first 6 weeks after giving birth, especially after a cesarean section, and remains raised until about 12 weeks postpartum. Risk is also increased throughout pregnancy, particularly in the third trimester.

Are blood thinners safe in pregnancy?

Yes. Low-molecular-weight heparin injections are the standard treatment and prevention in pregnancy because they do not cross the placenta. Warfarin and most direct oral anticoagulants are avoided during pregnancy. Heparin and warfarin are both safe while breastfeeding.

How can I tell normal pregnancy swelling from a clot?

Normal swelling usually affects both ankles and feet, builds up during the day and improves overnight or with raised legs. A clot more often causes swelling in one leg with pain, tenderness, warmth or redness. If one leg is more swollen or painful, get it checked the same day.