Deep vein thrombosis in pregnancy is a blood clot in a deep vein, usually in the leg, that forms because pregnancy naturally makes blood clot more easily. It typically causes pain and swelling in one leg, is diagnosed with a compression ultrasound that is completely safe for the baby, and is treated with daily injections of low-molecular-weight heparin (LMWH), which does not cross the placenta. Treatment continues for the rest of the pregnancy and for at least six weeks after birth, when the risk is actually at its highest.
Here I explain why the risk rises, which symptoms to act on, how doctors confirm the diagnosis and what treatment and delivery planning look like.
Why Pregnancy Raises the Risk of DVT
Clots form when three conditions come together, a concept known as Virchow’s triad: slow blood flow, a tendency of the blood to clot, and damage to the vessel wall. Pregnancy affects all three.
- Hypercoagulability: levels of several coagulation factors, including fibrinogen and factor VIII, rise, while natural anticoagulant protein S falls. This prothrombotic state protects against dangerous bleeding at delivery.
- Venous stasis: hormones, especially progesterone, relax vein walls, and the growing uterus presses on the pelvic veins, slowing return of blood from the legs.
- Vessel injury: delivery, particularly cesarean section or an instrumental birth, can damage pelvic veins.
The result is that clot risk is several times higher during pregnancy than in non-pregnant women of the same age, and higher still in the weeks after birth. DVT in pregnancy occurs more often in the left leg, because the left iliac vein is crossed and compressed by the right iliac artery, and clots are more likely than usual to start high up in the pelvic veins.
Risk Factors
Every pregnancy carries some extra risk, but these factors raise it further:
| Category | Examples |
|---|---|
| Personal or family history | Previous clot; close relative with unprovoked or hormone-related clot |
| Inherited thrombophilia | Factor V Leiden, prothrombin gene mutation, antithrombin, protein C or protein S deficiency |
| Acquired conditions | Antiphospholipid syndrome, active cancer, sickle cell disease, inflammatory bowel disease |
| Maternal factors | Age over 35, obesity, smoking, varicose veins, multiple previous pregnancies |
| Pregnancy events | IVF, twins, preeclampsia, severe vomiting with dehydration, cesarean delivery, postpartum hemorrhage |
| Temporary factors | Immobility, long-haul travel, hospital admission, infection |
Maternity teams assess these risks at booking, on admission and after delivery. Women at higher risk may be offered preventive (prophylactic) LMWH during pregnancy, after birth, or both.
Signs and Symptoms
Many pregnant women have some swelling in both ankles, which is normal. The features that point toward DVT are different:
- Pain, tenderness or cramping in one leg, often the calf or thigh
- Swelling of one leg noticeably greater than the other
- Warmth, redness or discoloration of the skin over the affected area
- Pain in the buttock, groin or lower abdomen, which can signal a clot in the pelvic veins
A DVT can break off and travel to the lungs, causing a pulmonary embolism (PE). Sudden breathlessness, chest pain that is worse on breathing in, coughing up blood, a racing heart or collapse are emergencies. Clots differ in seriousness depending on where they sit; our comparison of superficial vs deep vein thrombosis explains why deep clots carry this risk.
Diagnosis
The key test is a compression duplex ultrasound of the leg veins. It uses no radiation and is safe at any stage of pregnancy. A vein that will not squash flat under the probe contains a clot.
- If the first scan is normal but suspicion remains high, the scan is usually repeated within about a week, or the pelvic veins are examined specifically.
- If a pelvic clot is suspected and ultrasound is unclear, MRI without contrast can be used.
- D-dimer rises normally as pregnancy progresses, so it is much less useful for ruling out a clot than in non-pregnant patients.
If a clot is strongly suspected, doctors will often start LMWH straight away while waiting for the scan, because the risks of an untreated clot outweigh a short course of treatment. Suspected PE is investigated with a chest X-ray and either a lung perfusion scan or CT pulmonary angiogram; the radiation dose to the baby from either is very low.
Treatment and Delivery Planning
LMWH, such as enoxaparin, dalteparin or tinzaparin, is the treatment of choice. It is given as a daily or twice-daily injection under the skin, with the dose based on weight. Because heparin molecules are too large to cross the placenta, the baby is not anticoagulated.
- Warfarin is avoided during pregnancy because it crosses the placenta and can cause birth defects, particularly in the first trimester, and fetal bleeding later on.
- Direct oral anticoagulants (such as apixaban or rivaroxaban) are not used in pregnancy or while breastfeeding because safety data are lacking.
- Duration: treatment continues for the rest of the pregnancy and for at least six weeks after birth, with a minimum total of three months.
- Compression stockings and leg elevation can ease swelling.
Delivery needs planning with the obstetric, anesthetic and hematology teams. LMWH is usually paused before planned induction or cesarean section, and epidural or spinal anesthesia is timed safely: generally at least 12 hours after a preventive dose and 24 hours after a full treatment dose. After birth, LMWH is restarted, and women can then choose LMWH or switch to warfarin, both of which are compatible with breastfeeding.
Key Takeaways
- Pregnancy increases clot risk through hypercoagulability, slow venous flow and delivery-related vessel injury.
- One-sided leg pain and swelling needs same-day assessment; sudden breathlessness or chest pain needs emergency care.
- Compression ultrasound is safe and is the main diagnostic test; D-dimer is unreliable in pregnancy.
- LMWH is the safe, effective treatment and continues until at least six weeks after birth.
For more on how clots form and are treated, see our blood clotting guide.
Frequently Asked Questions
Are heparin injections safe for my baby?
Yes. LMWH does not cross the placenta, so it does not affect the baby’s clotting. It has been used in pregnancy for many years and is the recommended anticoagulant.
When is the risk of DVT highest?
The risk rises throughout pregnancy but peaks in the first weeks after delivery, especially after a cesarean section. That is why treatment and, for some women, prevention continue for at least six weeks postpartum.
Can I breastfeed while on blood thinners?
Yes. Both LMWH and warfarin are considered compatible with breastfeeding. Direct oral anticoagulants are generally avoided while breastfeeding.
Will I need blood thinners in a future pregnancy?
Usually yes. Women with a previous pregnancy-related clot are normally offered preventive LMWH in future pregnancies and after birth, with the plan made early by a hematologist or obstetric medicine specialist.
