What is VTE in Pregnancy?
Venous thromboembolism (VTE) encompasses deep vein thrombosis (DVT) and pulmonary embolism (PE). Pregnancy is a well-recognized hypercoagulable state due to physiological changes that evolved to prevent hemorrhage during childbirth. These changes include increased levels of clotting factors (I, VII, VIII, X, von Willebrand factor), decreased protein S activity, increased venous stasis from uterine compression of the inferior vena cava, and vascular endothelial injury during delivery.
VTE remains one of the leading causes of maternal death in developed countries. The risk of VTE is approximately 4–5 times higher during pregnancy and the postpartum period compared to non-pregnant women of similar age, with the highest risk occurring in the first 6 weeks after delivery.
Incidence & Types
The overall incidence of pregnancy-related VTE is approximately 1–2 per 1,000 pregnancies. Key facts:
- Deep Vein Thrombosis (DVT): Accounts for approximately 75–80% of pregnancy-related VTE. Occurs more commonly in the left leg (due to compression of the left iliac vein by the right iliac artery) and in the iliofemoral veins (more proximal than in non-pregnant patients).
- Pulmonary Embolism (PE): Accounts for 20–25% of cases and is the most dangerous form, potentially fatal if a large clot lodges in the pulmonary arteries. PE is the leading direct cause of maternal death in the UK.
- Cerebral Venous Sinus Thrombosis: Rare but recognized complication of pregnancy, particularly in the postpartum period.
Risk is elevated throughout pregnancy but is highest in the third trimester and the first 6 weeks postpartum. Cesarean delivery approximately doubles the risk compared to vaginal delivery.
RCOG Risk Assessment
The Royal College of Obstetricians and Gynaecologists (RCOG) Green-top Guideline No. 37a recommends systematic VTE risk assessment for all pregnant women at the following time points:
- Booking visit: First antenatal appointment (typically 8–12 weeks)
- Hospital admission: Any admission during pregnancy
- Intercurrent illness or complication: When new risk factors develop
- Postpartum: Within hours of delivery, before discharge
The scoring system assigns points to various risk factors, with higher-risk factors (such as previous unprovoked VTE or high-risk thrombophilia) receiving more points. The total score determines the recommended thromboprophylaxis strategy.
VTE Risk Stratification Diagram
Risk Factor Details
| Risk Factor | Points | Category |
|---|---|---|
| Previous VTE (unprovoked or estrogen-related) | 4 | Pre-existing |
| High-risk thrombophilia (AT deficiency, APS, compound/homozygous) | 3 | Pre-existing |
| Medical comorbidities (heart/lung disease, SLE, cancer, etc.) | 3 | Pre-existing |
| Emergency cesarean section / CS in labor | 2 | Obstetric |
| BMI >40 | 2 | Pre-existing |
| Previous VTE (provoked) | 1 | Pre-existing |
| Low-risk thrombophilia | 1 | Pre-existing |
| Family history of VTE | 1 | Pre-existing |
| Age >35 | 1 | Pre-existing |
| BMI >30 | 1 | Pre-existing |
| Parity ≥3 | 1 | Pre-existing |
| Smoking | 1 | Pre-existing |
| Gross varicose veins | 1 | Pre-existing |
| Multiple pregnancy | 1 | Obstetric |
| Pre-eclampsia | 1 | Obstetric |
| Elective cesarean section | 1 | Obstetric |
| Prolonged labor >24 hours | 1 | Obstetric |
| PPH >1 liter | 1 | Obstetric |
| Preterm birth | 1 | Obstetric |
| Stillbirth | 1 | Obstetric |
| Current systemic infection | 1 | Transient |
| Immobility / dehydration | 1 | Transient |
| Hyperemesis | 1 | Transient |
| OHSS | 1 | Transient |
| Long-distance travel >4 hours | 1 | Transient |
Scoring & Recommendations
Antenatal Assessment
| Score | Risk Level | Recommendation |
|---|---|---|
| ≥4 points | High | Refer to expert; commence LMWH prophylaxis immediately (from first trimester) and continue throughout pregnancy and for 6 weeks postpartum |
| 3 points | Intermediate | Consider LMWH prophylaxis from 28 weeks of gestation until at least 6 weeks postpartum |
| <3 points | Lower | Mobilization, hydration, reassess if hospitalized or new risk factors develop |
Postnatal Assessment
| Score | Risk Level | Recommendation |
|---|---|---|
| ≥2 points | Increased | LMWH prophylaxis for at least 10 days postpartum (minimum). Longer if risk factors persist or additional risk factors present |
| <2 points | Lower | Early mobilization, adequate hydration. No pharmacological prophylaxis unless prolonged admission |
LMWH Prophylaxis
Low molecular weight heparin (LMWH) is the agent of choice for thromboprophylaxis in pregnancy because:
- Does not cross the placenta: LMWH molecules are too large to cross the placental barrier, making it safe for the fetus.
- Predictable pharmacokinetics: Does not require routine monitoring (unlike unfractionated heparin).
- Low risk of HIT: Heparin-induced thrombocytopenia is rare with LMWH.
- Compatible with breastfeeding: Not excreted in breast milk in significant amounts.
Common LMWH Regimens (Prophylactic Doses)
| Weight (kg) | Enoxaparin | Dalteparin | Tinzaparin |
|---|---|---|---|
| <50 | 20 mg daily | 2,500 units daily | 3,500 units daily |
| 50–90 | 40 mg daily | 5,000 units daily | 4,500 units daily |
| 91–130 | 60 mg daily | 7,500 units daily | 7,000 units daily |
| 131–170 | 80 mg daily | 10,000 units daily | 9,000 units daily |
| >170 | 0.5 mg/kg daily | 75 units/kg daily | 75 units/kg daily |
Important considerations: LMWH should be discontinued at least 12 hours before planned delivery or regional anesthesia. Women receiving LMWH should be educated about injection technique, and platelet count should be checked within 2 weeks of starting treatment.
Other Preventive Measures
- Graduated compression stockings (GCS): Class 1 (14–17 mmHg) compression stockings should be considered for women at increased risk, especially during hospitalization and long-distance travel. They should be properly fitted and worn throughout pregnancy and for 6 weeks postpartum.
- Early mobilization: After cesarean section or any period of bed rest, women should be encouraged to mobilize as soon as safely possible.
- Adequate hydration: Dehydration increases blood viscosity and thrombosis risk. Maintain good oral intake, especially during hyperemesis, labor, and postpartum.
- Avoid prolonged immobility: During long-distance travel (>4 hours), take regular breaks to walk and stretch, perform ankle exercises, and consider compression stockings.
- Smoking cessation: Smoking increases thrombosis risk through endothelial damage and increased fibrinogen levels.
Worked Example
A 37-year-old woman at her booking visit with BMI 33, multiple pregnancy (twins), and a family history of DVT in her mother:
With 4 points at antenatal assessment, this patient should be referred to a specialist and commenced on LMWH prophylaxis immediately (from the first trimester), continuing throughout pregnancy and for at least 6 weeks postpartum. Graduated compression stockings should also be provided.
Frequently Asked Questions
When should VTE risk assessment be performed?
VTE risk should be assessed at booking (first antenatal visit), at any hospital admission during pregnancy, whenever new risk factors develop (e.g., infection, immobilization), and again postpartum before discharge. Risk assessment should be an ongoing process throughout pregnancy.
Is LMWH safe during pregnancy?
Yes. LMWH is the anticoagulant of choice in pregnancy. It does not cross the placenta and therefore does not pose a risk of fetal hemorrhage or teratogenicity. Unlike warfarin, which is teratogenic and crosses the placenta, LMWH can be safely used throughout all trimesters. Side effects include injection site bruising and, rarely, osteoporosis with prolonged use and heparin-induced thrombocytopenia.
What are the warning signs of VTE in pregnancy?
Women should seek urgent medical attention for: unexplained swelling in one leg (especially calf or thigh), leg pain or tenderness (particularly in the calf), redness or warmth in the affected leg, sudden breathlessness, chest pain (especially pleuritic), coughing up blood, or unexplained collapse. DVT in pregnancy is more commonly left-sided and iliofemoral.
Should all women having cesarean section receive thromboprophylaxis?
Emergency cesarean section scores 2 points and elective cesarean scores 1 point. Combined with any other risk factor(s) to reach the threshold (2 points postnatal), most women having cesarean delivery will qualify for at least 10 days of postpartum LMWH prophylaxis. All women undergoing cesarean section should receive graduated compression stockings and early mobilization regardless of their risk score.
How long should postpartum prophylaxis continue?
The minimum duration for postpartum LMWH prophylaxis is 10 days. Women with previous VTE or high-risk thrombophilia should continue for at least 6 weeks. Those with ongoing risk factors (e.g., wound infection, prolonged immobility) may need extended prophylaxis as determined by their clinical team.