13.3 Venous Thromboembolism in Women: Risk, Diagnosis, Treatment & Prevention in Pregnancy

Why VTE Matters in Women's Health

Pulmonary embolism is a leading cause of pregnancy-related death, and estrogen exposure from contraception, pregnancy, and hormone therapy makes VTE a recurring exam theme. Clots form through Virchow's triad: venous stasis, endothelial injury, and hypercoagulability. Pregnancy adds all three, through compression of pelvic veins, delivery trauma, and increases in clotting factors with lower protein S.

Absolute VTE Risk Comparison (ACOG)

This comparison is useful for counseling: a combined pill raises risk, but far less than the pregnancy it prevents. Risk is highest in the first 3–6 weeks postpartum, especially after cesarean birth.

Risk Factors

• Inherited thrombophilias: Factor V Leiden (most common), prothrombin G20210A, and deficiencies of protein C, protein S, and antithrombin (antithrombin deficiency carries the highest risk).
• Acquired: Antiphospholipid syndrome, prior VTE (the strongest single predictor), obesity, smoking, immobility, surgery, cancer, long-distance travel, oral estrogen, older age, preeclampsia, cesarean birth, postpartum hemorrhage, infection, and multiple gestation.

---

Diagnosis

Nonpregnant Patients

1. Estimate pretest probability (Wells score).
2. With low probability, a negative D-dimer excludes VTE.
3. With high probability or a positive D-dimer: compression ultrasound for suspected DVT and CT pulmonary angiography (CTPA) for suspected PE (a V/Q scan when contrast is contraindicated).

Pregnant Patients

• D-dimer rises normally in pregnancy, so it is less useful (pregnancy-adapted algorithms such as YEARS can still help).
• About 85% of pregnancy-related DVTs are left-sided, and many involve the iliofemoral veins. Start with compression ultrasound; if it is negative but suspicion for iliac thrombosis is high, use MR venography or serial ultrasound.
• Suspected PE: If leg symptoms are present, compression ultrasound first (a positive study justifies treatment). Otherwise, obtain a chest radiograph: if it is normal, a V/Q scan is preferred; if abnormal, CTPA. Radiation from either test is far below doses associated with fetal harm, and diagnosis should never be withheld.

Presentation

• DVT: Unilateral calf or thigh pain, swelling (a calf circumference difference of 2 cm or more), warmth, and erythema.
• PE: Sudden dyspnea, pleuritic chest pain, tachycardia, hypoxemia, hemoptysis, and syncope. In pregnancy, these overlap with normal symptoms, so maintain a low threshold for testing.

---

Treatment

Delivery planning: Many patients switch to unfractionated heparin near term or have a planned induction so neuraxial anesthesia is possible; neuraxial placement generally waits 24 hours after a therapeutic LMWH dose and 12 hours after a prophylactic dose. Massive PE with hemodynamic instability may require thrombolysis or embolectomy even in pregnancy.

---

Thromboprophylaxis in Pregnancy (ACOG Practice Bulletin 196)

• Prior VTE with a high-risk thrombophilia, or multiple prior VTEs: Prophylactic or intermediate-dose LMWH antepartum and for 6 weeks postpartum (therapeutic dosing if on long-term anticoagulation).
• Single prior VTE that was unprovoked or related to pregnancy or estrogen: Antepartum and postpartum prophylaxis.
• Single prior VTE with a transient non-hormonal cause (for example, trauma): Surveillance during pregnancy and postpartum prophylaxis.
• All cesarean births: Pneumatic compression devices before surgery and until ambulation for patients not already on anticoagulation; LMWH is added for higher-risk patients.
• Encourage early ambulation and hydration after every birth.

---

Contraception and Hormone Therapy After VTE

• Combined hormonal contraception: US MEC Category 4 for acute VTE or a history with higher recurrence risk, and Category 3 for a history with lower recurrence risk.
• Progestin-only methods (implant, LNG-IUD, POPs) are Category 2, and the copper IUD is Category 1. On long-term anticoagulation, heavy bleeding is a concern, which favors the LNG-IUD.
• Menopausal hormone therapy: Systemic estrogen is generally avoided after VTE; if it is strongly needed, a transdermal route is preferred after specialist input. Nonhormonal options (SSRIs, SNRIs, fezolinetant, elinzanetant) are first-line for hot flashes.
• Superficial thrombophlebitis: Warm compresses and NSAIDs (outside late pregnancy); obtain ultrasound if the clot is near the saphenofemoral junction or extends, because it can coexist with DVT.

---

Counseling Pearls

• Thrombophilia testing is not routine before prescribing combined contraception. Take a careful personal and family history instead; test when a first-degree relative had VTE at a young age or a known familial thrombophilia exists.
• Timing of thrombophilia tests: Protein C, protein S, and antithrombin levels are unreliable during an acute clot, during anticoagulation, and in pregnancy (protein S falls normally), so defer them or use genetic tests (factor V Leiden, prothrombin G20210A).
• Progestin type matters: Combined pills containing desogestrel, gestodene, cyproterone, or drospirenone carry roughly 1.5–2 times the VTE risk of levonorgestrel-containing pills, although absolute risk stays small.
• Air travel and surgery: Advise hydration, walking, and calf exercises on long flights, and assess VTE risk before major surgery in patients taking estrogen.