Should a pregnant woman older than 35 years with a BMI greater than 35 kg/m² receive low‑molecular‑weight heparin (enoxaparin) prophylaxis?

VTE Prophylaxis in Pregnant Women with BMI >35 and Age >35

No, a pregnant woman with BMI >35 and age >35 does not automatically require enoxaparin prophylaxis—these are both minor risk factors, and current guidelines require either one major risk factor OR at least two minor risk factors to justify pharmacologic prophylaxis. 1

Risk Stratification Framework

Minor Risk Factors (Require ≥2 for Prophylaxis)

• Advanced maternal age (≥35 years) 1
• Class I obesity (BMI 31-34.9 kg/m²) or higher 1
• Current smoking 1
• Family history of VTE 1
• Varicose veins 1

Major Risk Factors (Any Single Factor Warrants Prophylaxis)

• Prior personal history of DVT or PE 1
• High-risk inherited thrombophilia (antithrombin deficiency, homozygous Factor V Leiden, compound heterozygosity) 2
• Antiphospholipid antibody syndrome 1
• Antepartum immobility ≥1 week 1

Evidence-Based Threshold for Pharmacologic Prophylaxis

The American College of Chest Physicians (ACCP) establishes that pharmacologic prophylaxis is warranted when absolute VTE risk exceeds 3%, which occurs with one major risk factor OR two or more minor risk factors. 1

For your specific patient:

• Age >35 = 1 minor risk factor
• BMI >35 (Class II obesity) = 1 minor risk factor
• Total = 2 minor risk factors → prophylaxis IS indicated 1

Recommended Prophylaxis Regimen

Standard Dosing for BMI 35-39.9 kg/m²

• Enoxaparin 40 mg subcutaneously once daily 1, 3
• Continue throughout pregnancy if risk factors persist 1

Dosing for Class III Obesity (BMI ≥40 kg/m²)

• Enoxaparin 40 mg subcutaneously every 12 hours (intermediate dosing) 1, 3
• Alternative: weight-based dosing at 0.5 mg/kg every 12 hours 3
• Weight-based dosing achieves target anti-Xa levels (0.2-0.5 IU/mL) more reliably than fixed dosing in morbidly obese patients 4, 5

Duration of Prophylaxis

• Antepartum prophylaxis should continue throughout pregnancy when risk factors persist 1
• Postpartum prophylaxis for 6 weeks is mandatory 1
• The postpartum period carries the highest VTE risk, particularly after cesarean delivery 1

Monitoring Considerations

• Routine anti-Xa monitoring is NOT required for standard prophylactic dosing (40 mg daily) 3
• Consider anti-Xa monitoring for Class III obesity (BMI ≥40) with target levels 0.2-0.5 IU/mL 3
• Measure anti-Xa levels 4-6 hours after injection, after 3-4 doses 3

Guideline Consensus vs. Divergence

The American Society of Hematology (ASH) 2018 guidelines suggest AGAINST routine antepartum prophylaxis for low-risk thrombophilias without family history, but this recommendation does NOT apply when additional clinical risk factors (obesity, age) are present. 2

The Society for Maternal-Fetal Medicine (SMFM) and ACOG align with ACCP in recommending prophylaxis when two or more minor risk factors coexist. 1

Common Pitfalls to Avoid

• Do not withhold prophylaxis simply because thrombophilia testing is negative—clinical risk factors alone justify treatment 1
• Do not use standard 40 mg once-daily dosing in patients with BMI ≥40—this results in subtherapeutic anti-Xa levels in the majority of patients 1, 6, 5
• Do not discontinue prophylaxis at delivery—extend for 6 weeks postpartum when risk factors persist 1
• Avoid initiating enoxaparin within 12 hours of neuraxial anesthesia (24 hours for intermediate dosing) 3

Practical Implementation

1. Calculate BMI and identify all VTE risk factors at first prenatal visit 1
2. If BMI 35-39.9 + age >35: start enoxaparin 40 mg daily 1, 3
3. If BMI ≥40 + age >35: start enoxaparin 40 mg every 12 hours 1, 3
4. Continue throughout pregnancy and extend 6 weeks postpartum 1
5. Hold enoxaparin 12-24 hours before planned delivery 3
6. Resume 4-12 hours after delivery depending on bleeding risk 3