Obstetric · Thrombosis
VTE Prophylaxis in Pregnancy
Pregnancy and puerperium VTE risk score + LMWH dose calculator. Per RCOG Green-top 37a and ACOG PB 196.
Last reviewed 26 May 2026
VTE prophylaxis — pregnancy + puerperium
RCOG GTG 37a risk score + LMWH dose
Phase
Risk factors (tick all that apply)
Risk score (antenatal)
0
Low risk — early mobilisation, hydration, graduated compression stockings if hospitalised. Reassess at each visit.
Educational tool only — not medical advice. RCOG Green-top 37a (2015, minor updates 2023). ACOG PB 196 (2018). Local protocols may differ (some US centres use ASH 2018 thresholds). Therapeutic LMWH dose for women with prior recurrent / OE-related VTE is 1 mg/kg BD enoxaparin (or anti-Xa-guided equivalent), not the prophylactic doses shown.
What does this mean?
Pregnancy increases VTE risk ~4–5 fold (~1.7 / 1,000 pregnancies) and the postpartum period is the single highest-risk 6 weeks of a woman’s reproductive life (~20-fold — Sultan 2014 BMJ). PE remains a leading direct cause of UK maternal mortality (MBRRACE-UK). Identification of risk and timely LMWH prophylaxis matters. The RCOG GTG 37a point system is the most widely-used pragmatic risk-stratifier worldwide: tally pre-existing and obstetric factors, threshold at ≥ 4 antenatal (1st-trimester start) or 3 (28-wk start), and ≥ 2 postnatal (10 days) or ≥ 3 (6 weeks). LMWH (enoxaparin, dalteparin, tinzaparin) is weight-banded and doesn’t cross the placenta. Women with prior recurrent or oestrogen-related VTE get THERAPEUTIC dose throughout. Mechanical: early mobilisation, hydration, graduated stockings — especially in hospital. Aspirin is NOT VTE prophylaxis in pregnancy (it’s for PE prevention, a different pathway).
Introduction
Venous thromboembolism (VTE) is a leading direct cause of maternal mortality. Risk assessment at booking, on hospital admission, and at delivery, with timely low-molecular-weight heparin (LMWH) for at-risk women, prevents most pregnancy and postpartum VTE deaths.
How to use
- Select antenatal or postnatal phase.
- Tick all applicable risk factors.
- Enter weight for the prophylactic enoxaparin band.
- Threshold logic and dose appear in the result card.
Thresholds
- Antenatal score ≥ 4 → LMWH from 1st trimester.
- Antenatal score = 3 → LMWH from 28 weeks.
- Postnatal score ≥ 2 → LMWH for ≥ 10 days.
- Postnatal score ≥ 3 → LMWH for 6 weeks.
LMWH dosing (prophylactic)
- Enoxaparin: < 50 kg = 20 mg SC od; 50–90 kg = 40 mg od; 91–130 kg = 60 mg od; 131–170 kg = 80 mg od; > 170 kg = 0.6 mg/kg/day split BD.
- Dalteparin and tinzaparin: equivalent unit-per-kg bands.
Anaesthesia and delivery
- Prophylactic LMWH: last dose ≥ 12 h before regional anaesthesia.
- Therapeutic LMWH: last dose ≥ 24 h before regional anaesthesia.
- Restart 4–12 h post-delivery once haemostasis confirmed.
Limitations
- Educational tool — not a prescribing aid; local protocols supersede.
- Therapeutic-dose patients need haematology / MFM input; this calculator addresses prophylaxis.
- Renal impairment requires anti-Xa monitoring and dose adjustment.
- Heparin-induced thrombocytopenia (HIT) is rare with LMWH but possible — monitor.
Sources
- RCOG Green-top Guideline 37a. Reducing the Risk of Venous Thromboembolism during Pregnancy and the Puerperium. April 2015 (minor updates 2023).
- ACOG Practice Bulletin 196. Thromboembolism in Pregnancy. 2018.
- Bates SM, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: venous thromboembolism in the context of pregnancy. Blood Adv 2018.
- Sultan AA, et al. Risk factors for first venous thromboembolism around pregnancy. Blood 2013.
- MBRRACE-UK confidential enquiry reports.
Frequently asked questions
Why is VTE such a big deal in pregnancy?
Pregnancy and the postpartum period are uniquely thrombogenic. Background pregnancy VTE rate is ~1.7 per 1,000 pregnancies — 4–5 × non-pregnant baseline — driven by venous stasis (uterine compression), procoagulant changes (increased factors II, VII, VIII, X, fibrinogen; decreased Protein S), and endothelial injury at delivery. The first 6 weeks postpartum is the single highest-risk 6-week window of a woman's reproductive life (~20-fold). MBRRACE-UK confidential enquiries have repeatedly cited pulmonary embolism among the top direct causes of UK maternal mortality.
Which guideline does this calculator use?
RCOG Green-top Guideline 37a (April 2015, with minor updates through 2023) — the most comprehensive and widely-adopted obstetric VTE risk-scoring system globally. The companion US guideline is ACOG Practice Bulletin 196 (2018), which uses slightly different thresholds but the same general framework. ASH 2018 also has thresholds. Local protocols vary; your unit will have a local adaptation of one of these.
What's the score threshold for starting LMWH antenatally?
RCOG GTG 37a: score ≥ 4 → start prophylactic LMWH from the FIRST trimester. Score = 3 → start from 28 WEEKS gestation. Score < 3 → no pharmacological prophylaxis but emphasise mobilisation, hydration, and graduated compression stockings if hospitalised. Women with prior recurrent or oestrogen-related VTE get THERAPEUTIC (treatment-dose) LMWH throughout pregnancy and 6 weeks postpartum.
What's the threshold postpartum?
RCOG GTG 37a postpartum thresholds (assessed at delivery): score ≥ 2 → prophylactic LMWH for at least 10 days; score ≥ 3 (or any single major risk factor like CS in labour) → prophylactic LMWH for the full 6 weeks postpartum. Some risk factors only count postnatally (caesarean in labour, prolonged labour, PPH > 1 L).
How is LMWH dosed?
PROPHYLACTIC (weight-banded enoxaparin example): < 50 kg = 20 mg/day; 50–90 kg = 40 mg/day; 91–130 kg = 60 mg/day; 131–170 kg = 80 mg/day; > 170 kg = 0.6 mg/kg/day in 2 divided doses. Dalteparin and tinzaparin use equivalent unit-per-kg bands. THERAPEUTIC (treatment-dose) enoxaparin is 1 mg/kg twice daily; anti-Xa-guided in extreme weights or renal impairment. LMWH does NOT cross the placenta — safe for the baby.
What about delivery and epidural with LMWH?
Standard guidance: prophylactic LMWH last dose ≥ 12 hours before planned regional anaesthesia (epidural / spinal) or induction. THERAPEUTIC LMWH last dose ≥ 24 hours before regional block. Hold for spontaneous labour and check the timing with anaesthesia at presentation. Convert to twice-daily UFH closer to term if anaesthesia timing is uncertain. Restart 4–12 h post-delivery once haemostasis confirmed (specific local protocols vary).
Is aspirin used for VTE prophylaxis in pregnancy?
No. Aspirin is used in pregnancy for PRE-ECLAMPSIA prevention (low-dose, 75–150 mg/day from < 16 wk in high-risk women — see /calculators/aspirin-pe-prevention), NOT for VTE prophylaxis. Different mechanism (antiplatelet vs anticoagulant), different evidence base, different indication. The 2018 ASH guideline mentions aspirin as a weak alternative for some specific scenarios but RCOG/ACOG do not recommend it as routine VTE prophylaxis.
How does this relate to other calculators on BumpBites?
BumpBites companion tools: /calculators/aspirin-pe-prevention for the different aspirin-PE pathway; /calculators/preeclampsia-diagnosis for the related severe-morbidity differential; /calculators/cmqcc-pph-risk for postpartum bleeding risk that PPH > 1 L (an input here) overlaps with; /calculators/meows for the daily maternal monitoring that catches early VTE warning signs (tachycardia, tachypnoea).