VTE prophylaxis
Written 2nd May 2020 - Last Update 31st August 2026
VTE prophylaxis
Pregnancy is a hypercoagulable state — hormonal change, increased blood volume and altered flow all raise VTE risk. Thromboprophylaxis decisions are made with a risk-factor scoring tool rather than on any single factor.
The tool below is the HNE Local Health District version, adapted from RCOG Green-top Guideline 37a. Reassess on admission, and again postnatally, as risk factors can appear or resolve through pregnancy.
NICE’s own evidence review deliberately does not endorse one particular tool. It directs clinicians to published tools while noting that the RCOG tool never reported its own accuracy, calibration or performance measures, and that the underlying evidence for individual risk factors is generally low to very low quality.
A related caution from the same review: risks from continuous variables such as age or BMI get split into discrete categories for guideline writing, so a woman just below a threshold and one just above may in practice have very similar absolute risk.
The risk factors with a consistent and independent association in that review were antenatal haemorrhage, blood group A, deprivation, history of VTE, history of blood transfusion, hospitalisation, hyperemesis, infection, inflammatory bowel disease, some medications, miscarriage, pre-existing diabetes, and varicose veins.
Risk factors and scores
| Risk factor | Score |
|---|---|
| Pre-existing | |
| Previous VTE (except a single event related to major surgery) | 4 |
| Previous VTE provoked by major surgery | 3 |
| Known high-risk thrombophilia | 3 |
| Medical comorbidity (cancer, heart failure, active SLE, inflammatory polyarthropathy or IBD, nephrotic syndrome, type 1 diabetes with nephropathy, sickle cell disease, current IV drug use) | 3 |
| Family history of unprovoked or oestrogen-related VTE in a first-degree relative | 1 |
| Known low-risk thrombophilia (no personal VTE) | 1 |
| Age >35 years | 1 |
| Obesity (BMI >30 kg/m²) | 1 or 2* |
| Parity ≥3 | 1 |
| Smoker | 1 |
| Gross varicose veins | 1 |
| Obstetric | |
| Pre-eclampsia in current pregnancy | 1 |
| ART/IVF (antenatal only) | 1 |
| Multiple pregnancy | 1 |
| Caesarean section in labour | 2 |
| Readmission/prolonged admission (≥3 days) in the puerperium | 1 |
| Elective caesarean section | 1 |
| Mid-cavity or rotational operative delivery | 1 |
| Prolonged labour (>24 hours) | 1 |
| PPH (>1 L or transfusion) | 1 |
| Preterm birth <37+0 weeks in current pregnancy | 1 |
| Stillbirth in current pregnancy | 1 |
| Transient | |
| Any surgical procedure in pregnancy/puerperium except immediate perineal repair (e.g. appendicectomy, postpartum sterilisation) | 3 |
| Hyperemesis | 3 |
| Current systemic infection | 1 |
| Immobility, long-distance travel, dehydration | 1 |
* BMI 30–39 = 1; BMI ≥40 = 2.
Thresholds
- Antenatal score ≥4 → thromboprophylaxis from the first trimester, continuing to 6 weeks postnatal, with LMWH. Four or more current risk factors other than previous VTE or thrombophilia means prophylactic LMWH throughout the antenatal period and usually 6 weeks postnatally, with a postnatal reassessment.
- Antenatal score 3 → thromboprophylaxis from 28 weeks. Three current risk factors other than previous VTE or thrombophilia means prophylactic LMWH from 28 weeks, again usually with 6 weeks postnatally.
- Postnatal score ≥2 → thromboprophylaxis for at least 10 days. Two current risk factors other than previous VTE or thrombophilia means prophylactic LMWH for at least 10 days postpartum.
- If admitted to hospital antenatally, consider thromboprophylaxis; if risk factors persist or there are >3, consider extending LMWH duration.
- Anyone requiring antenatal LMWH needs at least 6 weeks’ postnatal LMWH.
- For patients with an identified bleeding risk, discuss the bleeding vs thrombosis balance with a haematologist experienced in thrombosis and bleeding in pregnancy.
Which anticoagulant
Low molecular weight heparin (enoxaparin or dalteparin) is the usual choice for both treatment and prophylaxis in pregnancy, on the strength of its efficacy, predictable pharmacokinetics and safety. It comes in pre-filled syringes and needs no routine laboratory monitoring.
Unfractionated heparin is the alternative when LMWH is contraindicated or when rapid reversibility matters, since its half-life is shorter. It does require frequent monitoring of the activated partial thromboplastin time.
Warfarin crosses the placenta and is associated with embryopathy, particularly first-trimester exposure, so it is avoided except where the benefit clearly outweighs the risk — most often a mechanical heart valve, where the plan has to be individualised.
Direct oral anticoagulants are not recommended. Safety data in pregnancy are limited, and they should also be avoided while breastfeeding.
Monitoring and delivery
Prophylactic-dose LMWH rarely needs monitoring — optimal anti-Xa levels for prophylactic dosing have never been established. Therapeutic weight-based dosing may be tracked with anti-Xa levels, but the need for it is uncertain and comes down to local protocol. Check platelets to pick up heparin-induced thrombocytopenia.
Anticoagulation raises the risk of spinal or epidural haematoma with neuraxial anaesthesia. Timing therefore matters:
- Prophylactic LMWH — stop at least 12 hours before a planned induction or caesarean.
- Adjusted (therapeutic) dose LMWH — stop at least 24 hours before a planned delivery or neuraxial anaesthetic.
- Restart prophylactic LMWH at least 12 hours after neuraxial blockade and at least 4 hours after catheter removal; for intermediate or adjusted doses, wait at least 24 hours after the blockade.
Tell the paediatric team, since neonates born to mothers on anticoagulants may be at increased risk of bleeding complications.