Clinical Resources for Clinicians · For clinicians
Thrombophilia Testing and VTE Prophylaxis in Pregnancy: ACOG 196/197
Test for inherited thrombophilia in pregnancy only when the result will change management. ACOG Practice Bulletin 197 supports testing in patients with a personal history of VTE who haven't been tested, and in those with a first-degree relative with a high-risk thrombophilia. It recommends against testing for pregnancy loss, preeclampsia, abruption or growth restriction. Prophylaxis decisions then combine thrombophilia type with personal and family VTE history, as laid out in ACOG Practice Bulletins 196 and 197.
The common problem is less often missed testing than the opposite: patients arriving at a first prenatal visit with a full panel drawn after a loss, an MTHFR result, and a lot of anxiety. Much of the clinical work is un-doing that, so this guide covers counseling as well as the algorithms.
Why testing should be selective
Inherited thrombophilias are common, and most carriers never have a VTE. Testing has real costs:
- False positives and misclassification, especially for protein S, protein C and antithrombin, which shift with pregnancy, acute thrombosis and anticoagulation
- Overtreatment: months of injections with bleeding, bruising and cost burdens
- Anxiety and insurance implications for patients and relatives
ACOG PB 197 concluded the evidence doesn't support a causal link between inherited thrombophilias and recurrent loss, preeclampsia, abruption, fetal growth restriction or stillbirth, and prophylactic heparin hasn't been shown to improve those outcomes in carriers.
Who to test
| Clinical situation | Test? (ACOG PB 197) |
|---|---|
| Personal history of VTE, not previously tested | Reasonable, if results will affect management |
| First-degree relative with a high-risk thrombophilia | Reasonable |
| First-degree relative with VTE, thrombophilia unknown | Consider, after discussion |
| Recurrent pregnancy loss, stillbirth, abruption, FGR, preeclampsia | Not recommended for inherited thrombophilia |
| MTHFR polymorphisms or fasting homocysteine | Not recommended in any scenario |
Antiphospholipid syndrome is different. Lupus anticoagulant, anticardiolipin and anti-β2-glycoprotein I testing is appropriate for some obstetric indications, such as certain patterns of pregnancy loss and early severe preeclampsia, and for thrombosis. ACOG PB 132 covers those indications. APS is acquired and has its own management framework, so keep the two conversations separate with patients.
Which tests, and when
| Thrombophilia | Test | Reliable during pregnancy / anticoagulation? |
|---|---|---|
| Factor V Leiden | DNA analysis (or activated protein C resistance screen) | DNA: yes |
| Prothrombin G20210A | DNA analysis | Yes |
| Antithrombin deficiency | Antithrombin activity | Can be lowered by acute thrombosis and heparin; interpret cautiously |
| Protein C deficiency | Protein C activity | Lowered by warfarin and acute thrombosis |
| Protein S deficiency | Free protein S antigen | Falls physiologically in pregnancy; testing in pregnancy is unreliable without pregnancy-specific ranges |
Genetic tests are valid any time. Functional assays are best drawn remote from pregnancy, acute thrombosis and anticoagulation. If they have to be drawn in pregnancy, bring hematology in on interpretation.
Risk classification
ACOG groups inherited thrombophilias by VTE risk:
- High-risk: homozygous factor V Leiden, homozygous prothrombin G20210A, compound heterozygosity for FVL and prothrombin G20210A, antithrombin deficiency
- Low-risk: heterozygous factor V Leiden, heterozygous prothrombin G20210A, protein C deficiency, protein S deficiency
Prophylaxis framework
PB 196 and 197 pair each risk category with a range of acceptable antepartum and postpartum strategies. The table below reflects their general structure; use the bulletins' full tables for individual decisions.
| Scenario | Antepartum | Postpartum (6 weeks) |
|---|---|---|
| Low-risk thrombophilia, no personal or family VTE | Surveillance | Surveillance, or prophylaxis if additional risk factors |
| Low-risk thrombophilia, first-degree relative with VTE | Surveillance or prophylactic LMWH/UFH | Prophylaxis or intermediate-dose |
| Low-risk thrombophilia, single prior VTE, not on long-term anticoagulation | Prophylactic or intermediate-dose, or surveillance | Prophylaxis or intermediate-dose |
| High-risk thrombophilia, no prior VTE | Surveillance or prophylactic LMWH/UFH | Prophylaxis |
| High-risk thrombophilia with prior VTE or affected first-degree relative | Prophylactic, intermediate or adjusted-dose | Prophylactic, intermediate or adjusted-dose |
| No thrombophilia, single prior VTE from a transient non-hormonal risk factor | Surveillance | Prophylaxis |
| No thrombophilia, prior VTE that was unprovoked or pregnancy/estrogen-associated | Prophylactic LMWH/UFH | Prophylaxis |
| Two or more prior VTEs, or on long-term anticoagulation | Therapeutic-dose | Resume long-term anticoagulation |
The ASH 2018 pregnancy VTE guideline is broadly similar. It recommends antepartum prophylaxis after unprovoked or hormone-associated VTE and is more selective about antepartum prophylaxis in asymptomatic carriers, weighing family history and thrombophilia type.
Postpartum risk factors that tip the balance
VTE risk peaks early postpartum. For a borderline patient, cesarean delivery (especially emergency), postpartum hemorrhage, transfusion, infection, preeclampsia, obesity, immobility and preterm delivery can push toward postpartum prophylaxis. A structured institutional risk assessment at delivery keeps this from depending on who's on call.
Mechanical prophylaxis
PB 196 recommends pneumatic compression devices before cesarean for patients not already on pharmacologic prophylaxis.
Choice of agent
- LMWH is preferred for most patients over unfractionated heparin (ACOG, ASH).
- Warfarin crosses the placenta and is avoided in pregnancy, apart from selected patients with mechanical valves. It's compatible with breastfeeding.
- DOACs aren't recommended in pregnancy or breastfeeding given limited data.
- Heparins are compatible with breastfeeding.
More in anticoagulation in pregnancy.
Peripartum timing
Coordinate with anesthesia using ASRA guidance:
- Neuraxial anesthesia is generally deferred at least 12 hours after a prophylactic LMWH dose and 24 hours after a therapeutic dose.
- Many teams switch from LMWH to unfractionated heparin at about 36 weeks, or earlier if preterm delivery is expected, for more flexibility.
- PB 196 suggests resuming prophylaxis 4–6 hours after vaginal delivery or 6–12 hours after cesarean, adjusted for neuraxial catheter timing and bleeding.
A common failure mode: a patient on LMWH presents in early labor with no written plan, and nobody is sure when the last dose was. A one-page plan in the chart and in the patient's hands by 34 to 36 weeks prevents most of it.
Counseling patients who've already been tested
- Heterozygous FVL or prothrombin G20210A with no personal or family VTE usually doesn't call for antepartum anticoagulation. Tell the patient postpartum risk will be reassessed at delivery, so it doesn't feel like the result was ignored.
- MTHFR variants aren't a thrombophilia for clinical purposes. No anticoagulation, and no special folate formulation beyond standard prenatal recommendations.
- Isolated low protein S drawn in pregnancy may just be the normal pregnancy fall. Retest after pregnancy, off hormones and anticoagulants, before applying the label.
- Relatives: a confirmed high-risk thrombophilia may matter for family members, particularly first-degree female relatives considering estrogen-containing contraception or pregnancy. A patient-facing explainer like our Factor V Leiden and pregnancy guide can help with that conversation.
Practical tips for the LMWH patient
- Teach injection technique at the visit where LMWH starts, and give written instructions for what to do when labor begins.
- Get a baseline platelet count. Routine platelet monitoring for HIT generally isn't needed with LMWH alone unless there's been recent unfractionated heparin exposure.
- Send the plan to the delivery hospital and anesthesia well before term. In New York, where many patients see an MFM at one site and deliver at another, confirm the receiving L&D actually has it.
Referral partnership in New York
Patients with high-risk thrombophilia, prior VTE, APS or ambiguous functional results benefit from a shared antenatal and peripartum plan. Premier Hematology & Oncology, which sponsors our professional resources, offers OB practices referral pathways and co-management for thrombophilia and anticoagulation, as well as fast-track IV iron scheduling. See our hematology partners page, and our hematology referral checklist for criteria.
Frequently asked questions
Who should be tested for inherited thrombophilia in pregnancy?
ACOG considers testing reasonable for patients with a personal history of VTE who haven't been tested, and for those with a first-degree relative with a high-risk thrombophilia. Testing is not recommended for recurrent pregnancy loss, stillbirth, abruption, fetal growth restriction or preeclampsia.
Should MTHFR be tested in pregnancy?
No. ACOG does not recommend testing for MTHFR polymorphisms or fasting homocysteine, because they haven't been shown to increase VTE risk or adverse pregnancy outcomes.
Which thrombophilias are considered high-risk?
ACOG classifies homozygous factor V Leiden, homozygous prothrombin G20210A, compound heterozygosity for both, and antithrombin deficiency as high-risk thrombophilias.
Is LMWH preferred over unfractionated heparin in pregnancy?
Yes, for most patients. ACOG and ASH prefer LMWH for more predictable dosing and lower rates of HIT and osteoporosis. Unfractionated heparin is sometimes substituted near delivery to make neuraxial timing easier.
Sources
- ACOG — Thromboembolism in Pregnancy (Practice Bulletin No. 196)
- ACOG — Inherited Thrombophilias in Pregnancy (Practice Bulletin No. 197)
- ACOG — Antiphospholipid Syndrome (Practice Bulletin No. 132)
- ASH — 2018 guidelines for management of venous thromboembolism: venous thromboembolism in the context of pregnancy
- ASRA — Regional Anesthesia in the Patient Receiving Antithrombotic or Thrombolytic Therapy (4th edition)
This article is for general education and is not medical advice. It does not replace an evaluation by a licensed clinician who knows your history. If you think you have a medical emergency, call 911.