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For Secondary Care| Key Learning Points

Key Learning Points: British Society of Haematology Guideline on Anticoagulation Management During Pregnancy for Patients with Mechanical Heart Valves

Newly Published Guidance from the British Society of Haematology Addresses the Risks Faced by Pregnant Women with Mechanical Heart Valves; Dr Niki Walker and Dr Lorna Swan Highlight Seven Key Learning Points for Cardiologists on the Anticoagulation Management of These Patients

Read This Article to Learn More About:
  • the nature and prevalence of risks in pregnancy for women with mechanical heart valves and their babies in the UK 
  • British Society of Haematology guidance on anticoagulation strategies for pregnant women who have mechanical heart valves
  • recommendations on planning, counselling, and monitoring from pre-pregnancy to postpartum.
Key Points can be found at the end of this article 

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Pregnancy in individuals with a mechanical heart valve (MHV) is very high risk. According to the modified World Health Organization classification of maternal cardiovascular risks, mechanical valves rank as category III (on a scale of I–IV of ascending risk), and are associated with a significantly increased risk of maternal mortality or morbidity.1 Key maternal risks associated with MHVs in pregnancy stem from the need for anticoagulation therapy, notably to prevent valve thrombosis and haemorrhagic complications.2 The fetus is also exposed to significant hazards, including miscarriage, stillbirth, fetal haemorrhage, and warfarin-induced embryopathy.3

An Urgent Need for New Guidance

Results from the UK Obstetric Surveillance System (UKOSS) study published in 2017 shone a spotlight on the true extent of the risks faced by pregnant women with MHVs in this country.4 The UK-specific data noted that only 28% of these pregnancies resulted in good maternal and fetal outcomes—vastly inferior to the 58% of uncomplicated pregnancies found in the European Registry of Pregnancy and Cardiac Disease (RoPAC) study in 2015, although it should be noted that this is based on a relatively small number of cases.5 Maternal mortality stood at a shocking 9% in the UKOSS study, with a 41% risk of severe morbidity, including a 16% rate of valve thrombosis and 9% risk of cerebrovascular accident.4 This was a watershed moment, highlighting the urgent need to improve pregnancy care in women with MHVs in the UK. 

In response, new guidance from the British Society for Haematology (BSH) for the anticoagulant management of pregnant individuals with mechanical heart valves3 addresses the key risks encountered, and covers pre-pregnancy counselling, antenatal care, delivery management, and postpartum complications. The BSH recommendations were developed using observational data and expert opinion, as well as evidence from non-pregnancy situations.3 

Compared to the top-line advice offered by NICE and the European Society of Cardiology (ESC), the BSH guideline affords a deep dive into the key issues facing cardiologists when managing pregnant individuals with MHVs.2,3,6 BSH recommendations were designed to be more practical and useful for clinicians, providing a detailed ‘how to’ guide for navigating the complexities and challenges of pregnancy in this group of patients. Recommendations align closely with other leading guidance in the field—notably the 2021 ESC/European Association for Cardiothoracic Surgery (EACTS) guideline for the management of valvular heart disease.7

Perhaps most importantly, BSH guidance was developed by a diverse group of specialists, including obstetricians, neonatologists, cardiologists, anaesthetists, and haematologists, thereby providing the overarching multidisciplinary team (MDT) perspective that is critical to improving maternal and fetal outcomes in this very high-risk setting.3 

This article focuses on seven key learning points for cardiologists from the BSH guideline for anticoagulant management of pregnant individuals with mechanical heart valves.

1. Plan Ahead by Providing Pre-pregnancy Counselling for All Women of Reproductive Age

The BSH stresses that women with an MHV who are of childbearing age should be offered pre-pregnancy counselling (Box 1) as soon as appropriate.3 In advance of cardiac surgery, cardiologists should be talking to their female patients about the different valve replacement options available and their potential impact on future pregnancy risk. When there is a viable alternative, mechanical valves should not be used in young women who may wish a future pregnancy. 

Box 1: Key Elements of Pre-pregnancy Counselling3 
  • Evaluation of patient’s current condition, and clarification of valve status
  • Adherence with existing anticoagulation therapy and current dosing 
  • Risk stratification for thrombotic events
  • Risks of pregnancy and advantages/disadvantages of different anticoagulation regimens
  • Importance of early pregnancy testing and urgent specialist review
    • protocol for making early contact with the specialist team
  • Planning of pregnancy anticoagulation regimens
  • Importance of adherence and awareness of associated monitoring burden
  • Risks of unplanned pregnancy and need for effective contraception
  • General aspects of preconception counselling (folic acid, etc)
  • Risks of congenital heart disease in offspring
  • Review of existing medications and optimisation for pregnancy
  • Written pregnancy management plan (copy to be provided to patient and GP).
One of the key pre-pregnancy counselling points for women with MHVs should be a frank discussion about the benefits and risks of different anticoagulation regimens for a mother and her fetus. A clear dichotomy exists in this regard. Although vitamin K antagonists (VKAs) are a superior anticoagulant in preventing mechanical valve thrombosis (MVT), they carry greater risks for a developing fetus.3 Warfarin crosses the placenta, increasing the risk of fetal, placental, and neonatal haemorrhage, and also causes embryopathy, a fetal warfarin syndrome that occurs in up to 12% of cases.3 Conversely, low molecular weight heparin (LMWH) anticoagulants do not cross the placenta, and are therefore safer for a fetus, but their use is associated with poorer maternal outcomes.3

2. Urgently Refer all Confirmed Pregnancies to the Specialist Team 

Echoing guidance from other societies, the BSH urges that all pregnant individuals with MHVs should be managed in a tertiary specialist centre that has the relevant expertise.3 Referral to the specialist team should be done as a matter of urgency—as soon as pregnancy is confirmed—with review of the patient by the MDT ideally taking place before 6 weeks’ gestation.3 Patients should have been informed how to self-refer as part of their pre-pregnancy counselling, but any healthcare professional who encounters a newly pregnant woman with an MHV can—and should—initiate a referral if this has not already been done.

3. Review Risk–Benefit Ratio and Develop an Anticoagulation Strategy

All anticoagulants carry risks to both a mother and the developing fetus. An anticoagulation plan should therefore be individualised, considering specific maternal risk factors and patient preferences, and ideally developed in the pre-pregnancy setting. This should be a written document that is widely circulated, with a copy also provided to the patient. 

The BSH guideline states that no single anticoagulation strategy can be recommended above others, based on current evidence.3 This mirrors ESC/EACTS guidance, which acknowledges that, although therapeutic anticoagulation during pregnancy is ‘of upmost importance’, no single anticoagulation regimen is ideal, and the choice must be a balance between maternal and fetal risks.7 

VKAs are superior to LMWH at preventing valve thrombosis in pregnancy, and constitute the optimal therapy for improving maternal outcomes.3 According to the BSH, it is therefore reasonable to weight recommendations towards warfarin for women at higher risk of MVT (Box 2). Although the instinct may be to reduce the dose to minimise harm, this is not recommended. There is insufficient evidence that keeping the international normalised ratio (INR) target below range so as to maintain warfarin at less than 5 mg averts adverse fetal outcomes;3 low-dose warfarin may in fact lead to poorer fetal outcomes.8 

Box 2: Risk Factors for Mechanical Valve Thrombosis3,7 
  • Prosthesis thrombogenicity
  • Mitral, tricuspid, or pulmonary valve replacement
  • Previous thromboembolism
  • Valve dysfunction or mismatch
  • Left ventricular dysfunction
  • Atrial fibrillation
  • Poor medication adherence. 
Given the potential for warfarin embryopathy, in clinical practice most mothers-to-be opt to switch to LMWH. In this case, as soon as a positive pregnancy test is confirmed, the individual should stop taking VKAs and start on twice-daily LMWH; it is not necessary for the INR to already be within the normal range.3 When switching to LMWH, the BSH advises starting on a dose higher than the standard therapeutic dose, because the risk of MVT appears to be front-loaded in the first trimester during transition from VKA to LMWH.3 Despite the perception that MVT risk is highest after birth, in reality, the majority of events occur antenatally rather than postpartum.3 In the international RoPAC study, half of all MVTs in pregnant women occurred during the transition from VKA to LMWH in the first trimester.5 Very careful attention is therefore required during this transition in the early stages of pregnancy. The addition of low-dose aspirin to LMWH anticoagulation is also recommended, assuming there are no contraindications.3 

Once an anticoagulation regimen has been decided upon, the priority is to optimise ongoing management of the patient during pregnancy. Table 1 provides a summary of anticoagulation strategies.

Table 1: Anticoagulation Strategies at a Glance3 

Option 1: Switch to Low Molecular Weight HeparinOption 2: Vitamin K Antagonist Second Trimester to 36 Weeks, then Heparin
  • Twice daily
  • Start high
    • 2.5 mg/kg/day enoxaparin
    • 250 IU/kg/day dalteparin
    • 250 IU/kg/day tinzaparin
  • Peak anti-Xa level 1–1.4 
    • 3–4 hours post-dose
    • at least weekly until stable 
  • Intensive and fast titration due to high rate of valve thrombosis during first trimester switch
  • Add aspirin (75 mg/day) in the absence of contraindications
  • INR same target range as without pregnancy
  • INR monitoring 1–2 times weekly
  • Better for women at higher risk of valve thrombosis, but higher fetal risks
INR=international normalised ratio

4. Monitor Anticoagulants Throughout Pregnancy and Take Steps to Maximise Adherence

For patients who opt to remain on warfarin, non-pregnancy INR targets can be used for monitoring. At a minimum, INR should be checked weekly when it is unstable and fortnightly when stable, with women given the option to self-test when appropriate.3 Warfarin crosses the placenta so avoid a high INR, otherwise a fetus will be more anticoagulated than its mother.3

Data suggest that standard therapeutic doses of LMWH in pregnancy are inadequate and that it is prudent to monitor, but controversy persists on whether to use peak or trough testing.3 Currently, evidence to support the routine use of specific trough levels is lacking.3 As a ‘reasonable compromise’, the BSH therefore recommends using an anti-Xa assay—with a peak anti-Xa target between 1.0 and 1.4 IU/ml, taken 3–4 hours after a twice-daily LMWH dose.3 Monitoring should be carried out at least weekly until the target level is achieved—or when levels are below target at any stage—moving to regular checks every 2–4 weeks thereafter, depending on stability.3

The BSH advises a frequency of LMWH injection of not more than twice daily—given that the physical burden and injection site toxicity of such a regimen could prove counterproductive to adherence.3 Low adherence to the chosen anticoagulation strategy is a leading cause of morbidity and mortality in pregnant women with MHV, underscoring the importance of measures to boost adherence both during pregnancy and the postpartum period.3 Women should be made aware, even before embarking on pregnancy, of the need for frequent hospital appointments and the fact that travel to a tertiary specialist centre will be required. This, in turn, may have implications for their work, finances, or ability to care for other children. 

5. Maintain a High Index of Suspicion for Valve Thrombosis in Any Pregnant Women with a Mechanical Heart Valve

Given the high risk of MVT in pregnancy, clinicians should remain alert to potential warning signs and symptoms, and act accordingly (Box 3).3
Box 3: Red Flags for Mechanical Heart Valve Dysfunction3
  • Muffled mechanical sounds
    • unable to hear clicks from the valve closure
  • New murmur
  • Shortness of breath
  • Heart failure
  • Cardiogenic shock
  • Embolic events 
    • stroke
    • acute abdominal pain—renal, splenic infarcts
  • Pulmonary embolism (when patients have a right-sided MHV).
MHV=mechanical heart valve
If an MVT should unfortunately occur, the patient must be managed in a specialist centre under the care of an MDT, with the use of thrombolysis or emergency cardiac surgery when appropriate.3 

6. Be Ready for Labour and Delivery—and Beyond

Planning is central to a successful outcome of a pregnancy in women with MHV. Birth can prove a hazardous time due to the need to balance the risk of a haemorrhage with that of an MVT from prolonged periods of anticoagulant reduction. Currently, there is an absence of data to support any specific mode of delivery or anticoagulation regimen as optimal, so the decision should be individualised—taking into account benefits and risks.3 A plan for timing and mode of delivery should be agreed in advance, with input from all members of the MDT and from the patient.3

The BSH makes several key recommendations on anticoagulation in the run up to birth and the immediate postpartum period (Box 4).3  

Box 4: Managing Labour and Delivery3


  • Switch patients on VKAs to heparin a minimum of 2 weeks before anticipated delivery, and by 36 weeks at the latest
  • Pause therapeutic LMWH for at least 24 hours before surgical delivery to permit neuraxial analgesia/anaesthesia
  • Stop aspirin at least 3 days before delivery given the high risk of PPH.
  • Consider a caesarean section for women on VKAs presenting in labour ≤2 weeks from their scheduled delivery date to reduce fetal bleeding risks from labour
  • Avoid prolonged periods of interruption to anticoagulation—e.g. during induction of labour
  • Consider use of intermediate or prophylactic doses of LMWH or intravenous UFH during labour induction.
  • Use prophylactic or intermediate doses of LMWH for the first 24–48 hours after delivery, before re-introducing therapeutic doses
  • Do not restart VKAs until day 5–7 at the earliest
  • If using UFH, gradually increase intensity for the first few days.
VKA=vitamin K antagonists; LMWH=low molecular weight heparin; PPH=postpartum haemorrhage; UFH=unfractionated heparin
In contrast to NICE and other guidance which recommends resumption of therapeutic anticoagulation almost immediately after birth, the BSH advises a more cautious approach—reintroducing anticoagulation in a stepwise fashion.2,3,6 BSH advice is therefore to give prophylactic or intermediate doses of LMWH only for the first 24–48 hours after delivery.3 The rationale is that, although the risk of MVT exists throughout pregnancy, the risk of major haemorrhage is concentrated in the period around delivery. If postpartum haemorrhage occurs, further extended pauses in anticoagulation will be required which may be counterproductive. Furthermore, evidence from subjects who are not pregnant argues against immediate anticoagulant escalation, even in patients with MHVs.7,9

Postpartum, women should not leave hospital without a firm plan for contraception in place. Insertion of an intrauterine contraceptive device or implant can be undertaken before discharge, or even during delivery, if appropriate. Women should be followed up postnatally to review pregnancy outcomes and discuss plans for any future pregnancies.

7. Recognise that Multidisciplinary Collaboration and Individual Ownership are Key to Optimising Outcomes

This new guideline from the BSH fills a clear gap in current care for pregnant women with MHVs, and aims to optimise management for both mother and baby. Communication and collaboration are key to achieving the improvement in outcomes that are clearly needed, given the stark findings from the UKOSS study.4 The importance of MDT input is reiterated in ESC/EACTS guidelines, which recommend close collaboration with a Pregnancy Heart Team, both when choosing the initial prosthesis and when managing any subsequent pregnancy.7 Similarly, the WHO advocates the need for expert-led preconception counselling and care during pregnancy to mitigate maternal cardiovascular risk posed by mechanical valves.1 The MDT team charged with managing pregnant women with MHVs should include representation from obstetrics, cardiology, cardiac surgery, anaesthetics, neonatology, haematology, and specialist midwives.

Clinicians must also recognise that pregnancy is an emotive time for women and, although it is not possible to make the process entirely safe, patients can be empowered to take charge of their own care by being involved in proactive planning, anticoagulation choices, and adherence. Many cardiologists will have direct experience of adverse outcomes with MHVs during pregnancy, so individual ownership is important, despite the constraints of workload. By reading and referring to these BSH guidelines—and reaching out to MDT colleagues as and when required—cardiologists can put themselves in the best possible position to help their pregnant patients with MHVs. 


Pregnancy represents a high-risk time for women with MHVs, but proactive intervention, specialist care, and guidelines-led anticoagulation management are key tools for minimising risk and maximising outcomes. To improve the prospects of pregnant women with MHVs in the UK, rapid optimisation of management according to the latest guidance is crucial.

Caring for women with MHVs in pregnancy is a huge clinical challenge that can only be improved through raised awareness of the risks, education on the current guidance, and advocacy within the NHS on behalf of women of childbearing age with MHVs. 

Key Points
  • The BSH guideline provides recommendations to address the key risks in individuals with MHVs during pregnancy, at delivery, and postpartum
  • Proactive pre-pregnancy counselling is essential, and should encompass a full and frank discussion of risks to both mother and fetus
  • No single strategy for anticoagulation can be recommended; treatment should be planned and documented on an individual basis
  • Individuals with MHVs who are pregnant should be managed in a designated specialist centre under the care of an experienced MDT
  • The risk of MVT is predominantly antenatal; poor adherence to anticoagulants is a key theme
  • High starting doses of LMWH should be used when switching from VKAs to LMWH in early pregnancy; peak anti-Xa levels should be monitored, to a target of 1–1.4 IU/ml
  • Aspirin should be considered for women at increased risk of MHV thrombosis
  • Postpartum anticoagulation requires careful consideration and consultation 
  • A plan for contraception should be in place before discharge from hospital 
  • Planning and communication are vital to help women with MHVs successfully navigate their pregnancy journey—from preconception through to the postpartum period.
BSH=British Society of Haematology; MHV=mechanical heart valve; MDT=multidisciplinary team; MVT=mechanical valve thrombosis; LMWH=low molecular weight heparin; VKA=vitamin K antagonist