Warfarin use in pregnancy

(Date: April 2023. Version: 4)

This factsheet has been written for members of the public by the UK Teratology Information Service (UKTIS). UKTIS is a not-for-profit organisation funded by the UK Health Security Agency (UKHSA) on behalf of UK Health Departments. UKTIS has been providing scientific information to health care providers since 1983 on the effects that medicines, recreational drugs and chemicals may have on the developing baby during pregnancy.

Quick read

Warfarin is avoided in pregnancy where possible as it can cause birth defects. However, warfarin may sometimes be recommended in patients with mechanical heart valves.

What is it?

Warfarin acts as a ‘blood thinner’. Medicines that work in this way are called anticoagulants. Warfarin is used to treat blood clots, for example, in people with deep vein thrombosis. It can also be used long-term to prevent blood clots for a variety of different conditions.

What are the benefits of using warfarin in pregnancy?

Warfarin helps to treat blood clots and to prevent further blood clots from forming. Blood clots can be serious. If you have a metal heart valve, warfarin is used to prevent blood clots in the valve which can be life-threatening.

Are there any risks of using warfarin in pregnancy?

Yes. Warfarin use in early pregnancy can affect the baby’s bones, as well as development of the face. There are also possible effects on learning and behaviour in children who were exposed to warfarin in the womb.

Warfarin use in early pregnancy has been linked to an increased chance of miscarriage, and in later pregnancy can increase the risk of bleeding in the baby’s brain.

When taking warfarin, it is important to use reliable contraception to avoid an unplanned pregnancy. Women who are planning to conceive should speak to their specialist about whether they need to change from warfarin to a different anticoagulant.

Please do not stop taking warfarin unless advised to do so by a doctor, as this can be dangerous.

Are there any alternatives to using warfarin in pregnancy?

Yes. Switching to a different anticoagulant (such as low molecular weight heparin) is safer for the baby and may be an option, depending on your circumstances.

If you have a metal heart valve you should seek urgent advice from your specialist or GP within a day or two of a positive pregnancy test. Do not stop taking warfarin whilst waiting to be seen as this could be dangerous.

What if I prefer not to take medicines during pregnancy?

If a doctor advises that warfarin or other blood thinning medicines should be continued during pregnancy, this is because it is safer for both mother and baby than stopping treatment.

A medicine will only be prescribed during pregnancy if it is needed.

Your doctor or specialist will be happy to discuss the benefits and risks with you.

Will my baby need extra monitoring?

As part of routine antenatal care, all pregnant women in the UK are offered a detailed scan at around 20 weeks of pregnancy. If warfarin is continued, you may receive more detailed scanning of the baby. As pregnancy progresses, you will have extra scans to monitor your baby’s growth and wellbeing.

Are there any risks to my baby if the father has used warfarin?

We would not expect any increased risk to the baby if the father took warfarin before or around the time you became pregnant.

Who can I talk to if I have questions?

If you have any questions regarding the information in this leaflet please discuss them with your health care provider. They can access more detailed medical and scientific information from www.uktis.org

How can I help to improve drug safety information for pregnant women in the future?

Our online reporting system allows women with a current or previous pregnancy to create a digitally secure ‘my bumps record’. You will be asked to enter information about your health, whether or not you take any medicines, and your pregnancy outcome. You can update your details at any time during pregnancy or afterwards. This information will help us better understand how medicines affect the health of pregnant women and their babies. Please visit https://www.medicinesinpregnancy.org/Login/ to register.

Feedback request 

WE NEED YOUR HELP! Do you have 3 minutes to complete a short, quick and simple 12 question user feedback form about our bumps information leaflets? To have your say on how we can improve our website and the information we provide please visit here.

General information 

Up to 1 out of every 5 pregnancies ends in a miscarriage, and 1 in 40 babies are born with a birth defect. These are referred to as the background population risks.  They describe the chance of these events happening for any pregnancy before taking factors such as the mother’s health during pregnancy, her lifestyle, medicines she takes and the genetic make up of her and the baby’s father into account.

Medicines use in pregnancy

Most medicines used by the mother will cross the placenta and reach the baby. Sometimes this may have beneficial effects for the baby.  There are, however, some medicines that can harm a baby’s normal development.  How a medicine affects a baby may depend on the stage of pregnancy when the medicine is taken. If you are on regular medication you should discuss these effects with your doctor/health care team before becoming pregnant.

If a new medicine is suggested for you during pregnancy, please ensure the doctor or health care professional treating you is aware of your pregnancy.

When deciding whether or not to use a medicine in pregnancy you need to weigh up how the medicine might improve your and/or your unborn baby’s health against any possible problems that the drug may cause. Our bumps leaflets are written to provide you with a summary of what is known about use of a specific medicine in pregnancy so that you can decide together with your health care provider what is best for you and your baby.   

Every pregnancy is unique. The decision to start, stop, continue or change a prescribed medicine before or during pregnancy should be made in consultation with your health care provider. It is very helpful if you can record all your medication taken in pregnancy in your hand held maternity records.



Disclaimer: This information is not intended to replace the individual care and advice of your health care provider. New information is continually becoming available. Whilst every effort will be made to ensure that this information is accurate and up to date at the time of publication, we cannot cover every eventuality and the information providers cannot be held responsible for any adverse outcomes following decisions made on the basis of this information. We strongly advise that printouts should NOT be kept for any length of time, or for “future reference” as they can rapidly become out of date.

My bump’s record

Create your own ‘My bump’s record’.

Provide information about your pregnancy to help women in the future.

Login to my bumps

Join my bumps