Warfarin use in pregnancy

(Date: December 2017. Version: 3)

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.

What is it?

Warfarin is an anticoagulant medicine that is used to treat and prevent the formation of blood clots. Warfarin is used to treat deep vein thromboses (DVTs) (a type of blood clot that most commonly occur in the leg), to prevent blood clots in people with heart and circulatory conditions, and following surgery.

Is it safe to take warfarin in pregnancy?

Use of warfarin in the first trimester of pregnancy can cause fetal warfarin syndrome. This is a pattern of structural problems in the baby including underdevelopment of the nasal bone, resulting in a flat nose and facial profile, abnormalities of the spinal bones, heart defects, brain defects, and altered development of the leg and arm bones. It is thought that these problems can occur if warfarin is taken between week 6 and 12 of pregnancy.

Exposure to warfarin in later pregnancy can lead to internal bleeding in the baby. This can be a particular problem if bleeding occurs in the baby’s brain.

Most women who take warfarin and who are planning a pregnancy, or who have discovered they are pregnant, will be switched to a different medicine (usually heparin) to prevent blood clots. However, for some women (particularly those with mechanical heart valves), continued treatment with warfarin may be considered the safer option overall, despite increased risks to the baby.

Doctors will be able to help you weigh up your personal risks and benefits of staying on warfarin or switching to a different medicine, taking into account factors such as the reason for warfarin treatment and the severity of the condition. You should expect to be referred to specialist hospital doctors to have these discussions.

What if I have already taken warfarin during pregnancy?

If you are taking warfarin and are pregnant you should arrange to see your GP or hospital doctor as soon as possible so that a review of your medicines can be carried out.

It is likely that your GP will refer you to hospital in order to discuss the pros and cons of warfarin treatment with specialist doctors.

It is important that you do not suddenly stop taking warfarin without medical supervision as this could be dangerous to you and to your baby if you are already pregnant. Do not make any changes to your medication without first talking to your doctor.

Can taking warfarin in pregnancy cause birth defects in my baby?

A baby’s body and most internal organs are formed during the first 12 weeks of pregnancy. It is mainly during this time that some medicines are known to cause birth defects.

It is estimated that up to 12 in every 100 babies exposed to warfarin between weeks 6 and 12 of pregnancy will show signs of fetal warfarin syndrome, and around one to two in every 100 babies exposed during the later stages of pregnancy will have a brain/central nervous system problem. The chance of having a baby with birth defects is greatest in women who take higher doses of warfarin (more than 5mg/day).

Can taking warfarin in pregnancy cause miscarriage?

Use of warfarin in early pregnancy is thought to increase the chance of miscarriage. However, the actual chance of having a miscarriage following warfarin use is unclear. Ideally, warfarin should be stopped prior to planning a pregnancy.

Can taking warfarin in pregnancy cause stillbirth, low infant birth weight, or preterm birth?

Use of warfarin in pregnancy can cause bleeding behind the placenta. This type of bleeding can cause reduced fetal growth, placental abruption, and stillbirth (if severe). The chance of preterm delivery is also higher. The actual chance of these complications occurring is unclear as the studies that have investigated these problems do not all agree.

Can taking warfarin in pregnancy cause other health problems in the baby/child?

Bleeding problems in the newborn baby
There are case reports of babies exposed to warfarin in pregnancy experiencing bleeding problems after birth. The chance of this happening can be minimised by stopping warfarin prior to delivery (see below).

Learning and behavioural problems
A baby’s brain continues to develop right up until the end of pregnancy. It is therefore possible that taking certain medicines at any stage of pregnancy could have a lasting effect on a child’s learning or behaviour. Babies exposed in the womb to warfarin are at increased risk of learning and behavioural problems due to the risk of bleeding on the brain posed by warfarin exposure. However, the actual chance of these complications occurring is unclear as learning and development has been studied in only a small number of children exposed in the womb to warfarin.

Will my baby need extra monitoring?

Most women will be offered a scan at around 20 weeks of pregnancy to look for problems with the baby as part of their routine antenatal care. Women who continue taking warfarin in pregnancy will be offered extra scans to pick up certain problems and to assess their baby’s growth in the womb.

If vaginal delivery is planned then warfarin should be stopped four weeks prior to delivery so that the warfarin doesn’t affect the baby during birth. If you are still taking warfarin around the time of delivery it is likely that you will need a caesarean section. You should discuss delivery options with your obstetrician as you may be able to switch medication prior to delivery. Closer monitoring of the baby after delivery will be required.

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

We would not expect any increased risk to your 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 healthcare provider. They can access more detailed medical and scientific information from www.uktis.org

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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.

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