(Date: July 2015. Version: 2)

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 Public Health England 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?

Natalizumab (Tysabri®) is a type of medicine called a monoclonal antibody and may also be referred to as a biologic. It is used to treat certain forms of multiple sclerosis and works by blocking molecules in the immune system that cause inflammation.

Is it safe to take natalizumab in pregnancy?

There is no yes or no answer to this question. Natalizumab is quite a new medicine and so far no studies have investigated whether it is safe to use during pregnancy. When deciding whether to take natalizumab during pregnancy it is important to weigh up how necessary natalizumab is to your health against any possible risks to you or your baby, some of which may depend on how many weeks pregnant you are. Your doctor is the best person to help you decide what is right for you and your baby.

This leaflet summarises the scientific studies relating to the effects of natalizumab on a baby in the womb. 

What if I have already taken natalizumab during pregnancy?

If you have taken any medicines it is always a good idea to let your doctor know that you are pregnant so that you can decide together whether you still need the medicines that you are on and to make sure that you are taking the lowest dose that works.

Can taking natalizumab in pregnancy cause my baby to be born with birth defects?

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.

There are currently no large scientific studies that have investigated whether natalizumab use during pregnancy increases the risk of birth defects in the baby. There is a small study of 29 babies and case reports of a further 2 who were born to pregnant women who took natalizumab.  Thirty of these babies did not have birth defects (with the remaining baby having an extra finger). Although this is reassuring, large scientific studies need to be carried out before we can say whether natalizumab is safe to use in pregnancy.

Can taking natalizumab in pregnancy cause miscarriage, stillbirth, preterm birth, or my baby to be small at birth (low birth weight)?

To date, no studies have specifically assessed rates of miscarriage or stillbirth in pregnant women taking natalizumab. A single small study of 29 pregnant women taking natalizumab provided no strong evidence of an increased risk of preterm birth, or altered birth weight in their babies, although much larger studies need to be carried out to confirm these findings. More research into the possible effects of taking natalizumab in pregnancy is therefore required.

Can taking natalizumab in pregnancy cause other health problems in the child?

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.

No link with learning and behavioural problems e.g. autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD) is known about in children who were exposed to natalizumab while in the womb, however no studies have been carried out to specifically investigate this.

Infection in the baby after birth 
Monoclonal antibodies such as natalizumab work by reducing or inactivating molecules in the body that are important to fight off infections. In non-pregnant people being treated for multiple sclerosis, natalizumab is known to increase the risk of a serious viral infection that affects the brain called progressive multifocal leukoencephalopathy (PML). It is currently unclear whether treatment during pregnancy with natalizumab might increase the risk of PML in the baby after birth.

Will my baby need extra monitoring during pregnancy?

Most women will be offered a scan at around 20 weeks of pregnancy to look for birth defects as part of their routine antenatal care. Because natalizumab is a new medicine and the effects of taking it in pregnancy are unknown, your doctor may suggest extra monitoring of your baby, including more detailed scans for birth defects and monitoring of your baby’s growth in the womb. Women with multiple sclerosis may also be more closely monitored during pregnancy to ensure that they remain well throughout and that their baby is growing and developing as expected. 

Natalizumab works by blocking molecules in the body that are important to fight off infections. In non-pregnant people being treated for multiple sclerosis, natalizumab is known to increase the risk of a viral infection that affects the brain called progressive multifocal leukoencephalopathy (PML). Because pregnancy itself also reduces the body’s capacity to fight infection, pregnant women taking natalizumab may be closely monitored for symptoms of PML.

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

Most experts agree that the majority of medicines used by the father are unlikely to harm the baby through affects on the sperm. However, more research on the effects of natalizumab specifically and medicine use in men around the time of conception generally is needed.

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  

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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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