Use of carbimazole or methimazole in pregnancy

(Date: February 2019. 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.


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What is it?

Carbimazole (Neo-Mercazole®) and the related medicine methimazole are antithyroid medicines used to treat women with an overactive thyroid gland (hyperthyroidism). This is sometimes caused by Graves’ disease, an autoimmune condition.

Is it safe to take carbimazole or methimazole in pregnancy?

Carbimazole and methimazole use in early pregnancy is thought to slightly increase the chance of certain birth defects occurring in the baby. If you are trying to conceive you should speak to your doctor who may suggest that you change to a different medicine. When deciding whether or not to take carbimazole during pregnancy your doctor will help you to weigh up how necessary carbimazole is to your health against the possible risks to you or your baby, some of which will depend on how many weeks pregnant you are. It is important that normal thyroid function is maintained during pregnancy as the effects of an overactive thyroid can be harmful to both mother and unborn child. For some women carbimazole or methimazole may be considered the best medicines to maintain normal thyroid function during pregnancy.

What if I have already taken carbimazole or methimazole during pregnancy?

If you have taken or are taking any medicines it is always a good idea to let your doctor know that you are pregnant so that you can decide together whether this is still the best medicine to treat your condition, and if so, to make sure that you are taking the correct dose. Do not make any change to your medication without first talking to your doctor.

Can taking carbimazole or methimazole in pregnancy cause birth defects in the 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.

Carbimazole and methimazole use in early pregnancy appears to occasionally cause certain birth defects in the baby. These include choanal atresia (where the opening at the back of one or both nostrils is blocked by extra bone or tissue), problems with the development of the heart or digestive system and abdominal wall, and a scalp defect called aplasia cutis (where there are gaps in the skin on the scalp). Some affected babies have only one of these defects while others have more. Studies suggest that fewer than two in every hundred babies who are exposed to carbimazole in the womb are born with these defects. In other words, at least 98 out of every 100 babies who are exposed to carbimazole in the womb do not have these birth defects.

Can taking carbimazole or methimazole in pregnancy cause miscarriage or stillbirth?

No links between carbimazole or methimazole use in pregnancy and miscarriage were shown in either of two small studies but more research is required to confirm this finding.

No scientific studies have investigated the chance of stillbirth following use of carbimazole or methimazole in pregnancy.

Can taking carbimazole or methimazole in pregnancy cause preterm birth or my baby to be small at birth (low birth weight)?

There is no strong evidence from a number of studies that use of carbimazole or methimazole in pregnancy is linked to preterm birth.

It is currently unclear whether taking carbimazole or methimazole in pregnancy is linked to low birth weight in the baby because the results of the studies that have investigated this do not all agree. However, an overactive thyroid in pregnancy has been linked to low birth weight in the baby and this may explain why some studies have found that babies of pregnant women taking carbimazole or methimazole tended to be smaller. More research is required before we can say whether taking carbimazole in pregnancy is specifically linked to low birth weight in the baby.

Can taking carbimazole or methimazole in pregnancy cause other health problems in the baby/child?

Reduced thyroid function at birth (neonatal hypothyroidism)
When used after week 10 of pregnancy, carbimazole, methimazole and other antithyroid medicines can affect the unborn baby’s thyroid gland. Newborn babies of women with Graves’ disease may also experience other thyroid problems. If you have taken carbimazole or methimazole after week 10 of pregnancy, and/or if you have Graves’ disease, your baby’s thyroid function should be monitored after delivery.

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.

There are isolated reports of some children with structural birth defects thought to have been caused by exposure to carbimazole or methimazole in the womb also having learning or behavioural problems. However, none of four small scientific studies have shown that children exposed to carbimazole or methimazole in the womb are more likely to have problems with learning and behaviour than children not exposed to these medicines. Most of the children who were exposed to carbimazole or methimazole in the womb in these four studies did not have structural birth defects.

More research is required before we can say whether carbimazole or methimazole use during pregnancy increases the chance of learning and behavioural problems in the child, and whether learning and behavioural problems can occur in carbimazole/methimazole-exposed children who do not have structural birth defects.

Will my baby need extra monitoring?

Women who have been treated with carbimazole or methimazole during the first trimester may be offered the option of having earlier and more detailed anomaly scans to look for birth defects in addition to the main scan for birth defects which is generally offered at around 20 weeks. It is important to understand that scans are unlikely to detect some of the birth defects that have been linked to carbimazole and cannot rule out that a baby will have problems with learning.

Treatment with carbimazole, methimazole or other antithyroid medicines after week 10 of pregnancy can affect the development of the unborn baby’s thyroid gland. Therefore, women treated with carbimazole or methimazole at this stage of pregnancy are likely to be offered additional ultrasound scans to assess the growth and well-being of the baby.

If you have taken carbimazole, methimazole or other antithyroid medicines) after week 10 of pregnancy your baby will require extra monitoring after birth because of the risk of altered thyroid function.

Are there any risks to my baby if the father has taken carbimazole or methimazole?

We would not expect any increased risk to your baby if the father took carbimazole or methimazole 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

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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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