(Date: December 2013. Version: 2.1)

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?

Trimethoprim (Monotrim®, Trimopan®) is an antibiotic used to treat bacterial infections of the bladder (often referred to as ‘UTIs’) and lungs (such as pneumonia).

Is it safe to take trimethoprim in pregnancy?

There is no ‘yes’ or ‘no’ answer to this question. When deciding whether or not to take trimethoprim during pregnancy it is important to weigh up how necessary trimethoprim 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.

Untreated bacterial infections during pregnancy can harm both mother and baby. Pregnant women are at increased risk of kidney damage from UTIs and must therefore be treated with an antibiotic that will clear an infection properly. UTIs during pregnancy have also been linked to premature labour (before 37 weeks). Many different types of antibiotics are available and you will usually be prescribed the antibiotic that is considered the best to treat your infection quickly and completely. If trimethoprim is considered to be the antibiotic that is most likely to fully treat your infection you may be prescribed it in pregnancy.

This leaflet summarises the scientific studies relating to the effects of trimethoprim on a baby in the womb. Your doctor is the best person to help you decide what is right for you and your baby.

What if I have already taken trimethoprim during pregnancy?

If you are taking trimethoprim or any other medicine it is 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, if so, to make sure that you are taking the lowest dose that works and only for as long as you need to. If you need to take trimethoprim during the first trimester it is recommended that you also take high dose folic acid (see below).

Can taking trimethoprim 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. Taking trimethoprim after 12 weeks of pregnancy will not cause birth defects in the baby.

Trimethoprim treats infections by preventing bacteria from producing folic acid, which is essential for their survival. However, this means that trimethoprim could also theoretically reduce a woman’s folic acid levels. In pregnancy, folic acid is needed for the normal formation of a baby’s spine and skull, and possibly lip and palate (the ‘roof’ of the mouth), and heart.

Some studies have found that women who took trimethoprim during early pregnancy were more likely to have a baby with a spine or skull defect (neural tube defect), a cleft lip and/or palate, or a heart malformation. However, other studies have not found links between taking trimethoprim during early pregnancy and having a baby with a birth defect.

Birth defect rates in babies of pregnant women taking trimethoprim have not been extensively studied, and some of the research dates back to a time before it was recommended that pregnant women take folic acid. More research is therefore needed before any firm conclusions can be drawn about whether taking trimethoprim in pregnancy is linked to birth defects in the baby.

In the meantime, it is recommended that women who need to take trimethoprim during the first trimester also take high dose folic acid (5 milligrams per day) until week 12 of pregnancy, which will need to be prescribed by a doctor. Taking high dose folic acid alongside trimethoprim may be particularly important for women who have previously had a child with a neural tube defect, and women who are already at risk of having low folic acid levels (e.g. women with diabetes, obesity, or those taking certain other medicines), although it is likely that women with these risks will be taking high dose folic acid already.

Can taking trimethoprim in pregnancy cause miscarriage?

It can be very difficult to accurately study possible links between medicines that are used to treat infections and miscarriage.

A study of women who took trimethoprim during the first three months of pregnancy showed that they were about twice as likely to have a miscarriage compared to pregnant women who were not taking trimethoprim. The study also looked at women treated with another antibiotic used to treat UTIs and showed no increased risk of miscarriage in this group. More research is therefore needed before it is possible to say whether trimethoprim increases the risk of miscarriage, or whether the increased risk seen in this study is due to other factors such as the infections or underlying illnesses in the mothers taking trimethoprim.

Can taking trimethoprim in pregnancy cause stillbirth?

One large study investigated rates of stillbirth in women who had used trimethoprim or other medicines that reduce the amount of folic acid, either during or in the three months before pregnancy. A possible increased risk of stillbirth was seen  in these women compared to women in the general population who were not taking these medicines.

Because this risk has only been investigated in a single study which could not rule out the possibility that the mother’s illness (and not the medicine) increased the risk of stillbirth, there is currently no good scientific evidence that use of trimethoprim in pregnancy increases the chance of stillbirth.

It is however, well known that untreated bacterial infections can result in severe illness and even death. If trimethoprim is the antibiotic that is needed to treat an infection in pregnancy, taking the full course as prescribed by your doctor is likely to be the safest option for both you and your baby.

Can taking trimethoprim in pregnancy cause preterm birth?

There is currently no convincing scientific evidence that taking trimethoprim during pregnancy is linked to having a preterm birth (before 37 weeks of pregnancy). Two studies which examined this found opposite results. However, urinary tract infections (UTIs) during pregnancy are strongly linked to premature birth and it is therefore possible that the increased risk seen in the one study was due to underlying infection in the mothers, rather than to trimethoprim.

Can taking trimethoprim in pregnancy cause my baby to be small at birth (low birth weight)?

It is currently unclear whether women who take trimethoprim during pregnancy have an increased chance of having a baby with a low birth weight (<2500g). While two studies have shown that babies of women who took trimethoprim during pregnancy were more likely to have a baby that was smaller than expected, one further study did not agree with this. The number of pregnant women taking trimethoprim who have been studied so far is small which can lead to inaccurate findings. More research is therefore required.

Can taking trimethoprim in pregnancy cause learning or behavioural problems in the child?

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 is no known link between taking trimethoprim in pregnancy and learning or behavioural problems (such as ADHD or autism spectrum disorder) in the child later on in life. There are, however, no scientific studies that have specifically investigated a link with these problems.

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. Taking trimethoprim during pregnancy is not expected to cause any problems that would require extra monitoring of your baby. However, if you are at risk of having low folate levels due to e.g. diabetes, obesity, or being on certain other medicines, and have taken trimethoprim during pregnancy, your doctor may wish to arrange a more detailed scan of your baby’s spine.  

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

We would not expect any increased risk to your baby if its father took trimethoprim 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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