Trimethoprim

Print
Date: June 2026, Version 5.0

Quick read

Trimethoprim might be used in pregnancy if another antibiotic has not worked to treat your urinary tract infection (UTI).

What is it? 

Trimethoprim is an antibiotic mainly used to treat urinary tract infections (UTIs). It may be prescribed on its own or combined with another antibiotic called sulfamethoxazole. This combination is known as co-trimoxazole.

Benefits

What are the benefits of taking trimethoprim in pregnancy?

It is important to treat a UTI effectively during pregnancy, as infection can lead to complications for both you and your baby. Correct treatment of a UTI reduces these risks and supports a healthy pregnancy.

Risks

What are the risks of taking trimethoprim during pregnancy?

Trimethoprim is usually avoided in early pregnancy because it can lower folate levels in your body. Folate is important for your baby’s development. Low levels have been linked to a small increased risk of certain birth defects, including spina bifida.

If trimethoprim is needed in early pregnancy, it is usually prescribed with a high-dose folic acid supplement to help maintain folate levels.

Alternatives

Are there any alternatives to taking trimethoprim in pregnancy? 

Yes. Other antibiotics are usually used first to treat UTIs during pregnancy. However, trimethoprim may be recommended if:

•    Other antibiotics have not worked, or
•    Test results show that trimethoprim is the most suitable option

Your healthcare professional will choose the safest and most effective treatment for you.

No treatment

What if I prefer not to take medicines in pregnancy? 

It is important to correctly treat a UTI. An untreated UTI can lead to serious illness, including kidney infection or, in rare cases, sepsis. It can also increase the chance of pregnancy complications such as preterm birth.

Taking antibiotics as prescribed helps reduce these risks. If you are unsure, speak to your doctor who will be happy to discuss your concerns and help you make an informed decision.

Will my baby need extra monitoring during pregnancy?

Most women who take trimethoprim during pregnancy will not need any additional monitoring.

In the UK, you will be offered a detailed scan at around 20 weeks of pregnancy. This scan checks your baby’s growth and development, including the brain and spine.

If trimethoprim was taken in early pregnancy without high-dose folic acid, you may be offered additional or more detailed scans to look more closely at your baby’s spine.

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

There is no evidence that trimethoprim taken by the father before or around the time of conception affects the baby.

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 (MyBump Portal) allows women who are currently pregnant to create a secure record of their pregnancy, collected through a series of questionnaires. You will be asked to enter information about your health, whether or not you take any medicines, your pregnancy outcome and your child's development. 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 the MyBump Portal to register.

General information
Sadly, miscarriage and birth defects can occur in any pregnancy.

Miscarriage occurs in about 1 in every 5 pregnancies, and 1 in every 40 babies are born with a birth defect. This is called the ‘background risk’ and happens whether medication is taken or not.

Most medicines cross the placenta and reach the baby. For many medications this is not a problem. However, some medicines can affect a baby’s growth and development.

If you take regular medication and are planning to conceive, you should discuss whether your medicine is safe to continue with your doctor/health care team before becoming pregnant. If you have an unplanned pregnancy while taking a medicine, you should tell your doctor as soon as possible.

If a new medicine is suggested for you during pregnancy, please make sure that the person prescribing it knows that you are pregnant. If you have any concerns about a medicine, you can check with your doctor, midwife or pharmacist.

Our Bumps information leaflets provide information about the effects of medicines in pregnancy so that you can decide, together with your healthcare provider, what is best for you and your baby.