Date: March 2024, Version 4.0

Quick take

Trimethoprim might be used in pregnancy if another antibiotic has not worked to treat a UTI.

What is it? 

Trimethoprim is an antibiotic most commonly used to treat and prevent urinary tract infections (UTIs). Trimethoprim is sometimes prescribed on its own or in combination with another antibiotic called sulfamethoxazole. This combination of drugs is called co-trimoxazole.


What are the benefits of taking trimethoprim in pregnancy?

Trimethoprim treats some UTIs. It is important to effectively treat a UTI in pregnancy as it can cause complications for both the woman and her baby.


What are the risks of taking trimethoprim during pregnancy?

Trimethoprim is avoided in early pregnancy where possible as it can affect levels of folic acid in the body. Folic acid is important for the baby to develop normally, and lower levels are linked to an increased risk of some birth defects including spina bifida. Women who need to take trimethoprim in early pregnancy will be offered a high dose folic acid supplement.


Are there any alternatives to taking trimethoprim in pregnancy? 

Possibly. Other antibiotics will usually be tried first for a pregnant woman with a UTI. However, trimethoprim may be required if other antibiotics have not worked, or if tests on a urine sample show it is the only antibiotic to treat a woman’s UTI.

No treatment

What if I prefer not to take medicines in pregnancy?

Untreated UTI in pregnancy can lead to serious illness in the woman (including sepsis) and pregnancy complications such as preterm delivery. It is therefore very important to take any drugs recommended by a doctor to treat UTI.

Will my baby need extra monitoring during pregnancy?

Women in the UK should be offered a very detailed scan at around 20 weeks of pregnancy to look for birth defects as part of their routine antenatal care. Women who took trimethoprim in early pregnancy without high dose folic acid may be offered additional and/or more detailed scans to examine their 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 the 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

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.