This sheet talks about using sulfamethoxazole/trimethoprim in a pregnancy and while breastfeeding. This information should not take the place of medical care and advice from your healthcare provider.

What are sulfamethoxazole and trimethoprim?

Sulfamethoxazole and trimethoprim are medications that are used to treat bacterial infections. These two medications are usually given together and called Bactrim® or Septra®. Sulfamethoxazole is a member of the sulfonamide class of medications that are sometimes also called “sulfa drugs”

The combination of these antibiotics is used to treat a variety of infections, including urinary tract infections (UTIs). UTIs are common among women during pregnancy.

I am taking sulfamethoxazole/trimethoprim. Can it make it harder for me to become pregnant?

There are no studies looking at whether sulfamethoxazole/trimethoprim can make it harder for women to become pregnant.

Does taking sulfamethoxazole/trimethoprim increase the chance for miscarriage?

Miscarriage can occur in any pregnancy. One study with 25 women found an increased chance for miscarriage but more studies need to be done.

Does taking sulfamethoxazole/trimethoprim in the first trimester increase the chance of birth defects?

In every pregnancy, a woman starts out with a 3-5% chance of having a baby with a birth defect. This is called her background risk. If there is an increased chance for birth defects with sulfamethoxazole/trimethoprim use during pregnancy, it appears to be small.

There are not many well controlled studies on sulfamethoxazole use alone in human pregnancy. Some studies have suggested the use of sulfonamides (in general) during the first trimester may be associated with an increased chance for birth defects while other studies have not.

There are more studies on trimethoprim use in pregnancy. Some studies have not found an increased chance for birth defects. A few studies looking at trimethoprim and sulfonamide during the first trimester have found an increased chance for birth defects. The birth defects that were seen included heart defects, neural tube defects (opening in the spine), cleft lip or palate (lip or roof of mouth do not form correctly), and urinary tract defects.

Trimethoprim might decrease the level of folic acid in your body. Folic acid is a B vitamin that may help lower the chance of certain birth defects, like spina bifida, oral clefts and heart defects. It is recommended that pregnant women consume folic acid each day from foods or vitamin supplements.

Could taking sulfamethoxazole/trimethoprim in the second or third trimester cause other pregnancy complications?

One study has suggested that women who take medications that can lower levels of folic acid might have a higher chance for pregnancy complications such as preeclampsia, placenta abruption (when the placenta breaks away from the wall of the uterus) and poor growth of the baby. Exposure to sulfamethoxazole/trimethoprim has been associated with preterm delivery and low birth weight. However, this medication is frequently used to treat UTIs, and pregnant women with UTIs are at a greater chance for some of the same complications. Therefore, it is difficult to determine whether it is the medication, the decrease in folic acid, the underlying infection, or other factors which are increasing the chance for these problems.

I am in my 3rd trimester, is there anything I should know about if I want to take sulfamethoxazole/trimethoprim?

Some authors have recommended not taking sulfonamides such as sulfamethoxazole after 32 weeks gestation. There is a theoretical concern that sulfonamide use near the end of pregnancy can increase the chance for severe jaundice (a problem with liver function) and related complications in the baby. In this situation, your health care provider can help to suggest a medication that is right for you.

I was prescribed sulfamethoxazole/trimethoprim for a UTI. Should I take this medication?

It is important to treat most infections during pregnancy. Untreated UTIs could lead to kidney infection for the mother, preterm delivery (birth before 37 weeks gestation) and preeclampsia (dangerously high blood pressure).*

Does taking sulfamethoxazole/trimethoprim in pregnancy cause long-term problems in behavior or learning for the baby?

At this time, there are no studies on the possible long-term effects of sulfamethoxazole/trimethoprim on the developing baby.

Can I breastfeed while taking sulfamethoxazole/trimethoprim?

Sulfamethoxazole and trimethoprim pass into breast milk in small amounts. There is some concern about taking sulfamethoxazole and trimethoprim while breastfeeding if the baby is premature (born before 37 weeks of pregnancy), has severe jaundice (caused by a buildup of bilirubin, a yellow pigment of red blood cells), or a condition known as glucose-6- phosphate dehydrogenase (G6PD) deficiency. However, it is not always necessary to stop breastfeeding while taking these medications. Talk to your health care provider about all your breastfeeding questions.

If a man takes sulfamethoxazole/trimethoprim, does it increase the chance of infertility or birth defects?

Men who took a combination of a sulfamethoxazole and trimethoprim combination for one month were found to have decreased production of sperm. A lowered sperm count may affect a man’s ability to father a child.

There are no studies looking at chance for birth defects when the father takes sulfamethoxazole/trimethoprim. In general, exposures that fathers have are unlikely to increase chances to a pregnancy. For more information, please see the MotherToBaby fact sheet Paternal Exposures at*

* Section Updated May 2020

References Available By Request.