Fosfomycin is generally considered safe during pregnancy. It carried an FDA pregnancy category B rating before that system was retired, meaning animal studies showed no harm and limited human data supported its safety. Multiple studies have now examined its use in pregnant women, and none have found an increased risk of birth defects, miscarriage, or preterm birth.
What the Safety Data Shows
The strongest evidence comes from studies looking at first-trimester exposure, which is the window when birth defects are most likely to occur. In a German observational study of 146 infants exposed to fosfomycin during early pregnancy, only 1 was born with a major birth defect (0.7%), compared to 3.8% in unexposed pregnancies. Miscarriage rates were also lower in the fosfomycin group (6.2%) than in the comparison group (23.1%), though that likely reflects other differences between the groups rather than a protective effect of the drug.
A large 2023 French study reinforced these findings. Among women who took fosfomycin in the first trimester, the rate of major congenital anomalies was 2.0%, virtually identical to the 2.1% rate in unexposed pregnancies. A 2025 review in the journal Pharmacotherapy classified fosfomycin as “safe and effective in the first trimester” and grouped it alongside other antibiotics considered safe in pregnancy, such as penicillins and cephalosporins.
Less data exists on use during the second and third trimesters, but experts generally consider it safe throughout pregnancy based on the available evidence and its mechanism of action.
How It Crosses the Placenta
Fosfomycin does cross the placental barrier and reaches the fetus. Research in women during active labor found a clear relationship between the mother’s blood levels of the drug and fetal blood levels, with measurable amounts in fetal circulation about 90 minutes after administration. The amount that reaches the fetus also correlates with placental weight. Despite this transfer, the studies above show no signal of fetal harm, which is why its safety rating has remained favorable.
Why It’s Prescribed for UTIs in Pregnancy
Urinary tract infections are common during pregnancy, affecting up to 8% of pregnant women. Untreated UTIs, including infections that cause no symptoms (called asymptomatic bacteriuria), can lead to kidney infections and pregnancy complications. That makes treatment important, but the list of antibiotics considered safe in pregnancy is shorter than the general list.
Fosfomycin has a practical advantage: it works as a single dose. You dissolve one 3-gram sachet in about half a cup of cool water and drink it once. Most other antibiotics used for UTIs in pregnancy require multiple days of treatment. The American College of Obstetricians and Gynecologists (ACOG) notes that fosfomycin has shown good efficacy as a single-dose treatment for both symptomatic bladder infections and asymptomatic bacteriuria, making it a reasonable choice for one-day treatment.
The bacteria that most commonly cause UTIs in pregnant women, primarily E. coli (responsible for about 66% of cases), are highly susceptible to fosfomycin. One study found that 89% of bacteria isolated from pregnant women with urinary infections were sensitive to it. This is particularly relevant as antibiotic resistance continues to rise among common UTI-causing bacteria.
How It Compares to Other Options
The most common alternatives for treating UTIs in pregnancy are nitrofurantoin and certain penicillin-type antibiotics. Each has trade-offs.
- Nitrofurantoin requires a 5 to 7 day course rather than a single dose. A meta-analysis comparing the two found similar effectiveness, though fosfomycin had a slightly higher rate of minor side effects like nausea and diarrhea. Nitrofurantoin is typically avoided near the end of pregnancy (after 36 weeks) due to a small risk of a blood disorder in newborns.
- Penicillin-type antibiotics (like amoxicillin) have a long safety record in pregnancy, but resistance rates among UTI-causing bacteria are higher, which can make them less effective.
Fosfomycin’s single-dose convenience and strong effectiveness against resistant bacteria make it a particularly useful option when adherence to a multi-day course is a concern or when the bacteria involved are resistant to other first-line drugs.
What to Know About Taking It
If you’re prescribed fosfomycin, it comes as a powder in a single-use sachet. Pour the entire contents into about half a cup of water (not hot), stir to dissolve, and drink it right away. You can take it with or without food. Never take the powder dry.
Common side effects are mild and short-lived: diarrhea, nausea, and headache are the most frequently reported. Because it’s a single dose, side effects typically resolve quickly. Your provider may want a follow-up urine test to confirm the infection has cleared, which is standard practice for UTIs in pregnancy regardless of which antibiotic is used.