Is D-Mannose Safe During Pregnancy?

No controlled human trial has tested whether D-mannose is safe for pregnant women, so there is no evidence-based green light from any regulatory or medical authority. At the same time, animal studies from the 1980s and 2010s found that high-dose mannose can cause birth defects and reduced survival in genetically susceptible mice and rats, which is the main reason researchers and clinicians remain cautious. The gap between those alarming animal findings and the near-total absence of human pregnancy data makes this one of those questions where the honest answer is “we don’t know enough,” and that uncertainty itself matters.

Why Pregnant Women Look Into D-Mannose in the First Place

Urinary tract infections are the single most common bacterial infection during pregnancy. Hormonal shifts, a growing uterus pressing on the bladder, and changes in urine composition all raise the risk. If a bladder infection goes untreated or isn’t caught early, up to 30 percent of pregnant women with asymptomatic bacteriuria go on to develop acute pyelonephritis, a kidney infection that can trigger preeclampsia, preterm birth, growth restriction, and low birth weight.¹ Acute pyelonephritis during pregnancy has also been linked to increased maternal complications in its own right.²

Standard treatment is antibiotics, but many women who get recurrent UTIs are understandably wary of repeated antibiotic courses during pregnancy. Concerns about antibiotic resistance, disruption to the gut and vaginal microbiome, and potential effects on the developing baby drive interest in alternatives. D-mannose, a sugar sold as a dietary supplement, has gained popularity as a non-antibiotic option for UTI prevention. The question is whether using it while pregnant trades one set of risks for another.

How D-Mannose Works

D-mannose is a simple sugar, closely related to glucose. Your body absorbs it from the gut but doesn’t metabolize it in any meaningful way; most of it passes into the urine unchanged.³ Once in the urinary tract, mannose molecules stick to the hair-like structures (called fimbriae) on the surface of E. coli bacteria. These fimbriae normally latch onto the bladder lining, which is how E. coli colonizes and causes infection. When mannose occupies those binding sites, the bacteria can’t attach to the bladder wall and get flushed out during urination.⁴

This mechanism is specific to E. coli strains that use mannose-sensitive fimbriae, which happen to be responsible for the majority of uncomplicated UTIs. Against other bacterial species, D-mannose would have little effect. That specificity is worth knowing, because UTIs caused by organisms other than E. coli sometimes become more common during pregnancy or after antibiotic use reshuffles the microbial landscape.

What Animal Studies Found

The animal data is where the real concern comes from, and it’s hard to wave away even though it doesn’t translate directly to human pregnancies.

A study in rats infused pregnant dams with enough D-mannose to raise plasma levels to roughly 150–200 mg/dl. The mothers showed no obvious toxicity: their behavior, eating, and blood chemistry stayed normal. But about 9 percent of the embryos developed abnormalities, most involving the brain or neural tube, with incomplete neural tube closure in nearly half of those affected. Some embryos also had abnormal heart development or eye defects, and the mannose-exposed group showed significant growth retardation compared to controls.⁵

A separate study in mice with reduced activity of a mannose-processing enzyme (phosphomannose isomerase) found even more dramatic effects. When pregnant dams were given 1–2 percent mannose in their drinking water, litter size dropped and survival to weaning fell by roughly two-thirds. About half of the surviving pups developed eye defects that began around mid-gestation. When mannose was given to pups after birth, eye defects still occurred, but only if exposure started before eye development was complete. The researchers traced the problem to a buildup of mannose-6-phosphate and related metabolites in the developing embryo and placenta.⁶

Two things are important about these findings. First, the doses used were far higher than what a person would get from a typical supplement (usually 1–2 grams per day). Second, the mouse study involved animals with a genetic deficiency in mannose processing, which amplified the toxic buildup. Healthy mice or rats without that mutation may handle mannose differently. But the findings establish that, under certain conditions, excess mannose during embryonic development can cause serious structural defects. No one has proven that this cannot happen in humans, and that missing proof is the crux of the safety concern.

D-Mannose Crosses the Placenta

If mannose stayed entirely in the mother’s bloodstream, the animal data would be less worrying. It doesn’t. Research on perfused human placentas has shown that D-mannose crosses from the maternal side to the fetal side via the same carrier-mediated transport system that moves glucose. Transfer rates of D-mannose were about 1.5 to 4 times higher than those of molecules that cross only by passive diffusion, and increasing the concentration of glucose competitively inhibited mannose transport, confirming they share the same pathway.⁷

A study using stable isotopes in uncomplicated term pregnancies confirmed this picture in living women. The ratio of labeled mannose in fetal blood compared to maternal blood was essentially 1.0, meaning the fetus receives mannose directly from the mother’s circulation in proportion to what’s available.⁸ So any mannose you take orally that ends up in your bloodstream will reach the fetus. The question then becomes whether the amounts from a typical supplement dose are large enough to matter.

Mannose Levels in Normal and Diabetic Pregnancies

Your body maintains circulating mannose at low levels even without supplementation. In late pregnancy, fasting plasma mannose in women with normal blood sugar regulation averaged about 9.8 micrograms per milliliter, compared to roughly 790 micrograms per milliliter for glucose. Mannose was, in other words, a tiny fraction of total circulating sugars, never exceeding 3 percent of the glucose concentration.⁹

In pregnant women with diabetes, though, fasting mannose levels were significantly higher, averaging about 16.9 micrograms per milliliter, even when glucose was only modestly elevated. Both the fetus and the mother appear to be exposed to measurable amounts of mannose during late gestation, and faulty glucose regulation amplifies that exposure.⁹ This is relevant because taking oral D-mannose supplements could, in theory, push circulating mannose levels well above the natural baseline. Whether those levels would approach the concentrations that caused harm in animal models is unknown.

The Very Limited Human Evidence

No randomized controlled trial has ever tested D-mannose for UTI prevention or treatment specifically in pregnant women. The human evidence that exists is razor-thin.

A 2025 case report described a woman with a rare genetic condition called MPI-CDG, which impairs mannose metabolism and requires D-mannose supplementation as a medical treatment. When she became pregnant, clinicians stopped her mannose at six weeks’ gestation because of the animal data on potential fetal toxicity. Severe digestive symptoms and dangerously low blood sugar returned, and the medical team decided to restart D-mannose at ten weeks’ gestation despite the lack of human teratogenicity data. Her symptoms resolved quickly. The dose was gradually increased throughout pregnancy, monitored by blood tests. She delivered at 38 weeks, with intrauterine growth retardation noted. The infant weighed about 2,390 grams at birth and had a low initial Apgar score but recovered rapidly.¹⁰

This case offers cautious reassurance but also illustrates the dilemma. The baby survived and recovered, and the authors concluded that low-dose D-mannose from 10 weeks could be a safe option for women with this specific condition. But one case report from a woman with a rare metabolic disorder cannot establish general safety for otherwise healthy pregnant women. The growth retardation and low Apgar score, while they could have many explanations including the underlying disease, aren’t entirely reassuring either.

A broader review of complementary medicines for UTIs in pregnant women and children acknowledged the problem directly: limited information is available on the safety of natural products in these populations, though based on what historical data does exist, these remedies appear to be safe and well tolerated.¹¹ That “appear to be safe” reflects an absence of reported harm rather than rigorous safety testing, and there’s an important difference between those two things.

How Effective Is D-Mannose for UTI Prevention Generally

Even outside of pregnancy, the evidence for D-mannose is promising but not yet definitive. A systematic review and meta-analysis comparing D-mannose to placebo and antibiotics in adult women found that D-mannose appeared protective against recurrent UTIs when compared to placebo, and possibly similar in effectiveness to preventive antibiotics, though the pooled comparison with antibiotics didn’t reach statistical significance and had high variability across studies.¹² A Cochrane review on D-mannose for UTI prevention and treatment reached a similar conclusion about the mechanism but noted the overall quality and quantity of evidence was limited.⁴

A more recent randomized trial comparing antibiotic prophylaxis, D-mannose, and increased water intake in premenopausal women found that antibiotics produced the fewest UTI recurrences (about 0.2 episodes per year), D-mannose came in second (about 0.32 episodes per year), and increased hydration alone was least effective (about 1.08 episodes per year). The mean time to first UTI was longest in the antibiotic group at 4.5 months, compared to 2.5 months for D-mannose and just one month for the hydration group.¹³ These results suggest D-mannose does something real, but antibiotics still outperform it for prevention.

None of these trials included pregnant participants, so extending the efficacy findings to pregnancy requires an additional leap of faith on top of the safety question.

Side Effects Outside of Pregnancy

In the general adult population, D-mannose is well tolerated. The Cochrane review noted that adverse events across the included studies were very few and poorly reported, with none classified as serious. The most commonly mentioned side effects were diarrhea and vaginal burning.⁴ Loose stools make sense given that mannose is a sugar that isn’t well absorbed at higher doses, so it draws water into the intestine the same way other sugar alcohols do.

These mild side effects are a separate question from the pregnancy-specific concern, which centers on whether the fetus can safely metabolize the mannose that crosses the placenta. A supplement that causes nothing worse than diarrhea in an adult could still, in theory, pose developmental risks to an embryo during vulnerable windows. That disconnect between adult tolerability and embryonic safety is common across many substances and is the reason pregnancy categories exist for medications in the first place.

What About Combination Products

Many D-mannose supplements on the market don’t contain mannose alone. They’re frequently combined with cranberry extract, probiotics (often Lactobacillus strains), or vitamin C. Research into these combinations suggests they may be effective for uncomplicated UTIs, with some studies reporting results comparable to antibiotics.¹⁴ But combining ingredients complicates the safety picture during pregnancy, because each additional component brings its own set of unknowns.

Cranberry extract, for example, has a somewhat better safety profile in pregnancy than D-mannose based on longer use history, but it can interact with blood thinners and isn’t without concerns at high doses. Probiotics are generally considered low-risk during pregnancy, but the specific strains and formulations matter. If you’re considering any supplement during pregnancy, reading the full ingredient list and discussing each component with your provider is more useful than assuming the product is as simple as the front label suggests.

Supplement Quality and Regulation

D-mannose is sold as a dietary supplement, not a pharmaceutical. In most countries, that means it isn’t subject to the same pre-market testing for purity, potency, and safety that prescription drugs undergo. The dose listed on the label may not match what’s inside the capsule or powder. Contaminants, fillers, and inconsistent batch quality are documented problems across the supplement industry. For anyone, but especially during pregnancy, this regulatory gap adds another layer of uncertainty on top of the already-limited safety data.

If you do take D-mannose during pregnancy after discussion with your healthcare provider, choosing a product from a manufacturer that submits to third-party testing (look for USP, NSF, or ConsumerLab seals) at least reduces the contamination risk, even though it doesn’t address the fundamental question of whether mannose itself is safe for the developing fetus.

Why Clinicians Usually Default to Antibiotics

Given all the unknowns, most obstetricians and midwives will recommend antibiotics rather than D-mannose for UTI treatment and prevention during pregnancy. Antibiotics like nitrofurantoin and cephalosporins have decades of pregnancy safety data behind them. The risks of untreated UTIs during pregnancy, including kidney infection, preterm labor, and low birth weight, are well documented and serious enough that clinicians generally don’t feel comfortable trading a known-safe treatment for an unproven one.¹

That said, antibiotic resistance is a growing concern, and recurrent courses of antibiotics can cause their own problems. For a woman with repeated UTIs who has already been through several antibiotic rounds, the calculus might look different, and some clinicians may be open to discussing D-mannose as an adjunct or as a prophylactic measure between infections. These conversations are highly individual and depend on how far along the pregnancy is, the severity and frequency of infections, and the specific bacterial strains involved.

The First Trimester Question

Timing within pregnancy matters when weighing any potential risk. The first trimester, particularly weeks 3 through 8, is when organogenesis occurs and the embryo is most vulnerable to developmental disruption. The animal studies that found neural tube defects and eye abnormalities specifically implicated exposure during the equivalent of early organ formation. The MPI-CDG case report, notably, stopped D-mannose at 6 weeks’ gestation and did not restart it until 10 weeks, after the most critical window for neural tube closure had passed.¹⁰

This doesn’t mean D-mannose is safe after the first trimester. It means the theoretical risk is probably highest during early development, and clinicians managing the one published case clearly thought so too. If you took D-mannose before discovering you were pregnant, the exposure would have been at a dose designed for UTI prevention (much lower than the animal study doses), and your body’s mannose-processing enzymes were presumably functioning normally (unlike the mutant mice). Those are reassuring differences, even though they don’t eliminate all uncertainty.

The Gap Between “Natural” and “Safe in Pregnancy”

D-mannose is sometimes marketed as a natural, gentle alternative to antibiotics, and the word “natural” can create a false sense of safety during pregnancy. Mannose is indeed a naturally occurring sugar present in fruits like cranberries and peaches, and your body produces small amounts of it. But the dose in a supplement is many times what you’d get from food, and “natural” has no bearing on whether a substance is safe for a developing embryo. Vitamin A is natural, and at high doses it’s a well-known teratogen. Arsenic is natural. The category just isn’t informative for safety decisions.

The responsible framing is that D-mannose is a biologically active substance with a plausible mechanism for UTI prevention, limited but encouraging efficacy data in non-pregnant adults, no human safety data during pregnancy, and concerning findings in animal models at high doses. Whether you consider that profile acceptable during your pregnancy is a conversation to have with your provider, armed with the actual evidence rather than supplement marketing.

1
PubMed. Urinary tract infection during pregnancy: current concepts on a common multifaceted problem
2
PubMed. Urinary tract infections in pregnancy
3
PubMed Central. Considerations on D-mannose Mechanism of Action and Consequent Classification of Marketed Healthcare Products
4
PubMed Central. D‐mannose for preventing and treating urinary tract infections
5
JCI Insight. Fuel-mediated teratogenesis. Use of D-mannose to modify organogenesis in the rat embryo in vivo
6
PubMed Central. Mannose supplements induce embryonic lethality and blindness in phosphomannose isomerase hypomorphic mice
7
PubMed. Evidence for a specific transport of D-hexoses across the human term placenta in vitro
8
The Journal of Clinical Endocrinology & Metabolism. Transplacental Supply of Mannose and Inositol in Uncomplicated Pregnancies Using Stable Isotopes
9
The Journal of Clinical Endocrinology & Metabolism. Relationships between Glucose and Mannose during Late Gestation in Normal Pregnancy and Pregnancy Complicated by Diabetes Mellitus
10
PubMed. Oral D-mannose therapy during pregnancy in a woman with MPI-CDG: A case report and management review
11
PubMed Central. Examination of Complementary Medicine for Treating Urinary Tract Infections Among Pregnant Women and Children
12
American Journal of Obstetrics & Gynecology. D-Mannose vs other agents for recurrent urinary tract infection prevention in adult women: A systematic review and meta-analysis
13
PubMed. Comparison of increased hydration, D-mannose, and antibiotic prophylaxis for recurrent urinary tract infection prevention in premenopausal women: a three-arm randomized-controlled study
14
Obstetrics, Gynecology and Reproduction. Regarding the prospects of using Lactobacillus-based probiotics, D-mannose and cranberry extracts in therapy of urinary tract infections