Infections account for an estimated 15% of early miscarriages (before 12 weeks) and up to 66% of late miscarriages (between 12 and 24 weeks). While chromosomal abnormalities remain the leading cause of pregnancy loss overall, a range of bacterial, viral, and parasitic infections can independently trigger miscarriage, often through inflammation or by crossing the placenta and directly infecting the fetus.
TORCH Infections: The Major Group
The most well-studied infections linked to miscarriage are collectively called TORCH infections: toxoplasmosis, rubella, cytomegalovirus (CMV), and herpes simplex virus (HSV). Together, these four pathogens account for roughly 20% of pregnancy loss cases where an infectious cause is identified. Each one works differently, but they share one dangerous trait: the ability to cross from a pregnant person’s bloodstream into the placenta.
Rubella contributes to the highest proportion of incomplete miscarriages among the group, at nearly 20%. CMV follows at about 9%, with toxoplasmosis at around 7% and herpes at 7%. Many of these infections produce mild or no symptoms in the pregnant person, which is part of what makes them dangerous. A rubella infection might feel like a mild cold with a rash. CMV often causes no symptoms at all. Toxoplasmosis, typically picked up from undercooked meat or contact with cat feces, can pass without notice.
The key factor with TORCH infections is timing. A first-time infection during pregnancy is far more dangerous than a reactivation of a virus the body has already learned to fight. This is why rubella vaccination before pregnancy is so effective at preventing these losses, and why pregnant people are often tested for immunity early in prenatal care.
Bacterial Vaginosis
Bacterial vaginosis (BV) is not a single infection but a shift in the balance of bacteria in the vagina. Normally, protective bacteria called lactobacilli make up about 95% of vaginal flora. They produce lactic acid and other compounds that keep harmful organisms in check. When lactobacilli decline, anaerobic bacteria take over, and BV develops.
BV during pregnancy carries a two- to threefold increase in the risk of miscarriage. The risk is particularly strong for second-trimester losses. In one study, 41% of miscarriages among women with BV occurred in the second trimester, compared to only 16% of miscarriages in women without BV. The bacteria associated with BV have been found in amniotic fluid and placental tissue, suggesting they can ascend from the cervix and vagina into the uterus, triggering inflammation that leads to pregnancy loss.
BV often produces no symptoms. When it does, the most common sign is a thin, grayish discharge with a fishy odor. But the absence of discharge, itching, or pain does not rule it out.
Listeria: The Foodborne Threat
Listeria is a bacterial infection typically contracted through contaminated food: unpasteurized soft cheeses, deli meats, smoked seafood, and ready-to-eat salads. It poses a uniquely high risk during pregnancy because the bacterium is one of the few pathogens that can efficiently cross the placental barrier by moving from cell to cell inside host tissue.
When listeria infection occurs in early pregnancy, 65% of cases result in miscarriage. In the second or third trimester, about 26% of infections lead to stillbirth, fetal loss, or miscarriage. What makes listeria especially tricky is its unusually long incubation period. While most foodborne illnesses cause symptoms within a day or two, listeria in pregnancy takes an average of 19 to 27 days to develop, with a range of 1 to 9 weeks. The bacterium needs time to colonize the placenta before reaching the fetus.
Symptoms in the pregnant person are typically mild and flu-like: fever (usually between 38°C and 39°C), headache, muscle aches, and sometimes diarrhea. Between 65% and 81% of infected pregnant people develop a fever, which is often the only noticeable sign. Because these symptoms overlap with so many common illnesses, listeria is easy to miss without a blood culture.
Sexually Transmitted Infections
Untreated syphilis is one of the most dangerous STIs during pregnancy, associated with up to an 80% increased risk of adverse pregnancy outcomes overall, including miscarriage, stillbirth, preterm birth, and congenital infection. The specific miscarriage risk is about 1.24 times higher than in uninfected pregnancies. Syphilis is routinely screened for early in prenatal care, and treatment with antibiotics before or during pregnancy dramatically reduces these risks.
Chlamydia and gonorrhea can also contribute to pregnancy complications, primarily through ascending infection that causes inflammation of the uterine lining and membranes. These infections are frequently asymptomatic, which is why screening during pregnancy is standard practice.
Parvovirus B19 (Fifth Disease)
Parvovirus B19, the virus that causes “fifth disease” (recognizable by its distinctive slapped-cheek rash in children), poses a specific risk during the first half of pregnancy. The virus can cross the placenta and attack the fetal red blood cells, causing severe anemia in the developing baby. In some cases this leads to miscarriage, particularly when infection occurs in the first trimester.
The CDC describes the miscarriage risk from parvovirus as a “small increase,” and most pregnant people who are exposed do not lose their pregnancies. About 50% of adults are already immune from childhood exposure. The risk is highest for people who work closely with young children, such as teachers and daycare workers, since outbreaks are common in schools.
Urinary Tract Infections
Urinary tract infections are among the most common infections during pregnancy, and even asymptomatic bacteriuria (bacteria in the urine with no symptoms) can lead to complications if untreated. The primary concern is that an undetected bladder infection can progress to a kidney infection (pyelonephritis), which is most common in the second trimester and has been linked to preterm birth and other adverse outcomes.
Current guidelines recommend a urine culture early in prenatal care to screen for asymptomatic bacteriuria. Treating it with antibiotics reduces the risk of preterm birth by roughly two-thirds and lowers the chance of low birth weight. The connection between UTIs and miscarriage specifically is less direct than with other infections, but the downstream complications of untreated urinary infections make screening a priority.
How Infections Cause Pregnancy Loss
Infections trigger miscarriage through two main pathways. The first is direct invasion: certain pathogens, particularly those with intracellular life cycles (meaning they live and reproduce inside human cells), can pass through the placental barrier and infect the fetus. This is the mechanism behind listeria and toxoplasmosis. These organisms essentially hitchhike inside the mother’s own cells to reach the placenta, then cross into fetal tissue.
The second pathway is inflammation. When the body detects an infection in or near the reproductive tract, it mounts an immune response that releases inflammatory signaling molecules. In the context of pregnancy, this inflammation can destabilize the uterine lining, weaken the membranes surrounding the fetus, or trigger contractions. BV-related miscarriages typically follow this pattern: bacteria ascend from the vagina into the uterus, and the resulting immune response disrupts the pregnancy rather than the bacteria themselves infecting the fetus.
COVID-19 and Miscarriage
A large meta-analysis published in Human Reproduction Update found no evidence that SARS-CoV-2 infection increases miscarriage risk. Among over 4,400 women studied, COVID-19 infection in the first or second trimester did not raise the odds of pregnancy loss compared to uninfected pregnancies. The miscarriage rate among women infected in the first trimester was 9.9%, which falls within the normal background rate. COVID-19 during pregnancy is associated with higher rates of preterm birth and stillbirth at later gestational ages, but early pregnancy loss does not appear to be affected.
What Gets Screened and What Doesn’t
Standard prenatal screening catches some of these infections but not all. Early pregnancy bloodwork typically checks for rubella immunity, syphilis, and hepatitis B. A urine culture screens for asymptomatic bacteriuria. HIV and sometimes hepatitis C are included depending on risk factors and local guidelines.
Notably, several infections linked to miscarriage are not part of routine screening. Toxoplasmosis, CMV, listeria, and parvovirus B19 are generally tested only when symptoms or known exposures raise suspicion. BV is not universally screened for in pregnancy either, despite its association with second-trimester loss. This means that for many of the infections on this list, prevention through food safety, hygiene, and awareness of symptoms plays a larger role than screening in reducing risk.