Miscarriage is far more common than most people realize, affecting an estimated 10-20 percent of recognized pregnancies. While chromosomal abnormalities in the embryo account for the majority of early pregnancy losses, infections are a distinct and important category of causes. Certain bacterial, viral, and parasitic infections can cross the placenta or trigger a severe maternal immune response that ends the pregnancy. Understanding which infections pose a real risk, and which ones are often blamed without evidence, can help you separate genuine medical concerns from internet fear-mongering.
What Infections Can Cause A Miscarriage
The infections most clearly linked to miscarriage are those that reach the fetus or the placenta directly. These include bacterial infections like syphilis and bacterial vaginosis, viral infections like parvovirus B19 and cytomegalovirus, and the parasitic infection toxoplasmosis. The timing of the infection matters enormously. Some pathogens are dangerous mainly in the first trimester, while others pose greater risk later in pregnancy. Many common infections, including the ordinary cold or a mild flu, do not directly cause miscarriage in an otherwise healthy pregnancy.
It is also critical to understand that a positive test for an infection does not automatically mean it caused a pregnancy loss. Many women carry these organisms without any adverse outcome. The distinction between infection and disease is a real one, and your doctor will consider the full clinical picture before drawing conclusions.
Which Bacterial Infections Are Linked to Pregnancy Loss
Syphilis is one of the most well-documented bacterial causes of miscarriage and stillbirth. The bacterium Treponema pallidum can cross the placenta at any stage of pregnancy. Untreated syphilis can cause fetal infection, placental damage, and pregnancy loss, particularly in the second half of pregnancy. This is why routine syphilis screening is a standard part of prenatal care in the United States. The infection is highly treatable with penicillin, and treatment dramatically reduces the risk of pregnancy complications.
Bacterial vaginosis is a more complicated story. This condition involves an overgrowth of certain bacteria in the vagina, and it is associated with preterm birth and late miscarriage in some studies. However, the evidence linking bacterial vaginosis to early first-trimester miscarriage is less consistent. Some research suggests a connection, while other studies have not confirmed it. What is clear is that treating bacterial vaginosis in women with a previous preterm birth may reduce their risk of recurrence. Routine screening and treatment for all pregnant women is not currently recommended by major medical organizations because the evidence does not show a clear benefit for everyone.
Listeria is another bacterial infection with serious pregnancy risks. It comes from contaminated food, particularly unpasteurized dairy products, deli meats, and soft cheeses. Listeriosis during pregnancy can cause miscarriage, stillbirth, or severe illness in a newborn. The infection is relatively rare, but the consequences are severe enough that pregnant women are advised to avoid high-risk foods.
Which Viral Infections Can Harm a Pregnancy
Parvovirus B19, sometimes called fifth disease, is a common childhood illness. In children it causes a mild rash and is usually harmless. In pregnancy, however, the virus can cross the placenta and infect fetal red blood cell precursors. This can lead to fetal anemia and, in severe cases, hydrops fetalis, a condition where fluid builds up in fetal tissues. Untreated severe fetal anemia can cause pregnancy loss. The risk is highest when infection occurs before 20 weeks of gestation. Fortunately, many infected fetuses recover without intervention, and fetal monitoring can identify cases that need treatment.
Cytomegalovirus, or CMV, is the most common congenital viral infection worldwide. Most adults have been exposed to CMV by middle age, and a primary infection during pregnancy carries the highest risk of transmission to the fetus. CMV can cause miscarriage, but it is more commonly associated with stillbirth and congenital disabilities including hearing loss and developmental delay. The challenge with CMV is that most infected pregnant women have no symptoms, making it difficult to identify who needs testing.
Genital herpes simplex virus is rarely a cause of miscarriage. The greater risk occurs during vaginal delivery, when active lesions can transmit the virus to the newborn, causing severe neonatal infection. This is why women with active herpes lesions at the time of labor are typically advised to have a cesarean section.
Influenza deserves mention because it is often feared during pregnancy. Severe influenza illness, particularly with high fever and pneumonia, can increase the risk of pregnancy complications. However, routine influenza infection does not directly cause miscarriage in most cases. The seasonal flu vaccine is recommended for all pregnant women because it protects both mother and baby from severe disease.
Parasitic Infections and Pregnancy Risk
Toxoplasmosis is caused by the parasite Toxoplasma gondii, which can be found in cat feces and undercooked meat. A primary infection during pregnancy can cross the placenta and cause fetal infection. The risk of transmission increases with gestational age, but the risk of severe fetal damage is highest when infection occurs early in pregnancy. Congenital toxoplasmosis can cause miscarriage, stillbirth, or long-term neurological problems in the infant. Routine screening is not performed in the United States, so prevention through food safety and hygiene measures is the primary strategy.
Malaria is a significant cause of pregnancy loss in endemic regions of the world, particularly sub-Saharan Africa. The parasite infects the placenta and can cause maternal anemia and fetal growth restriction. In the United States, malaria is rarely encountered outside of travel-related cases, but pregnant women traveling to endemic areas should take preventive measures seriously.
Infections Commonly Blamed for Miscarriage Without Good Evidence
Some infections are frequently discussed in online forums as miscarriage causes, but the evidence does not support these claims. A urinary tract infection that remains confined to the bladder does not directly cause miscarriage. The concern arises when a kidney infection develops, because the high fever and systemic illness can stress the pregnancy. Routine screening and prompt treatment of UTIs in pregnancy is standard care, but the rationale is prevention of kidney infection and preterm labor rather than miscarriage prevention.
COVID-19 has been studied extensively since 2020. The evidence shows that severe COVID-19 illness during pregnancy increases the risk of pregnancy complications, including preterm birth. However, the virus itself does not appear to be a direct cause of early miscarriage at rates higher than baseline. The larger risk comes from severe maternal illness, which is why vaccination is strongly recommended for pregnant women.
Some people worry about yeast infections causing miscarriage. Candida, the fungus responsible for most vaginal yeast infections, does not cross the placenta in an immunocompetent woman and is not a cause of miscarriage. It is uncomfortable and worth treating, but it does not threaten the pregnancy directly.
How Do Doctors Determine If an Infection Caused a Miscarriage
Determining whether an infection caused a specific miscarriage is often difficult. After a pregnancy loss, tissue can be sent for pathological examination and genetic testing. Testing for specific infections is not routine after a single miscarriage. It becomes more relevant after recurrent losses or when there are specific clinical signs pointing to an infectious cause.
Blood tests can identify past or current infections by measuring antibodies. However, antibody testing has limitations. The presence of IgG antibodies indicates past exposure and immunity, while IgM antibodies may indicate recent infection, but false positives occur. A four-fold rise in antibody titers between acute and convalescent samples is more reliable evidence of a recent infection.
For women who experience recurrent pregnancy loss, defined as two or more consecutive losses, a workup for underlying causes is standard. This workup typically includes genetic testing of both partners, evaluation of uterine anatomy, hormonal assessment, and testing for antiphospholipid syndrome. Infection screening is part of this evaluation in some cases, but it is not the first line of investigation because infections are a less common cause of recurrent loss than genetic or anatomical factors.
How Can You Reduce Your Risk of Infection-Related Miscarriage
Prevention strategies are straightforward and evidence-based. Wash your hands frequently, particularly after handling raw meat, gardening, or changing cat litter. Cook meat to safe internal temperatures to kill toxoplasma and listeria. Avoid unpasteurized dairy products and deli meats unless they are heated until steaming. These are standard food safety recommendations from the CDC and FDA.
Stay current on recommended vaccinations before and during pregnancy. The flu vaccine and Tdap vaccine are recommended during each pregnancy. Measles, mumps, rubella, and varicella vaccines should be given before pregnancy if you are not immune, because they contain live viruses and are not given during pregnancy.
If you have a known infection like syphilis or HIV, treatment during pregnancy dramatically reduces the risk of transmission to the baby. Prenatal care that includes routine screening for syphilis, hepatitis B, HIV, and chlamydia is standard and effective. Attending all prenatal appointments ensures these screenings happen at the right time.
The evidence does not support extreme measures like avoiding all public places or pets during pregnancy. The infections that genuinely threaten pregnancy are specific and largely preventable with standard hygiene and medical care. Fear is not a prevention strategy, and it does not serve you or your baby.
Frequently Asked Questions
Can a urinary tract infection cause a miscarriage?
A simple bladder infection does not directly cause miscarriage. The risk increases if the infection spreads to the kidneys and causes severe systemic illness, which is why prompt treatment is important.
Can a yeast infection cause a miscarriage?
No, vaginal yeast infections do not cross the placenta and are not linked to miscarriage. They are treatable during pregnancy and are more of a discomfort than a danger.
How long after an infection can a miscarriage occur?
Timing varies widely depending on the specific infection and how the maternal immune system responds. Some infections act within days, while others may affect the pregnancy over several weeks, which is why clinical evaluation is needed for accurate answers.
Should I be tested for infections after a miscarriage?
Testing for infections is not routine after a single miscarriage. Your doctor may recommend infection screening if you have recurrent losses, specific symptoms, or exposure to a known high-risk infection during the pregnancy.
