Infections during pregnancy can harm a developing fetus in ways that range from subtle neurodevelopmental delays to severe organ damage or pregnancy loss. The group of pathogens historically associated with these risks goes by the acronym TORCH, covering toxoplasmosis, rubella, cytomegalovirus (CMV), and herpes simplex virus, though the list of concerning infections has grown well beyond that shorthand. How much damage occurs depends on which pathogen is involved, how it reaches the fetus, and when during gestation the exposure happens, making prenatal infections a subject with far more nuance than a simple list of dangerous germs.
How Infections Reach the Fetus
A fetus is not directly exposed to the outside world, so any infection it acquires has to get past the mother’s body first. There are three main routes. The most feared is transplacental transmission, where a pathogen circulating in the mother’s blood crosses the placenta and enters the fetal bloodstream. This is how organisms like the syphilis bacterium, CMV, toxoplasma, rubella virus, and Zika virus typically reach the fetus. The second route is ascending infection, in which bacteria from the vagina or cervix travel upward through the cervical canal, sometimes breaching the membranes surrounding the fetus. This route is strongly linked to preterm premature rupture of membranes and neonatal sepsis. The third is peripartum transmission, meaning the baby picks up the infection during labor and delivery by coming into contact with infectious secretions in the birth canal. Herpes simplex virus is the classic example: roughly 85 percent of neonatal herpes cases are acquired during delivery, with only about 5 percent acquired while still in the womb.1PubMed. Vertical transmission of herpes simplex virus: an update
The Placenta as Both Shield and Target
The placenta is often described as a barrier, and for many pathogens it genuinely is one. Its outer layer, the syncytiotrophoblast, sits in direct contact with the mother’s blood and is surprisingly resistant to invasion. Research on Listeria, for instance, found that this surface layer was highly resistant to bacterial infection by both direct invasion and cell-to-cell spread.2PLoS Pathogens. Placental Syncytiotrophoblast Constitutes a Major Barrier to Vertical Transmission of Listeria monocytogenes IgG antibodies also cross the placenta in the opposite direction, from mother to fetus, providing passive immune protection. This transfer is mediated by specific receptors on the syncytiotrophoblast and is a key reason newborns arrive with some degree of immune readiness.3PubMed Central. IgG placental transfer in healthy and pathological pregnancies
But some pathogens have evolved mechanisms to get through. Malaria parasites during pregnancy express a surface protein called VAR2CSA that binds to a sugar molecule in the placental tissue, allowing infected red blood cells to pile up in the spaces between placental villi. This accumulation triggers inflammation and can restrict nutrient flow to the fetus, raising the risk of low birth weight and fetal growth restriction.4PubMed Central. Placental Malaria Other pathogens, like CMV, exploit different cellular entry points to cross into fetal tissue. The placenta is formidable, but not impenetrable.
When Timing Changes Everything
Gestational age at the time of maternal infection is one of the strongest predictors of what happens to the fetus, and the relationship is not always intuitive. For many infections, earlier exposure causes more severe fetal damage, but later exposure results in higher rates of transmission. CMV illustrates this clearly. When a mother has her first CMV infection in the third trimester, the chance of the virus reaching the fetus is high, yet the risk of serious neonatal disease is low. The reverse holds near conception and in the first trimester, where transmission is less common but the consequences for the fetus are far worse.5PubMed. Intrauterine transmission and clinical outcome of 248 pregnancies with primary cytomegalovirus infection in relation to gestational age
Parvovirus B19 shows its own timing-dependent pattern. In a prospective study of over a thousand pregnancies with confirmed maternal parvovirus infection, the fetal death rate for infections occurring before 20 weeks of gestation was about 11 percent. Fetal death was not observed when maternal infection happened after 20 weeks.6PubMed. Fetal morbidity and mortality after acute human parvovirus B19 infection in pregnancy: prospective evaluation of 1018 cases The reason ties back to biology: parvovirus targets rapidly dividing red blood cell precursors, and the fetus in earlier stages has a faster turnover of those cells combined with a less developed immune system. By mid-pregnancy, the fetus is better equipped to tolerate and recover from the viral hit.
Rubella follows the same general principle. Infection in the first trimester carries the highest risk of congenital rubella syndrome, which can involve cataracts, heart defects like patent ductus arteriosus and pulmonary stenosis, hearing loss, and neurological damage.7PubMed Central. Clinically Confirmed Congenital Rubella Syndrome: The Role of Echocardiography After the first trimester, the risk of these structural birth defects drops sharply, though it does not disappear entirely.
Major Pathogens and Their Fetal Effects
Cytomegalovirus
CMV is the most common congenital infection worldwide and the leading non-genetic cause of sensorineural hearing loss in children.8PubMed Central. Hearing and neurodevelopmental outcomes among children with congenital cytomegalovirus Most infected newborns appear healthy at birth, which makes CMV tricky: problems like hearing loss and developmental delays can surface months or years later. A smaller fraction of congenitally infected babies are born with obvious symptoms such as an enlarged liver and spleen, jaundice, small head size, or a distinctive rash. Because most adults have already been exposed to CMV at some point, many maternal infections during pregnancy are reactivations or reinfections rather than first-time encounters. First-time infections carry the highest risk of fetal transmission, which is one reason universal screening during pregnancy is debated but not universally recommended.
Toxoplasmosis
Toxoplasma gondii, a parasite commonly acquired from undercooked meat or cat feces, can cross the placenta and cause congenital toxoplasmosis. When the infection is clinically evident in the newborn, hallmark features include eye inflammation (retinochoroiditis), calcifications in the brain, hydrocephalus, and cognitive impairment.9PubMed Central. Congenital Toxoplasmosis: The State of the Art As with CMV, many congenitally infected infants show no symptoms at birth, and retinochoroiditis can appear later in childhood or even adolescence. The transmission rate to the fetus rises with gestational age, but the severity of disease drops, following the same paradox seen with CMV.
Syphilis
Congenital syphilis has been surging in many countries after decades of decline, and the consequences for the fetus are severe. The bacterium Treponema pallidum crosses the placenta readily, and untreated maternal syphilis can lead to miscarriage, stillbirth, preterm birth, or a range of neonatal problems affecting the skin, bones, internal organs, and nervous system.10PubMed. Congenital syphilis: adverse pregnancy outcomes and neonatal disorders Bone involvement deserves particular mention: the infection disrupts normal bone growth, producing fragile and disorganized bone tissue, especially in the metaphyses near the growth plates. These abnormalities make the bones more vulnerable to fractures and secondary infections like osteomyelitis during childhood.11PubMed Central. Bone abnormalities in congenital syphilis: A case report The encouraging part is that syphilis is both detectable and treatable: penicillin given to the mother during pregnancy prevents congenital infection in most cases, making routine prenatal syphilis screening one of the most cost-effective interventions in obstetric care.
Zika Virus
Zika brought the concept of prenatal infection into mainstream awareness during the 2015-2016 epidemic, primarily because of its association with microcephaly. The mechanism is more specific than many other congenital infections. Zika directly targets human neural progenitor cells, the precursor cells that give rise to neurons during brain development. Infection of these cells causes cell death, disrupted cell division, and interference with the Notch signaling pathway that governs how progenitor cells differentiate into mature neurons.12PubMed Central. Zika virus differentially infects human neural progenitor cells according to their state of differentiation and dysregulates neurogenesis through the Notch pathway In animal models, this produces cortical thinning and microcephaly.13PubMed. Zika Virus Disrupts Neural Progenitor Development and Leads to Microcephaly in Mice Lab studies confirmed that human neural progenitor cells are a direct target of the virus, with infection leading to reduced growth of these cell populations through both increased cell death and disrupted cell cycling.14Cell Stem Cell. Zika Virus Infects Human Neural Progenitor Cells and Causes Cell Death Because the brain is being actively built during the first and second trimesters, early infections carry the highest risk of structural brain abnormalities.
Parvovirus B19
Parvovirus B19, the virus behind “fifth disease” or “slapped cheek” rash in children, is usually mild in adults but can be dangerous to the fetus. The virus infects red blood cell precursors in the fetal bone marrow, potentially causing severe anemia. In the most serious cases, the anemia leads to hydrops fetalis, a condition where fluid accumulates abnormally in fetal tissues. Many infected fetuses show no abnormalities at all, but in severe cases, the outcome can include pregnancy loss.15PubMed Central. Parvovirus B19 Infection and Pregnancy: Review of the Current Knowledge When hydrops is detected on ultrasound, intrauterine blood transfusions can be lifesaving for the fetus.
Herpes Simplex Virus
Neonatal herpes is uncommon but carries high stakes. When herpes simplex virus reaches a newborn’s central nervous system or disseminates through the body, the illness can be devastating. As noted earlier, the vast majority of cases happen during delivery rather than in utero. This is why antiviral prophylaxis in late pregnancy and, when necessary, cesarean delivery are standard strategies for women with active genital herpes lesions at the time of labor.1PubMed. Vertical transmission of herpes simplex virus: an update A first-episode genital herpes infection near the time of delivery is the highest risk scenario, because the mother has not yet produced antibodies that would offer some passive protection to the baby.
Damage Without Direct Infection
One of the more unsettling findings in prenatal infection research is that the fetus does not always need to be directly infected to suffer harm. Maternal immune activation, the inflammatory response a mother’s immune system mounts against an infection, can itself disrupt fetal brain development. Elevated levels of inflammatory signaling molecules crossing the placenta appear to alter how the fetal brain wires itself, affecting processes like how immune cells in the brain prune synapses and how neurons develop protective mechanisms.16PubMed Central. Maternal Immune Activation and Neurodevelopmental Disorders: Integrating Molecular, Cellular and Systems Mechanisms
Epidemiological studies have implicated maternal infection during pregnancy as a risk factor for autism spectrum disorder and schizophrenia in offspring. Animal studies reinforce this: maternal immune activation alone, without the pathogen ever reaching the fetus, is enough to produce lasting behavioral and neurological changes in offspring.17PubMed Central. Maternal immune activation: Implications for neuropsychiatric disorders In rodent models, offspring born to mothers whose immune systems were experimentally activated during pregnancy showed increased anxiety, social behavior deficits, signs of oxidative stress in the hippocampus, and upregulated inflammatory gene expression, patterns that resemble features of schizophrenia.18PubMed. Maternal Immune Activation Causes Schizophrenia-like Behaviors in the Offspring through Activation of Immune-Inflammatory, Oxidative and Apoptotic Pathways, and Lowered Antioxidant Defenses and Neuroprotection
This does not mean every maternal cold or flu causes psychiatric illness. The risk is statistical and most pregnancies complicated by infection result in healthy children. But the finding reframes how we think about prenatal infections: the pathogen itself is only part of the story, and the mother’s inflammatory response is an independent source of potential harm to the developing brain.
Ascending Infections and the Vaginal Microbiome
Not all prenatal infections arrive through the blood. Bacteria from the vagina can ascend through the cervix and infect the membranes surrounding the fetus, a process closely tied to preterm birth. Research has shown that vaginal dysbiosis, a shift away from the normally protective Lactobacillus-dominated community, is present before membrane rupture in a substantial fraction of preterm cases. In one study, Lactobacillus depletion was present before fetal membrane rupture in roughly a third of preterm cases compared to none of the term deliveries.19PubMed Central. Vaginal dysbiosis increases risk of preterm fetal membrane rupture, neonatal sepsis and is exacerbated by erythromycin The implication is that the composition of vaginal bacteria may serve as an early warning sign for preterm membrane rupture and the ascending infections that follow. Interestingly, the same study found that erythromycin, an antibiotic commonly used prophylactically in this setting, could worsen the dysbiosis rather than correct it, a finding that complicates standard treatment approaches.
SARS-CoV-2 and Placental Inflammation
COVID-19 during pregnancy offered a real-time case study in how a novel pathogen interacts with the placenta. Direct fetal infection with SARS-CoV-2 turned out to be rare, but the virus could still cause serious harm through placental inflammation. Even without infecting the placenta itself, the virus triggered blood clotting and inflammation in the spaces between placental villi. When the placenta did become infected, a more severe condition called SARS-CoV-2 placentitis could develop, characterized by immune cell infiltration, fibrin deposits, and destruction of the outer placental layer. In a series of 68 cases of placentitis associated with stillbirth or neonatal death, the cause of death was most likely fetal oxygen deprivation from placental damage rather than fetal infection. Only 2 of those 68 cases showed confirmed fetal infection.20Nature Reviews Immunology. SARS-CoV-2 infection and COVID-19 vaccination in pregnancy This pattern reinforces the broader point: a pathogen does not have to infect the fetus directly to put it at serious risk. Damaging the placenta can be just as dangerous.
Long-Term Effects That Show Up After Birth
One of the hardest aspects of congenital infections is that a baby who looks perfectly healthy at birth may develop problems months or years later. This is well documented for CMV, where hearing loss can be progressive and may not appear until after the newborn screening period. But the phenomenon extends across many congenital infections. Children exposed to infections in utero, even those who were asymptomatic at birth, may develop neurodevelopmental concerns over time.21Current opinion in infectious diseases. Neurodevelopmental outcomes in congenital and perinatal infections This means that a negative neonatal exam does not always close the book. Follow-up hearing tests, developmental assessments, and vision screening may be warranted for children with known in-utero exposure to infections like CMV or toxoplasmosis, even when they appeared fine at delivery.
How Prenatal Infections Are Detected
Screening for prenatal infections is a mix of routine and targeted approaches. Syphilis and rubella immunity are checked through standard prenatal blood panels in most healthcare systems. HIV screening is similarly routine. But for infections like CMV and toxoplasmosis, universal screening is not standard in many countries, partly because the follow-up pathway after a positive result is complicated. A positive antibody test in the mother does not necessarily mean the fetus is infected. Confirming fetal infection often requires testing amniotic fluid, which involves amniocentesis. Even then, the predictive value is imperfect: not every fetus with a positive amniotic fluid test develops symptoms, and the severity of disease is hard to predict prenatally.
Ultrasound plays a supporting role, picking up signs like brain calcifications, enlarged ventricles, abnormal fluid collections, or growth restriction that may point to congenital infection. But ultrasound findings are often nonspecific. A finding like enlarged brain ventricles has a long list of possible causes, and infection is just one. For CMV specifically, some screening programs use a combination of antibody testing and avidity index measurements to distinguish recent from past infections, though the yield of confirmed recent infections can be quite low relative to the number of positive initial screens.22PubMed Central. Screening and prenatal diagnosis of fetal cytomegalovirus infection: experience in a western Chinese city
Maternal Antibodies as a Protective Tool
The same placental machinery that allows pathogens to occasionally cross also actively transports protective antibodies from mother to fetus. Maternal IgG antibodies cross the placenta throughout pregnancy, with the bulk of transfer happening in the third trimester. These antibodies are a crucial source of immune protection for the newborn in the first weeks and months of life, before the infant’s own immune system is mature enough to mount strong responses.23PubMed Central. The Protective Role of Maternal Immunization in Early Life This is the biological rationale behind vaccinating pregnant women against influenza, pertussis, COVID-19, and now RSV: the mother produces antibodies that cross to the fetus, providing the baby with protection during its most vulnerable window.
Conditions that compromise the placenta, including some of the infections discussed above, can impair this antibody transfer. Placental malaria, for instance, by disrupting the placental architecture, may reduce the efficiency of IgG transfer and leave the newborn with weaker passive immunity. Preterm birth also cuts this process short, since the heaviest antibody transfer happens in the final weeks of a full-term pregnancy. Premature infants often arrive with lower levels of maternal antibodies and are consequently more susceptible to infections in early life.
Ethical Terrain of Fetal Treatment
When a prenatal infection is detected, the question of what to do next is not always straightforward. Some interventions are well established: penicillin for maternal syphilis, antiretroviral therapy for maternal HIV, intrauterine transfusion for severe fetal anemia from parvovirus. Others remain experimental or carry their own risks. Fetal therapy in the setting of congenital infection raises genuine ethical questions about balancing potential benefit against potential harm, respecting maternal autonomy, and navigating the possibility that treatment might be only partially successful, leaving a child with significant disability.24PubMed. Ethical considerations of fetal therapy The principle that fetal therapy should never proceed without the pregnant person’s informed consent, and should not be framed coercively as an alternative to termination, is a cornerstone of ethical guidance in this area. For many prenatal infections, the honest conversation involves substantial uncertainty: the infection has been detected, the possible outcomes range from completely normal development to serious impairment, and the available interventions may shift the odds without eliminating the risk.