Toxoplasma gondii, a single-celled parasite carried by cats and found in undercooked meat and contaminated soil, can cross the placenta during a pregnant woman’s first infection and damage the developing fetus. The effects range from invisible subclinical infection to severe brain and eye disease, and the specific harm depends heavily on when during pregnancy the infection occurs. Roughly a third of maternal infections are transmitted to the fetus overall, but the likelihood climbs steeply as pregnancy progresses while the severity of damage to the baby does the opposite.
How the Parasite Reaches the Fetus
Fetal infection happens only when a woman catches Toxoplasma for the first time during pregnancy, or very shortly before conception. During this acute phase, the parasite circulates in her bloodstream and can reach the placenta. Once her immune system mounts an antibody response, the parasite shifts into a dormant cyst form and stops circulating. Women who were infected before pregnancy and carry these dormant cysts pose essentially no risk to their baby.
At the placenta itself, the parasite does something biologically crafty. Normally, certain placental cells called cytotrophoblasts fuse together to form a protective barrier (the syncytiotrophoblast) that resists infection. Research has shown that Toxoplasma prevents this fusion. Instead, it redirects the precursor cells toward a different fate that is more permissive to infection, essentially manipulating placental cell development to open a path for its own transmission to the fetus.1PubMed. Toxoplasma gondii infection misdirects placental trophoblast lineage specification
The parasite also disrupts the local immune environment at the boundary between mother and fetus. Immune cells in the uterine lining that normally maintain tolerance toward the pregnancy get thrown off balance by the infection, ramping up inflammatory signals and suppressing the anti-inflammatory ones needed for a healthy pregnancy. This immune disruption can contribute to adverse outcomes beyond just direct fetal infection, including pregnancy loss.2PubMed Central. Tim-3 downregulation by Toxoplasma gondii infection contributes to decidual dendritic cell dysfunction 3PubMed. The Role of Decidual PD-1(+) Treg Cells in Adverse Pregnancy Outcomes due to Toxoplasma gondii Infection
The Timing Paradox
One of the most important things to understand about congenital toxoplasmosis is a counterintuitive pattern: the later in pregnancy you get infected, the more likely the parasite is to reach the baby, but the less damage it tends to cause. Early infections are harder to transmit but far more destructive when they do get through.
A large analysis of transmission data found that the overall mother-to-fetus transmission rate was about 29%, but this masked a dramatic shift across pregnancy. At 13 weeks, the transmission rate was only about 6%. By 36 weeks, it had climbed to roughly 72%. Yet fetuses infected in early pregnancy were far more likely to show clinical signs of disease. When researchers factored both risks together, women who seroconverted around 24 to 30 weeks of gestation carried the highest combined risk, about 10%, of having a baby with early clinical signs and a likelihood of long-term complications.4PubMed. Mother-to-child transmission of toxoplasmosis: risk estimates for clinical counselling
This paradox exists because the fetal brain and other organs are most vulnerable to inflammatory damage during the first and early second trimesters, when they are still forming basic structures. By the third trimester, the major architecture is largely in place and can better tolerate the insult. Meanwhile, the placenta becomes more permeable to the parasite as it matures, which explains the rising transmission rate.
What Happens to the Fetal Brain
The brain takes the hardest hit. In a multicenter survey of 88 fetuses with confirmed congenital toxoplasmosis, more than 90% showed at least one sign of brain involvement on ultrasound. The most common finding was hyperechogenic nodular foci, essentially bright spots in the brain tissue caused by inflammation and calcification, seen in 60 of the cases. Fluid buildup in the brain’s ventricles (ventriculomegaly) was the second most common finding, appearing in 44 cases and tending to worsen over time. Twelve fetuses had periventricular abscesses, pockets of infection near the brain’s fluid-filled chambers.5PubMed. Ultrasound features of fetal toxoplasmosis: A contemporary multicenter survey in 88 fetuses
In the most severe cases, typically from first-trimester infections, the brain damage can be catastrophic. Hydrocephalus (massive fluid accumulation that expands the skull) and widespread calcifications throughout the brain tissue can develop. These infants may be born with microcephaly or macrocephaly, seizures, and profound intellectual disability. Less severe brain involvement may not show obvious symptoms at birth but can still produce measurable cognitive effects over time.
Eye Damage and Retinochoroiditis
The eyes are the other primary target. Toxoplasma has a particular affinity for retinal tissue, and the resulting condition, retinochoroiditis, is the hallmark eye lesion of congenital toxoplasmosis. In one study of 44 children with confirmed congenital infection, about 70% had eye involvement, and the vast majority of those had retinochoroiditis lesions. In three out of four affected children, the lesions were in both eyes. The damage concentrated overwhelmingly in the papillomacular area, the region of the retina responsible for central, sharp vision.6Eye. Ocular manifestations of congenital toxoplasmosis
Other eye problems found alongside retinochoroiditis included cataracts, abnormally small eyes (microphthalmia), and strabismus (misaligned eyes). Many lesions had already healed by the time children were evaluated, but healed does not mean harmless. A healed scar in the macula can permanently impair central vision. Retinochoroiditis can also reactivate later in life, causing fresh damage to tissue that was previously spared.
The Danger of “Normal” Newborns
Perhaps the most unsettling aspect of congenital toxoplasmosis is that most infected babies look perfectly healthy at birth. The infection is subclinical in the majority of cases, meaning there are no obvious symptoms in the newborn period. Without specific testing, these infections go unrecognized. But looking normal at birth does not mean the child will stay that way.
A follow-up study tracked 24 children who were born with subclinical congenital Toxoplasma infection. By an average age of about 8.5 years, 85% of the children whose infection was caught early through prospective testing had developed retinochoroiditis. Among those whose infection was only diagnosed after symptoms appeared, every single one developed eye lesions. In the latter group, nearly half had bilateral blindness. Children in both groups also developed neurological problems, intellectual decline, and hearing loss over time. Six children who were tested repeatedly showed a meaningful drop in IQ scores, falling from an average of 97 to 74 over about five and a half years.7Pediatrics. Development of Adverse Sequelae in Children Born with Subclinical Congenital Toxoplasma Infection
A systematic review of long-term outcomes confirmed this pattern: even in children who appear completely normal at birth, serious sequelae can emerge later in life.8PubMed Central. Long-Term Outcomes in Children with Congenital Toxoplasmosis—A Systematic Review This delayed onset is why postnatal follow-up is considered essential for any child born to a mother who seroconverted during pregnancy, even if the baby seems fine at delivery.
How Pregnant Women Actually Get Infected
The popular image of toxoplasmosis risk centers on cats, and while cats are indeed the definitive host for Toxoplasma, the actual infection risk during pregnancy is more nuanced. A large European case-control study found that the strongest risk factors for acute infection during pregnancy were eating undercooked lamb, beef, or game, and having contact with soil. Between 30% and 63% of infections across different study centers were attributed to eating undercooked or cured meat, while soil contact accounted for 6% to 17%. Direct contact with cats was not identified as a significant risk factor in this study.9PubMed Central. Sources of toxoplasma infection in pregnant women: European multicentre case-control study
A Norwegian study painted a somewhat different picture, finding that cleaning the cat litter box was associated with increased risk, but it also confirmed that food-related exposures dominated. Eating raw or undercooked minced meat, unwashed raw vegetables or fruits, undercooked mutton, and undercooked pork all independently raised the odds of infection. The study also flagged a less obvious risk: infrequently washing kitchen knives after preparing raw meat before using them for other food.10American Journal of Epidemiology. Risk Factors for Toxoplasma gondii Infection in Pregnancy: Results of a Prospective Case-Control Study in Norway
The practical takeaway is that while keeping your distance from cat litter is reasonable, food hygiene matters more. Cooking meat thoroughly, washing produce, and cleaning cutting surfaces after handling raw meat are the measures most likely to prevent infection.
Detecting Infection Before Birth
Figuring out whether a pregnant woman has a recent Toxoplasma infection starts with blood tests. If both IgM and IgG antibodies against Toxoplasma are detected, it could mean a new infection or it could be a false alarm. IgM antibodies are supposed to indicate recent infection, but they sometimes persist for months or years after the initial infection, and in some cases they appear even without true infection.11PubMed Central. Role of Toxoplasma gondii IgG Avidity Testing in Discriminating between Acute and Chronic Toxoplasmosis in Pregnancy
To sort this out, doctors use an IgG avidity test. Antibodies produced early in an infection bind weakly to their targets (low avidity), while antibodies that have matured over several months bind tightly (high avidity). If a first-trimester blood sample shows high avidity, the infection almost certainly happened before pregnancy and poses no threat to the fetus.12PubMed. Toxoplasma gondii-specific IgG avidity testing in pregnant women One study found that high-avidity antibodies were present in 75% of pregnant women who had tested positive for IgM, meaning three quarters of those positive IgM results were not actually recent infections.13PubMed Central. VIDAS test for avidity of Toxoplasma-specific immunoglobulin G for confirmatory testing of pregnant women
Once maternal infection is confirmed as recent, the next step is amniocentesis to test the amniotic fluid for parasite DNA using PCR. A meta-analysis found that when this test was performed within five weeks of the maternal diagnosis, it had about 87% sensitivity and 99% specificity.14PLOS ONE. Performance of Polymerase Chain Reaction Analysis of the Amniotic Fluid of Pregnant Women for Diagnosis of Congenital Toxoplasmosis: A Systematic Review and Meta-Analysis Another study reported even higher accuracy with newer real-time PCR methods, achieving about 92% sensitivity and 100% specificity, though it stressed that a negative result does not completely rule out fetal infection. Postnatal follow-up during the first year of life remains necessary for children whose prenatal PCR came back negative.15PubMed. Accuracy of real-time polymerase chain reaction for Toxoplasma gondii in amniotic fluid
Treatment During Pregnancy
When a pregnant woman is diagnosed with acute toxoplasmosis, treatment aims to reduce both the chance of the parasite crossing the placenta and the severity of damage if it does. The drug spiramycin is commonly used as a first-line treatment to lower transmission risk. A meta-analysis of observational studies found that the transmission rate in women treated with spiramycin alone was about 18%, compared with roughly 51% in untreated women.16PubMed Central. A fresh look at the role of spiramycin in preventing a neglected disease: meta-analyses of observational studies
Some centers use a combination of spiramycin with cotrimoxazole (a sulfonamide antibiotic combination) rather than spiramycin alone. In one referral center’s experience, the transmission rate in women who started this combined therapy promptly after diagnosis was about 3.4%, compared with roughly 8% in women who started late or had poor adherence. In completely untreated cases, the transmission rate was over 71%. All infected newborns of mothers who received timely combined treatment were asymptomatic at birth, while nearly 29% of infected infants born to mothers with inadequate treatment had postnatal complications.17PubMed. The prevention of congenital toxoplasmosis using a combination of Spiramycin and Cotrimoxazole: The long-time experience of a tertiary referral centre
If fetal infection is confirmed by amniocentesis, treatment is typically escalated to pyrimethamine plus sulfadiazine, a more aggressive antiparasitic combination that crosses the placenta more effectively. Comparative data suggest that spiramycin alone carries a higher risk of congenital infection than the combined spiramycin-cotrimoxazole or pyrimethamine-based regimens.18PubMed. Spiramycin/cotrimoxazole versus pyrimethamine/sulfonamide and spiramycin alone for the treatment of toxoplasmosis in pregnancy The specific protocol varies by country and center, and treatment decisions often depend on the gestational age at infection, whether fetal infection is confirmed, and the presence of ultrasound abnormalities.
Treatment After Birth
Infected newborns are treated for a full 12 months with a combination of pyrimethamine, sulfadiazine, and folinic acid (leucovorin), the last of which protects the bone marrow from the toxic effects of pyrimethamine. The pyrimethamine dose starts higher in the first two days and tapers over the year. If the baby has severe eye disease or elevated protein levels in the spinal fluid, corticosteroids may be added to control inflammation.19Pediatrics. Diagnosis, Treatment, and Prevention of Congenital Toxoplasmosis in the United States
This yearlong treatment is demanding for families. It requires regular blood counts to monitor for bone marrow suppression, careful dose adjustments, and consistent administration. But studies suggest it can reduce the severity and frequency of long-term complications, particularly eye and neurological disease.
Parasite Strains and Why Geography Matters
Not all Toxoplasma is the same. The parasite comes in multiple genetic strains, and the strain type influences how severe congenital disease is likely to be. In the United States, a study of 193 congenitally infected infants found that non-type-II strains were more common than type II strains (61% versus 39%) and were associated with more severe disease and worse eye involvement at birth.20PubMed Central. Strain hypothesis of Toxoplasma gondii infection on the outcome of human diseases In Europe, type II strains predominate and tend to cause somewhat milder congenital disease on average.
South America, particularly Brazil, harbors a wider diversity of strains, including atypical genotypes that can cause unusually aggressive disease. This is one reason why congenital toxoplasmosis in South American cohorts sometimes looks worse than in European cohorts even after accounting for differences in treatment timing and access to care. The global seroprevalence of Toxoplasma in pregnant women reflects these geographic patterns: a recent meta-analysis estimated it at about 37% worldwide, with the highest rates in South America (around 53%) and Africa (around 47%), and lower rates in Europe (about 25%) and North America (about 20%).21PubMed Central. Global seroprevalence of Toxoplasma gondii in pregnant women: a systematic review and meta-analysis
Higher seroprevalence does not directly mean higher rates of congenital infection. A woman who is already seropositive before pregnancy is protected; it is seronegative women in high-exposure environments who face the greatest risk. Lower-income countries tend to have both higher seroprevalence and fewer resources for prenatal screening and treatment, which compounds the problem.22PubMed. Global prevalence of latent toxoplasmosis in pregnant women: a systematic review and meta-analysis
The Screening Debate
Countries handle prenatal toxoplasmosis screening very differently. France and Austria have long required routine screening of all pregnant women, typically with monthly or trimester-based blood tests. The United States and the United Kingdom do not recommend universal screening, instead relying on targeted testing when there is clinical suspicion, such as abnormal ultrasound findings.
The arguments against universal screening are practical. One decision analysis found that in a low-prevalence setting, universal screening with treatment would cause roughly 18 additional pregnancy losses (from amniocentesis complications) for each case of congenital toxoplasmosis prevented. If infected pregnancies were terminated instead of treated, the ratio was about 12 additional losses per case avoided.23Obstetrics and Gynecology. Prenatal screening for toxoplasmosis The rarity of acute infection during pregnancy in these countries, combined with the imperfect accuracy of serology and the risks of invasive follow-up testing, makes the cost-benefit math unfavorable.
On the other side, an economic model incorporating the societal costs of untreated congenital toxoplasmosis, including lifelong disability, vision loss, and special education needs, estimated that universal screening would actually save money. The model predicted savings of about $620 per screened child, or roughly $2.5 billion annually across all U.S. births.24PLoS Neglected Tropical Diseases. Maternal Serologic Screening to Prevent Congenital Toxoplasmosis: A Decision-Analytic Economic Model These two analyses reach opposite conclusions largely because of different assumptions about treatment effectiveness, discount rates, and the value placed on preventing long-term disability versus preventing procedure-related losses. The debate remains unresolved, and screening practice continues to vary by country.
Psychological Toll on Parents
The uncertainty built into every step of the diagnostic process creates significant anxiety for pregnant women. A positive IgM test that turns out to be a false positive, an equivocal avidity result, a negative amniocentesis that still cannot completely exclude infection: each of these scenarios leaves families in limbo. Research into the psychological consequences of prenatal toxoplasmosis screening has highlighted that more frequent testing protocols may generate substantial and sometimes unnecessary anxiety, particularly when results are ambiguous. False positives alone can cause lasting distress even after the infection is ruled out.25PubMed. Prenatal screening and diagnosis of congenital toxoplasmosis: a review of safety issues and psychological consequences for women who undergo screening
For women whose babies are confirmed infected, the stress does not end at delivery. The prognosis of congenital toxoplasmosis is famously hard to predict. A baby with normal-appearing scans can still develop eye or brain problems years later. The yearlong treatment regimen with its blood test requirements adds ongoing medical burden. And for families navigating potential vision loss or developmental delay in their child, the emotional weight persists well beyond the newborn period. These realities make it all the more important that clinicians frame test results carefully and provide realistic counseling about both the risks and the genuine uncertainty involved.
Incidence in Countries That Track It
Pinning down exactly how often acute toxoplasmosis strikes during pregnancy is difficult because most countries do not systematically screen for it. Germany, which tracks the condition through healthcare claims data, reported an average of about 289 cases of toxoplasmosis during pregnancy per year between 2011 and 2016, corresponding to roughly 40 cases per 100,000 pregnancies. The rate varied by year and region, ranging from about 29 to 60 per 100,000 pregnancies.26Emerging Infectious Diseases. Estimates of Toxoplasmosis Incidence Based on Healthcare Claims Data, Germany, 2011–2016
These numbers are likely underestimates. Most maternal infections produce no symptoms, so cases are caught only when screening is performed or when fetal abnormalities prompt investigation. In countries without routine screening, the true incidence almost certainly exceeds reported figures. Seroprevalence data, which shows how many women have already been exposed, is easier to obtain but tells a different story: it reflects cumulative lifetime exposure rather than acute risk during any given pregnancy. A high seroprevalence in a region can actually mean fewer susceptible women, since those already infected are immune. The countries with the worst congenital toxoplasmosis burden tend to combine moderate seroprevalence (enough susceptible women remain) with high environmental exposure and limited access to prenatal care.