Toxoplasmosis during pregnancy can pass from mother to fetus through the placenta, potentially causing serious brain and eye damage in the baby. The good news: the infection is rare in the United States, affecting roughly 2 in every 10,000 pregnancies, and the risk of severe harm depends heavily on when during pregnancy the infection occurs. Most pregnant women who were previously infected carry no risk to their baby at all.
Why Timing Changes Everything
The relationship between gestational age and toxoplasmosis follows a counterintuitive pattern. Early in pregnancy, the parasite is less likely to reach the fetus, but if it does, the damage tends to be severe. Later in pregnancy, the parasite crosses the placenta much more easily, but the baby is better equipped to handle it.
In untreated women, the transmission rate is approximately 25% in the first trimester, 54% in the second trimester, and 65% in the third trimester. One European study found an even wider gap: women infected at 12 weeks transmitted the infection just 9% of the time, compared to 83% for those infected at 40 weeks. So while a third-trimester infection is far more likely to reach the baby, a first-trimester infection that does get through is far more likely to cause lasting harm.
How the Infection Affects the Baby
When the parasite reaches the fetus, it travels through the blood to target the brain, eyes, and other organs. The active form of the parasite has a particular affinity for nervous tissue, which is why the most serious consequences involve the brain and vision.
Severe congenital toxoplasmosis can cause fluid buildup in the brain (hydrocephalus), calcium deposits in brain tissue, and inflammation of the retina. Seizures, abnormal head size, and eye problems like crossed eyes or unusually small eyes can also occur. This full picture of severe disease appears in a limited number of infected newborns, but when it does, the effects are significant.
What makes congenital toxoplasmosis especially tricky is that many infected babies look perfectly healthy at birth. In one Brazilian study, 88% of children with congenital toxoplasmosis showed no obvious signs of disease as newborns. Yet half of those children went on to develop neurological problems over time, and 77% eventually showed abnormalities on brain imaging. A North American study of 24 untreated children who appeared fine at birth found that 40% developed serious neurological consequences during follow-up, including eye disease, intellectual disability, and cognitive impairment. Retinal inflammation (retinochoroiditis) is the single most common long-term complication, sometimes not appearing until years after birth.
Who Is at Risk
Only a first-time infection during pregnancy poses a threat. If you were infected before becoming pregnant, your immune system has already built defenses that protect the baby. In the United States, about 9% of women of childbearing age have been previously infected, meaning roughly 91% are susceptible to a new infection. That susceptibility rate has actually increased over time as seroprevalence has dropped from 15% in the late 1980s to 9% by 2010, likely due to changes in food handling and less exposure to the parasite overall.
Despite most women being susceptible, actual new infections during pregnancy remain uncommon, estimated at about 0.2 per 1,000 pregnancies.
How Infection Is Detected
Toxoplasmosis in the mother rarely causes noticeable symptoms. When it does, it typically feels like a mild flu with swollen lymph nodes. Diagnosis relies on blood tests that measure two types of antibodies.
The basic screening looks for IgG and IgM antibodies. IgG antibodies indicate you’ve been exposed at some point. IgM antibodies suggest a more recent infection, but they’re unreliable on their own because IgM can persist in the blood for months or even years after the initial infection. Simply testing positive for IgM does not mean you were recently infected. Adding an IgA antibody test improves accuracy considerably. In one study, 67% of women who tested positive for both IgM and IgA had a recent infection, compared to only 6% of women with IgM alone.
The most useful follow-up test measures IgG avidity, which essentially checks how mature your antibodies are. High-avidity antibodies indicate your infection happened more than four months ago, effectively ruling out a pregnancy-related risk. Low or borderline avidity suggests the infection may be recent and warrants further investigation.
If blood tests suggest a recent maternal infection, amniocentesis with PCR testing can determine whether the parasite has reached the fetus. This test is most accurate when performed at least 4 weeks after the suspected maternal infection and no earlier than 18 weeks of gestation. When done within five weeks of the mother’s diagnosis, PCR testing of amniotic fluid has a sensitivity of 87% and a specificity of 99%, meaning it catches most infections and very rarely gives a false positive.
Treatment During Pregnancy
Treatment depends on the stage of pregnancy and whether the fetus is infected. For infections acquired and diagnosed before 18 weeks, when fetal infection hasn’t been confirmed, the goal is to prevent the parasite from crossing the placenta. The medication used at this stage works to reduce transmission and is most effective when started within eight weeks of the mother’s initial infection.
If the infection is acquired at or after 18 weeks, or if fetal infection is confirmed or suspected, a different combination of medications targets the parasite more aggressively. Infected newborns are typically treated for a full 12 months after birth. Early and sustained treatment in infancy is important precisely because of the high rate of delayed complications in untreated children who seemed healthy at birth.
How You Get Infected
Despite the common association with cats, undercooked meat is actually a major source of infection. The parasite forms cysts in animal muscle tissue that survive unless the meat is properly cooked or frozen. You can also pick it up from contaminated soil, unwashed produce, untreated water, or raw shellfish.
Cats play a unique role because they’re the only animal in which the parasite completes its reproductive cycle. An infected cat sheds the parasite in its feces, but there’s a critical detail: the parasite doesn’t become infectious until one to five days after being shed. This means daily litter box cleaning effectively eliminates the risk from that route.
Practical Prevention Steps
You do not need to give up your cat during pregnancy. The key precautions are straightforward:
- Cook meat thoroughly. Use a food thermometer. Whole cuts of meat (not poultry) need to reach at least 145°F (63°C) with a three-minute rest before cutting. Ground meat needs 160°F (71°C). All poultry needs 165°F (74°C). Color and texture alone are not reliable indicators.
- Freeze meat before cooking. Freezing at 0°F for several days greatly reduces the chance of viable parasites, though cooking to proper temperature remains the safest approach.
- Wash produce. Rinse all fruits and vegetables under running water before eating.
- Avoid raw shellfish. Oysters, mussels, and clams can be contaminated with the parasite from runoff into seawater.
- Wear gloves when gardening. Soil and sand may contain cat feces with infectious parasite eggs. Wash hands thoroughly afterward.
- Have someone else change the litter box. If that’s not possible, change it daily and wear gloves. Daily cleaning prevents the parasite from reaching its infectious stage.
- Keep cats indoors and feed them commercial food. Cats become infected by hunting prey or eating raw meat. An indoor cat eating commercial food has minimal risk of carrying the parasite.
- Skip unpasteurized goat’s milk. It can carry the parasite.
Cover outdoor sandboxes to keep cats from using them as litter boxes, and avoid drinking untreated water from streams or other natural sources.