Congenital toxoplasmosis: when the parasite reaches the baby

What is congenital toxoplasmosis?

Congenital toxoplasmosis is what happens when a woman catches Toxoplasma gondii for the first time during pregnancy, or shortly before it, and the parasite crosses the placenta into the baby. It is not passed on by an infection the mother had years earlier: by then the parasite is walled up in dormant cysts and is no longer circulating in the blood⁵. That distinction is the whole of the risk, and it is explained in more detail on the pregnancy page.

How likely is the baby to be infected?

The chance that the parasite reaches the baby rises steeply through pregnancy, while the harm it does falls just as steeply. An infection caught in the first trimester is transmitted in roughly one case in six, but when it is transmitted the consequences are most severe. An infection caught near term is transmitted in more than half of cases, yet most of those babies look entirely normal at birth⁴⁵. This inverse relationship is the single most important thing to understand about congenital toxoplasmosis, and it is why the timing of the mother’s infection matters more than the fact of it.

What it looks like in the baby

Most infected newborns have no visible signs at all. The classic triad described in older textbooks, hydrocephalus, calcifications in the brain and inflammation at the back of the eye, is real but uncommon, and describes the most severely affected end of the spectrum rather than the typical case⁵.

The problem is that “no signs at birth” does not mean “no consequences”. The commonest late manifestation is retinochoroiditis, inflammation of the retina and choroid, which can appear in childhood or adolescence in a child who was asymptomatic as a baby. That is the reason treated children are followed with eye examinations for years⁴. The eye disease itself is covered on the ocular toxoplasmosis page.

How it is diagnosed

Before birth

When a mother’s first infection during pregnancy is confirmed, the question becomes whether the parasite has reached the baby. The main tool is a PCR test for parasite DNA in the amniotic fluid, performed after about 18 weeks of gestation. A positive result establishes fetal infection; a negative result makes it unlikely but does not exclude it entirely⁴. Ultrasound is used alongside, looking for hydrocephalus or intracranial calcification.

After birth

In the newborn, the mother’s IgG antibodies have crossed the placenta and will be present whether or not the baby is infected, so IgG alone proves nothing. What counts is the baby making its own antibodies, detected as IgM or IgA, and IgG that persists beyond about twelve months instead of fading as the maternal antibodies clear⁵. Examination of the eyes, a neurological assessment and brain imaging complete the picture.

Treatment, and how well it works

Two separate questions arise. The first is whether treating the mother prevents transmission. Spiramycin has been used for this for decades, and the French randomized trial comparing it with pyrimethamine plus sulfadiazine found that the combination reduced placental transmission more effectively³. The second is whether treatment improves the outcome for babies already infected. Here the honest answer is that the evidence is weaker than the confidence with which it is often stated: a meta-analysis of individual patient data from 26 European cohorts found no strong evidence that starting prenatal treatment earlier reduced the risk of clinical manifestations².

That uncertainty is about prenatal treatment of the mother. Treatment of the infected infant after birth, typically pyrimethamine and sulfadiazine with folinic acid continued through the first year, is standard practice and is supported by long-term follow-up of treated cohorts⁴. Which drugs are used, and why none of them clears the dormant stage, is explained on the treatment page.

What this means if you are pregnant

• A positive IgG test from before your pregnancy is reassuring, not alarming. It means the risk does not apply to you.
• A result suggesting a recent infection should be confirmed by a reference laboratory before any decision is taken. False positives are common, and the stakes of acting on one are high.
• If infection during pregnancy is confirmed, amniotic fluid PCR after 18 weeks is what establishes whether the baby is affected.
• An infected baby who looks normal at birth still needs long-term eye follow-up.
• Prevention works and is undramatic: cook meat through, wash produce, use gloves for gardening and the litter box.

Sources

1. Centers for Disease Control and Prevention. Toxoplasmosis. Accessed 15 October 2026.
2. Thiébaut R, Leproust S, Chene G, Gilbert R (SYROCOT Study Group). Effectiveness of prenatal treatment for congenital toxoplasmosis: a meta-analysis of individual patients’ data. Lancet 2007;369:115-22. PubMed · DOI
3. Mandelbrot L, Kieffer F, Sitta R et al. Prenatal therapy with pyrimethamine + sulfadiazine vs spiramycin to reduce placental transmission of toxoplasmosis: a multicenter, randomized trial. Am J Obstet Gynecol 2018;219:386.e1-386.e9. PubMed · DOI
4. McLeod R, Kieffer F, Sautter M, Hosten T, Pelloux H. Why prevent, diagnose and treat congenital toxoplasmosis? Mem Inst Oswaldo Cruz 2009;104:320-44. PubMed · DOI
5. Montoya JG, Liesenfeld O. Toxoplasmosis. Lancet 2004;363:1965-76. PubMed · DOI