Toxoplasmosis blood test: understanding your serology results during pregnancy

Why is a blood test done for toxoplasmosis?

In France, toxoplasmosis serology is part of the mandatory biological tests prescribed at the first prenatal consultation. This obligation is established in the decree of February 14, 1992 regarding mandatory prenuptial, prenatal and postnatal examinations, and confirmed by the French National Authority for Health (HAS) recommendations.

The purpose of this blood test is simple: to determine whether you have already been in contact with the Toxoplasma gondii parasite and whether you are therefore protected (immune) or not. This information is crucial because a primary toxoplasmosis infection during pregnancy can have consequences for fetal development. Early screening allows for appropriate monitoring and, if necessary, rapid treatment.

To learn everything about toxoplasmosis during pregnancy (symptoms, modes of transmission, prevention), see our complete guide on toxoplasmosis and pregnancy.

France is one of the few countries in the world to require this systematic screening. This public health policy has significantly reduced the incidence of congenital toxoplasmosis, now estimated at 2.5 per 10,000 births according to the French Institute for Public Health Surveillance (InVS).

When to have the toxoplasmosis serology?

The toxoplasmosis serology schedule depends on your immune status. Here is the protocol recommended by HAS and CNGOF:

First blood test: before 14 weeks

Toxoplasmosis serology is prescribed at the first prenatal consultation, ideally before 14 weeks of amenorrhea (WA). This first blood test determines your immune status and guides all subsequent monitoring.

If you are immune: no further testing

If the result shows positive IgG and negative IgM, the matter is settled. You are protected by longstanding immunity and no additional blood test for toxoplasmosis is needed throughout the pregnancy.

If you are not immune: monthly monitoring

If the result shows negative IgG and IgM, a serological check will be performed every month until delivery. A final check is also recommended one month after delivery to detect possible late seroconversion in the third trimester.

Understanding the results: IgG and IgM

Toxoplasmosis serology is based on the search for two types of antibodies in your blood. Understanding the difference between the two is essential for interpreting your results.

IgG (Immunoglobulin G): memory antibodies

IgG are antibodies produced by your immune system several weeks after the first contact with the parasite. They remain present in the blood for life and constitute the immune memory of your body. Their presence means you have already encountered Toxoplasma gondii in the past and your body is capable of defending itself against a new exposure.

The IgG level is expressed in IU/mL (international units per milliliter). The positivity threshold varies between laboratories, but a result is generally considered positive above 8 to 10 IU/mL. A high IgG level does not mean a more severe or more recent infection: it simply reflects the intensity of the past immune response.

IgM (Immunoglobulin M): alert antibodies

IgM are the first antibodies produced by the immune system during an infection. They appear quickly, within the first days following contact with the parasite, and reach a peak in 1 to 2 months. Unlike IgG, IgM eventually disappear, but this process can take several months, sometimes more than a year.

It is this particular kinetics that sometimes makes interpretation tricky. The presence of IgM does not necessarily mean a very recent infection: it may be residual IgM from an infection dating back several months. This is why the IgG avidity test is often necessary to clarify the timeline.

Results interpretation table

Here is a summary table of the four possible combinations of toxoplasmosis serology results. In France, approximately 45% of women of childbearing age are immune (Santé Publique France), meaning 55% discover during their first pregnancy that they are not.

If your result does not clearly match one of these four cases (result marked as “equivocal” or “doubtful” by the laboratory), a follow-up check at 2-3 weeks will be prescribed to observe the evolution of antibody levels.

Case 1: Not immune (IgG- IgM-)

This is the most common result: approximately 55% of pregnant women in France are not immune to toxoplasmosis. This simply means you have never been in contact with the Toxoplasma gondii parasite. This is neither abnormal nor worrying in itself, but it requires specific monitoring.

What this means in practice

• Monthly blood test: a toxoplasmosis serology will be performed every month to detect possible seroconversion as early as possible. This regular monitoring is your best ally: it allows rapid intervention if an infection occurs.
• Dietary precautions: you will need to adapt your diet throughout the pregnancy. The main foods to avoid are raw or rare meat, raw cured meats (dry sausage, cured ham), and poorly washed soil-grown fruits and vegetables. See our guide on prohibited foods for toxoplasmosis for the complete list.
• Precautions with cats: if you have a cat, have someone else clean its litter box and wash your hands after petting it. The risk is low, especially for an apartment cat, but caution is warranted. Our article toxoplasmosis and cats during pregnancy details all precautions.

Case 2: Immune (IgG+ IgM-)

If your IgG is positive and your IgM is negative, this is good news. This result means you contracted toxoplasmosis in the past, probably without even realizing it (the infection is asymptomatic in 80 to 90% of cases), and your immune system has developed lasting protection.

What this means for your pregnancy

• No risk of congenital toxoplasmosis: your immunity prevents the parasite from multiplying and crossing the placenta. Your baby is protected.
• No monthly blood test for toxoplasmosis. The first result is sufficient.
• No dietary restrictions related to toxoplasmosis: you can eat dry sausage, cured ham, carpaccio, and rare meat without risk regarding this parasite.

Immunity against toxoplasmosis is considered permanent. Cases of reinfection in healthy individuals are exceptional and documented only in severely immunosuppressed patients. For a healthy pregnant woman, the acquired immunity is reliable.

Case 3: Seroconversion (IgG+ IgM+)

When both IgG and IgM are positive, the situation requires particular attention. This result can correspond to two very different scenarios:

• A recent infection (seroconversion during pregnancy) — this is the scenario we seek to identify quickly.
• An old infection with residual IgM — IgM can persist for several months, even more than a year after the initial infection. This scenario is actually the most common and has no consequences for the pregnancy.

The IgG avidity test: the key examination

To distinguish between these two situations, the laboratory performs an IgG avidity test. This test measures the “maturity” of IgG antibodies, meaning the strength with which they bind to the parasite.

• High avidity (above 30-40% depending on techniques): the infection dates back more than 4 months. If you are in the first trimester and avidity is high, this means the infection predates conception. This is reassuring: you are immune and there is no risk to the baby.
• Low avidity (below 20%): the infection is probably recent (less than 4 months). This is the situation that requires urgent management.
• Intermediate avidity (gray zone): the result is not conclusive. Close monitoring and additional tests will be necessary.

Management in case of confirmed recent infection

If low avidity confirms a recent infection, your doctor will implement the following protocol:

1. Spiramycin (Rovamycine) treatment: prescribed immediately, this antibiotic reduces the risk of the parasite crossing the placenta by approximately 60%.
2. Amniocentesis: offered from 18 WA and at least 4 weeks after the estimated date of infection. PCR analysis of amniotic fluid looks for the parasite's DNA.
3. If PCR is negative: spiramycin is continued until delivery and ultrasound monitoring is intensified.
4. If PCR is positive: treatment is changed (pyrimethamine-sulfadiazine combination) and follow-up takes place at a specialized reference center.

Monthly monitoring when not immune

If you are among the 55% of non-immune women, the monthly blood test will become a regular appointment throughout your pregnancy. Here is what you can expect.

The practical process

Toxoplasmosis serology is a simple venous blood draw, performed at the medical laboratory. It is prescribed each month by your doctor or midwife, along with other pregnancy monitoring tests. Results are generally available within 2 to 5 business days.

Waiting for results

Let's be honest: waiting each month for the serology result can be a source of stress. This is a perfectly normal feeling, shared by very many pregnant women. Here are some elements to reassure you:

• The risk of seroconversion is low: approximately 2,500 seroconversions per year are estimated among pregnant women in France, out of more than 700,000 pregnancies. This represents less than 0.4% of pregnancies.
• Dietary precautions are effective: by following the rules for cooking meat and washing fruits and vegetables, you considerably reduce the risk of contamination.
• Screening is there to protect you: if seroconversion occurs, it will be detected quickly thanks to monthly monitoring, enabling early and effective management.

The final check after delivery

A final serological check is recommended one month after delivery. This test detects possible seroconversion occurring at the very end of pregnancy, which would not have been visible on the last prenatal test. If this check is negative, serological monitoring is permanently complete.

Toxoplasmosis and breastfeeding

Good news for breastfeeding mothers: there is no risk of toxoplasmosis transmission through breast milk. The Toxoplasma gondii parasite does not pass into breast milk, whether you are immune or not, and whether you experienced seroconversion during pregnancy or not.

Breastfeeding is therefore perfectly safe regarding toxoplasmosis. No special precautions are necessary. Note that this safety also applies to women who received spiramycin treatment during pregnancy: spiramycin passes in small amounts into breast milk but has no known harmful effect on the infant.

However, if you are not immune and breastfeeding, continue to follow the usual dietary precautions for yourself, at least until the postpartum check result (one month after delivery). After this final check, if the serology remains negative, restrictions related to toxoplasmosis are no longer necessary.

Our expert's opinion

“Monthly toxoplasmosis screening for non-immune women is one of the pillars of prenatal care in France. I understand that this repeated blood test may seem burdensome, even anxiety-inducing. But it is precisely this vigilance that allows us to intervene very early in case of seroconversion and achieve excellent results. In practice, I recommend my patients not to worry while waiting for results: the risk of seroconversion is statistically very low when dietary precautions are followed. And even if seroconversion occurs, current management — spiramycin, avidity test, amniocentesis if needed — is well established and effective. Trust the screening system, follow the three simple rules (cook meat thoroughly, wash vegetables well, delegate the cat litter box) and enjoy your pregnancy with peace of mind.”

— Dr. Elie Servan-Schreiber, physician