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PubMed This is a summary of 9 peer-reviewed journal articles Updated
Obstetrics

How the Timing of Infection Affects Your Baby

At a Glance

The timing of a toxoplasmosis infection during pregnancy determines the risk to your baby. Early in pregnancy, transmission risk is low but potential complications are severe. Late in pregnancy, the infection is highly likely to pass to the baby, but symptoms are usually mild or absent at birth.

One of the most important factors in understanding your baby’s health is the timing of the infection. When it comes to Toxoplasma gondii, the risk of passing the infection to the baby and the severity of the baby’s symptoms follow two very different paths. Doctors look closely at your gestational age—how many weeks pregnant you were at the time of infection—to determine the best care plan [1][2].

The Inverse Relationship

There is a unique “seesaw” effect between how likely a baby is to get the infection and how serious that infection might be. This is known as an inverse relationship [2].

  • Risk of Transmission: This refers to how likely the parasite is to cross the placenta and reach the baby. This risk increases as your pregnancy progresses [3].
  • Clinical Severity: This refers to how much the infection might affect the baby’s development. This risk decreases as your pregnancy progresses [1].
Trimester of Infection Risk of Passing it to Baby Typical Severity if Infected
First Trimester Low (roughly 5%–15%) [4][3] Higher risk of severe complications [5]
Second Trimester Moderate (roughly 23%–25%) [4] Variable; moderate risk [5]
Third Trimester High (roughly 60%–70%) [3][4] Often mild or no symptoms at birth [2]

First-Trimester Infections (Weeks 1–12)

In the early stages of pregnancy, the placenta is a very effective barrier, making it difficult for the parasite to reach the baby [3]. However, because the baby’s major organs and systems are just beginning to form, an infection during this time can be more disruptive [1]. If the parasite does cross the placenta in the first trimester, there is a higher risk of neurological sequelae (conditions affecting the brain or nervous system) or ocular sequelae (conditions affecting the eyes) [5].

Third-Trimester Infections (Weeks 28–Birth)

By the third trimester, the placenta is much more permeable, meaning it is much easier for the parasite to pass through to the baby [3]. While most babies infected during this time are born with “subclinical” infections—meaning they appear healthy and show no symptoms at birth—they still require medical follow-up to ensure they do not develop issues, such as eye inflammation, later in life [2][6].

The Importance of Timing

Knowing exactly when the infection occurred helps your doctors decide which medication to use and how often to monitor the baby [7].

  • Dating the Infection: Doctors use tests like IgG avidity to look at how strongly your antibodies are binding to the parasite. This helps them estimate if the infection is recent or occurred several months ago [8].
  • Early Intervention: Regardless of the timing, starting treatment as soon as possible is the most effective way to reduce the risk of transmission and protect the baby’s health [9][4].

Understanding these patterns can help you have a more focused conversation with your medical team about what to expect and how to best support your baby’s development [7].

Common questions in this guide

How does the trimester of a toxoplasmosis infection affect my baby?
The timing of the infection creates an inverse relationship between transmission risk and severity. Infections early in pregnancy are less likely to pass to the baby but can cause more severe complications, while late-pregnancy infections are highly likely to pass but usually cause milder symptoms.
What is the risk of my baby getting toxoplasmosis if I am infected in the first trimester?
In the first trimester, the placenta acts as a strong barrier, so the risk of passing the parasite to your baby is low, roughly 5 to 15 percent. However, if the infection does cross the placenta early on, the risk of serious neurological or eye complications is higher.
What happens if I get toxoplasmosis in the third trimester?
By the third trimester, the placenta is more permeable, making the risk of passing the infection to your baby high, at roughly 60 to 70 percent. Fortunately, most babies infected late in pregnancy show no symptoms at birth, though they still require medical monitoring to prevent later issues.
How do doctors know when I was infected with toxoplasmosis?
Doctors use specific blood tests, such as an IgG avidity test, to estimate when the infection occurred. This test measures how strongly your antibodies bind to the parasite, helping determine if the infection is recent or from several months ago.
Does the timing of the infection change how congenital toxoplasmosis is treated?
Yes, knowing your gestational age at the time of infection helps your medical team choose the right medication and decide how often to monitor your baby. Regardless of timing, starting treatment as early as possible is the best way to reduce risks.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Based on my antibody tests (specifically the IgG avidity), at what gestational week do you estimate I was infected?
  2. 2.Given the timing of my infection, what is the statistical likelihood that the parasite crossed the placenta?
  3. 3.If the baby was infected during this specific trimester, what are the most likely symptoms or health concerns we should be looking for?
  4. 4.How does the timing of my infection change our treatment plan or the choice of medication?
  5. 5.Does the timing of this infection mean we should increase the frequency of our fetal monitoring or ultrasounds?

Questions For You

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References

References (9)
  1. 1

    Ocular Findings in Infants with Congenital Toxoplasmosis after a Toxoplasmosis Outbreak.

    Conceição AR, Belucik DN, Missio L, et al.

    Ophthalmology 2021; (128(9)):1346-1355 doi:10.1016/j.ophtha.2021.03.009.

    PMID: 33711379
  2. 2

    Is Toxoplasma gondii type related to clinical outcome in human congenital infection? Systematic and critical review.

    Rico-Torres CP, Vargas-Villavicencio JA, Correa D

    European journal of clinical microbiology & infectious diseases : official publication of the European Society of Clinical Microbiology 2016; (35(7)):1079-88 doi:10.1007/s10096-016-2656-2.

    PMID: 27146878
  3. 3

    Treatment of Acute Toxoplasmosis in Pregnancy: Influence in the Mother-to-Child Transmission.

    Bartholo BBGR, Monteiro DLM, Rodrigues NCP, et al.

    Journal of obstetrics and gynaecology Canada : JOGC = Journal d'obstetrique et gynecologie du Canada : JOGC 2020; (42(12)):1505-1510 doi:10.1016/j.jogc.2020.04.021.

    PMID: 32912728
  4. 4

    Congenital toxoplasmosis: an observational retrospective study in the Eastern Sicily.

    Garozzo MT, Garozzo R, Betta P, et al.

    Frontiers in pediatrics 2025; (13()):1597001 doi:10.3389/fped.2025.1597001.

    PMID: 40656197
  5. 5

    Congenital Toxoplasmosis: A Review.

    Hampton MM

    Neonatal network : NN 2015; (34(5)):274-8 doi:10.1891/0730-0832.34.5.274.

    PMID: 26802827
  6. 6

    A Proinflammatory Immune Response Might Determine Toxoplasma gondii Vertical Transmission and Severity of Clinical Features in Congenitally Infected Newborns.

    Gómez-Chávez F, Cañedo-Solares I, Ortiz-Alegría LB, et al.

    Frontiers in immunology 2020; (11()):390 doi:10.3389/fimmu.2020.00390.

    PMID: 32231666
  7. 7

    One severe case of congenital toxoplasmosis in China with good response to azithromycin.

    Li J, Zhao J, Yang X, et al.

    BMC infectious diseases 2021; (21(1)):920 doi:10.1186/s12879-021-06619-1.

    PMID: 34488656
  8. 8

    IgG Avidity Test in Congenital Toxoplasmosis Diagnoses in Newborns.

    Fonseca ZC, Rodrigues IMX, Melo NCE, et al.

    Pathogens (Basel, Switzerland) 2017; (6(2)) doi:10.3390/pathogens6020026.

    PMID: 28629167
  9. 9

    Toxoplasmosis: A Timeless Challenge for Pregnancy.

    Damar Çakırca T, Can İN, Deniz M, et al.

    Tropical medicine and infectious disease 2023; (8(1)) doi:10.3390/tropicalmed8010063.

    PMID: 36668970

This page explains how the timing of a toxoplasmosis infection affects pregnancy for educational purposes only. Always consult your maternal-fetal medicine specialist or obstetrician for advice regarding your specific pregnancy and test results.

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