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PubMed This is a summary of 18 peer-reviewed journal articles Updated
Obstetrics

Diagnosis: Maternal Serology, Fetal Ultrasounds, and PCR Testing

At a Glance

Congenital toxoplasmosis is diagnosed in three phases: checking the mother's blood to date the infection, using amniocentesis PCR and ultrasounds to monitor the fetus, and testing the newborn after birth. An amniocentesis must be done at least four weeks post-infection for accuracy.

Navigating the diagnosis of toxoplasmosis involves a series of tests designed to answer two main questions: When did the mother get infected? and Did the infection reach the baby? To get these answers, doctors follow a diagnostic path that moves from the mother’s blood to the baby’s environment, and finally to the newborn [1][2].

Phase 1: Maternal Testing (The Bloodwork)

The first step is checking the parent’s blood for antibodies, which are proteins the immune system makes to fight the parasite [1].

  • IgG and IgM Antibodies: These are the first markers checked. While IgM usually appears first during a new infection, it can stay in your system for years, so its presence doesn’t always mean the infection is “new” [3][4].
  • IgG Avidity: This is a critical follow-up test. Avidity measures how “sticky” or strong the bond is between your IgG antibodies and the parasite [5].
    • High Avidity: This generally means the infection happened at least 3 to 4 months ago (likely before pregnancy) [2].
    • Low Avidity: This suggests a more recent infection, possibly occurring during pregnancy [6].

Phase 2: Fetal Testing (During Pregnancy)

If maternal tests suggest a recent infection, the focus shifts to checking the baby’s health while they are still in the womb.

  • Amniotic Fluid PCR: This is a test performed via amniocentesis (using a thin needle to take a small sample of the fluid surrounding the baby). This is typically performed after 15 weeks of pregnancy and at least 4 weeks after the suspected time of infection [7][8]. Waiting at least 4 weeks is critical; testing too soon can result in a false negative, providing false reassurance. A PCR (Polymerase Chain Reaction) test looks for the actual DNA of the parasite in that fluid to confirm if the baby has been infected [9][10].
  • Targeted Ultrasounds: Doctors use high-resolution ultrasounds to look for specific physical signs:
    • Intracranial Calcifications: These are small, bright spots of calcium in the baby’s brain. While they sound scary, their impact depends on their size and location [11][12].
    • Hydrocephalus (Ventriculomegaly): This is a buildup of fluid in the brain’s “ventricles” (open spaces). Monitoring these spaces helps doctors assess the baby’s neurological development [11][13].

Phase 3: Newborn Testing (After Birth)

Even if tests during pregnancy were negative, all babies at risk receive a thorough check-up after birth to be certain [14].

  • Blood and CSF Tests: The baby will have their own blood tested for IgG, IgM, and IgA antibodies. In some cases, a sample of Cerebrospinal Fluid (CSF) may be tested via PCR to check the central nervous system [15][16].
  • Placental Testing: Examining the placenta after delivery (via pathology or PCR) is a common and non-invasive way doctors look for evidence that the parasite crossed over to the baby [17].
  • Eye Exams (Fundoscopy): Because the parasite can affect the eyes, a pediatric ophthalmologist will perform a specialized exam to check for retinochoroiditis (inflammation of the back of the eye) [14][18].
  • Brain Imaging: A postnatal ultrasound or CT scan may be done to get a clearer picture of the brain than was possible before birth [11][13].

Diagnostic Checklist

Ensure these steps are discussed with your care team:

  1. [ ] Maternal Serology: IgG, IgM, and IgG Avidity completed.
  2. [ ] Infection Dating: Discussion with a specialist about when the infection likely occurred.
  3. [ ] Amniocentesis Timing: Verified that the procedure is scheduled at least 4 weeks after the suspected infection date.
  4. [ ] Fetal Anatomy Scan: Specialized ultrasound to check the brain and organs.
  5. [ ] Pediatric Plan: Scheduling a specialist eye exam and brain imaging for the baby after birth.

Common questions in this guide

What does IgG avidity mean on my toxoplasmosis blood test?
IgG avidity measures how strong the bond is between your antibodies and the parasite. High avidity means the infection likely happened months ago, usually before pregnancy. Low avidity suggests a more recent infection that may have occurred during pregnancy.
When should an amniocentesis be done to test for toxoplasmosis?
An amniocentesis for toxoplasmosis is typically performed after 15 weeks of pregnancy and at least four weeks after the suspected time of infection. Waiting at least four weeks is critical because testing too soon can result in a false negative.
What will doctors look for on my fetal ultrasound?
Doctors use high-resolution ultrasounds to check the baby for specific physical signs of infection. They primarily look for intracranial calcifications, which are small spots of calcium in the brain, and hydrocephalus, which is a buildup of fluid in the brain's ventricles.
Will my baby be tested for toxoplasmosis after birth?
Yes, even if your pregnancy tests were negative, all at-risk babies receive a thorough evaluation after delivery. This includes testing the baby's blood, checking the placenta, getting specialized eye exams, and sometimes performing brain imaging to ensure the parasite did not affect the newborn.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Was my IgG avidity 'high' or 'low', and what does that tell us about when I was infected?
  2. 2.Based on my results, is an amniocentesis necessary to confirm if the infection has reached the baby?
  3. 3.Can we review the ultrasound images together—are there any signs of intracranial calcifications or enlarged ventricles?
  4. 4.What specific blood tests (IgG, IgM, IgA, or PCR) will be run on the baby immediately after birth?
  5. 5.When should our first appointment with a pediatric ophthalmologist be scheduled to check the baby's eyes?

Questions For You

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References

References (18)
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    Frequency of anti- Toxoplasma gondii IgA, IgM, and IgG antibodies in high-risk pregnancies, in Brazil.

    Murata FH, Ferreira MN, Camargo NS, et al.

    Revista da Sociedade Brasileira de Medicina Tropical 2016; (49(4)):512-4.

    PMID: 27598642
  2. 2

    Evaluation of Toxoplasma gondii IgG avidity assays through a comparison of IgM serostatus.

    Ikuta K, Kanno R, Bessho T, et al.

    Diagnostic microbiology and infectious disease 2023; (105(4)):115901 doi:10.1016/j.diagmicrobio.2023.115901.

    PMID: 36889215
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    Role of Toxoplasma IgA as Part of a Reference Panel for the Diagnosis of Acute Toxoplasmosis during Pregnancy.

    Olariu TR, Blackburn BG, Press C, et al.

    Journal of clinical microbiology 2019; (57(2)) doi:10.1128/JCM.01357-18.

    PMID: 30463899
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    Anti-Toxoplasma gondii IgM Long Persistence: What Are the Underlying Mechanisms?

    Vargas-Villavicencio JA, Cañedo-Solares I, Correa D

    Microorganisms 2022; (10(8)) doi:10.3390/microorganisms10081659.

    PMID: 36014077
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    Toxoplasma gondii-specific IgG avidity testing in pregnant women.

    Garnaud C, Fricker-Hidalgo H, Evengård B, et al.

    Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases 2020; (26(9)):1155-1160 doi:10.1016/j.cmi.2020.04.014.

    PMID: 32334096
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    IgG avidity and placenta real-time PCR in detection of active maternal toxoplasmosis: relation to pregnancy outcomes.

    Allam AF, Shehab AY, Ahmed NA, et al.

    Experimental parasitology 2025; (279()):109065 doi:10.1016/j.exppara.2025.109065.

    PMID: 41260562
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    Associations between Fetal Symptoms during Pregnancy and Neonatal Clinical Complications with Toxoplasmosis.

    Tűzkő N, Bartek V, Simonyi A, et al.

    Children (Basel, Switzerland) 2024; (11(9)) doi:10.3390/children11091111.

    PMID: 39334643
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    Management of suspected primary Toxoplasma gondii infection in pregnant women in Norway: twenty years of experience of amniocentesis in a low-prevalence population.

    Findal G, Helbig A, Haugen G, et al.

    BMC pregnancy and childbirth 2017; (17(1)):127 doi:10.1186/s12884-017-1300-1.

    PMID: 28441952
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    Seronegative ocular toxoplasma panuveitis in an immunocompetent patient.

    Sigle M, El Atrouni W, Ajlan RS

    American journal of ophthalmology case reports 2020; (19()):100745 doi:10.1016/j.ajoc.2020.100745.

    PMID: 32566798
  10. 10

    Congenital toxoplasmosis: An overview of the neurological and ocular manifestations.

    Khan K, Khan W

    Parasitology international 2018; (67(6)):715-721 doi:10.1016/j.parint.2018.07.004.

    PMID: 30041005
  11. 11

    Patterns of Hydrocephalus Caused by Congenital Toxoplasma gondii Infection Associate With Parasite Genetics.

    Hutson SL, Wheeler KM, McLone D, et al.

    Clinical infectious diseases : an official publication of the Infectious Diseases Society of America 2015; (61(12)):1831-4 doi:10.1093/cid/civ720.

    PMID: 26405147
  12. 12

    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
  13. 13

    Outcomes of hydrocephalus secondary to congenital toxoplasmosis.

    McLone D, Frim D, Penn R, et al.

    Journal of neurosurgery. Pediatrics 2019; (24(5)):601-608 doi:10.3171/2019.6.PEDS18684.

    PMID: 31491752
  14. 14

    Late diagnosis of congenital toxoplasmosis based on serological follow-up: A case report.

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    Parasitology international 2017; (66(2)):186-189 doi:10.1016/j.parint.2016.12.004.

    PMID: 27956093
  15. 15

    Efficacy of amniotic fluid, blood and urine samples for the diagnosis of toxoplasmosis in pregnant women candidates for amniocentesis using serological and molecular techniques.

    Abedian R, Esboei BR, Kordi S, et al.

    BMC pregnancy and childbirth 2024; (24(1)):771 doi:10.1186/s12884-024-06979-x.

    PMID: 39578753
  16. 16

    Employing digital PCR for enhanced detection of perinatal Toxoplasma gondii infection: A cross-sectional surveillance and maternal-infant outcomes study in El Salvador.

    Lynn MK, Rodriguez Aquino MS, Cornejo Rivas PM, et al.

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  17. 17

    Mammalian placental explants: A tool for studying host-parasite interactions and placental biology.

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  18. 18

    Prognosis of Fetal Parenchymal Cerebral Lesions without Ventriculomegaly in Congenital Toxoplasmosis Infection.

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    PMID: 27093552

This page explains diagnostic tests for congenital toxoplasmosis for educational purposes only. Always consult your maternal-fetal medicine specialist or pediatrician for medical advice regarding your specific test results.

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