Skip to content
How It Works
Explore
Resources Ask a Question
Who It's For
Patients Caregivers Providers
Log In Ask a Question
PubMed This is a summary of 18 peer-reviewed journal articles Updated
Obstetrics and Gynecology

Diagnostic Testing During Pregnancy

At a Glance

Diagnosing fetal CMV requires a step-by-step approach, starting with maternal IgG avidity blood tests. To confirm if the virus reached the baby, an amniocentesis is performed at least 6-8 weeks post-infection and after 21 weeks of pregnancy, followed by detailed ultrasound and MRI imaging.

Navigating diagnostic testing for CMV involves a series of steps designed to answer two main questions: Was the mother recently infected, and has the virus passed to the fetus? Because the virus takes time to move through the body and into the amniotic fluid, this process often requires a “wait and see” approach that can be difficult for parents [1][2].

Maternal Blood Testing: IgG Avidity

When a blood test shows CMV antibodies, the next step is often an IgG avidity test [3]. Avidity refers to how “sticky” or strong the bond is between the antibody and the virus [4].

  • Low Avidity: This suggests a primary infection (a first-time infection) occurred recently, typically within the last 3 to 4 months [4][5].
  • High Avidity: This usually indicates an old infection that occurred at least 3 to 6 months ago [4][5]. If high avidity is found in the first trimester, it often provides reassurance that a primary infection did not occur during the early, most vulnerable stages of pregnancy [6].

The Gold Standard: Amniocentesis

An amniocentesis is the most accurate way to determine if the virus has reached the fetus [7]. During this procedure, a small amount of amniotic fluid is collected and tested using a PCR (Polymerase Chain Reaction) test to look for CMV DNA [1].

Timing is critical for the accuracy of this test:

  • The 6-8 Week Lag: It takes approximately 6 to 8 weeks after the mother is infected for the virus to pass through the placenta, be excreted by the fetal kidneys, and appear in the amniotic fluid in detectable amounts [1][8]. Understanding this biology makes the wait more purposeful.
  • The 21-Week Mark: Doctors typically recommend waiting until at least 20 to 21 weeks of gestation to perform the amniocentesis [1][8]. Performing the test too early significantly increases the risk of a false negative result, where the test says the baby is uninfected even if the virus is present [7][2].

Understanding the Risks: It is important to know that an amniocentesis carries a small physical risk. Common side effects include mild cramping or a small amount of vaginal fluid leakage. There is also a very rare risk of miscarriage, generally estimated to be less than 1 in 300 to 1 in 900 [9][10]. Discuss these risks openly with your doctor to make an informed decision.

Ultrasound and Imaging Markers

Ultrasound is used to look for physical signs (markers) that the virus may be affecting the baby’s development. It is important to know that an infected fetus can have a completely normal ultrasound [11][12].

When signs are present, specialists look for:

  • Brain Findings: These include ventriculomegaly (enlargement of the fluid-filled spaces in the brain), small cysts, or calcifications (bright spots) [13][12].
  • Extracranial Findings: These may include echogenic bowel (the bowel appearing brighter than usual), microcephaly (small head size), or growth restriction [14][12].
  • Fetal MRI: Because ultrasound cannot see everything, a fetal MRI is often recommended in the late second or third trimester [11][15]. MRI is much more sensitive at detecting subtle changes in the brain’s white matter or “migration” (how brain cells move into place) [15][16].

Test Report Checklist

When reviewing your medical records, look for these specific details to ensure a complete evaluation:

Test Type What to Look For Why it Matters
Maternal Blood IgG Avidity Index (%) Determines if the infection is new or old [4].
Amniocentesis Quantitative PCR Provides the “viral load” (number of copies). Caveat: While viral load is a data point, normal ultrasound and MRI findings are far more predictive of a positive outcome than the viral load number alone. Do not panic over a high number if imaging is clear [17][18].
Ultrasound Neurosonogram A specialized, detailed look at the fetal brain structures [13].
Fetal MRI Cortical Development Checks if the brain is “folding” and developing correctly for its age [11].

Common questions in this guide

What does an IgG avidity test mean for CMV?
The IgG avidity test measures how strong the bond is between your antibodies and the CMV virus. Low avidity suggests a recent, first-time infection within the last few months, while high avidity usually indicates an older infection.
When is the best time to have an amniocentesis for CMV?
Doctors typically recommend waiting until at least 20 to 21 weeks of pregnancy, and at least 6 to 8 weeks after you were exposed to the infection. Testing too early can cause a false negative result, meaning the test misses the virus even though it is there.
Can my baby have CMV even if the ultrasound is normal?
Yes, a fetus with a CMV infection can have a completely normal ultrasound. Because ultrasound cannot see everything, doctors often recommend an amniocentesis and a fetal MRI to get a more accurate picture of the baby's health.
Why might I need a fetal MRI if I already had an ultrasound?
A fetal MRI is highly sensitive and can detect subtle changes in brain development that a standard ultrasound might miss. It is often recommended in the late second or third trimester to check if the brain's white matter and cells are developing correctly.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What was my IgG avidity score, and does it suggest a primary infection in the first trimester?
  2. 2.Is the amniocentesis scheduled at least 6 weeks after my suspected infection and after I reach 21 weeks of pregnancy?
  3. 3.Will the amniotic fluid be tested with a quantitative PCR to determine the viral load?
  4. 4.Are we doing a specialized 'fetal neurosonogram' or just a standard anatomy scan?
  5. 5.When is the ideal time for us to schedule a fetal MRI to look for findings ultrasound might miss?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (18)
  1. 1

    Amniocentesis for prenatal diagnosis of cytomegalovirus infection: challenging the 21 weeks' threshold.

    Enders M, Daiminger A, Exler S, Enders G

    Prenatal diagnosis 2017; (37(9)):940-942 doi:10.1002/pd.5107.

    PMID: 28677115
  2. 2

    Editorial Commentary: Amniocentesis for Detection of Congenital Cytomegalovirus Infection: What Is the Point?

    Schlesinger Y

    Clinical infectious diseases : an official publication of the Infectious Diseases Society of America 2016; (63(1)):39-40 doi:10.1093/cid/ciw238.

    PMID: 27114377
  3. 3

    Positive predictive values of CMV-IgM and importance of CMV-IgG avidity testing in detecting primary infection in three different clinical settings. A French retrospective cohort study.

    Périllaud-Dubois C, Bouthry E, Jadoui A, et al.

    Journal of clinical virology : the official publication of the Pan American Society for Clinical Virology 2020; (132()):104641 doi:10.1016/j.jcv.2020.104641.

    PMID: 32947202
  4. 4

    Cytomegalovirus infection in pregnancy - An update.

    Navti OB, Al-Belushi M, Konje JC,

    European journal of obstetrics, gynecology, and reproductive biology 2021; (258()):216-222 doi:10.1016/j.ejogrb.2020.12.006.

    PMID: 33454510
  5. 5

    Deciphering Positive Cytomegalovirus Immunoglobulin M Test Results in Immunocompetent Adults Hospitalized With Illness Suspicious for Acute Cytomegalovirus Infection: A Multihospital Study.

    Samuel S, Thomas S, Asleson L, et al.

    Open forum infectious diseases 2025; (12(3)):ofaf144 doi:10.1093/ofid/ofaf144.

    PMID: 40134634
  6. 6

    Evaluation of the indirect and IgM-capture anti-human cytomegalovirus IgM ELISA methods as confirmed by cytomegalovirus IgG avidity.

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

    Microbiology and immunology 2019; (63(5)):172-178 doi:10.1111/1348-0421.12683.

    PMID: 31012489
  7. 7

    Closing the Brief Case: Nystagmus in a 3-Month-Old Leading to a Diagnosis of Congenital Cytomegalovirus Infection.

    Uprety P, Graf EH

    Journal of clinical microbiology 2020; (58(7)) doi:10.1128/JCM.01471-19.

    PMID: 32580968
  8. 8

    Risk of congenital disease in 46 infected fetuses according to gestational age of primary human cytomegalovirus infection in the mother.

    Zavattoni M, Rustico M, Tassis B, et al.

    Journal of medical virology 2016; (88(1)):120-6 doi:10.1002/jmv.24313.

    PMID: 26118464
  9. 9

    Fetal Cytomegalovirus Infection in the Absence of Maternal Cytomegalovirus-IgM Seropositivity.

    Erenel H, Tuna G, Alpay V, Polat İ

    Reproductive sciences (Thousand Oaks, Calif.) 2024; (31(6)):1533-1540 doi:10.1007/s43032-024-01487-x.

    PMID: 38388923
  10. 10

    Clinical Implications for Children Born With Congenital Cytomegalovirus Infection Following a Negative Amniocentesis.

    Bilavsky E, Pardo J, Attias J, et al.

    Clinical infectious diseases : an official publication of the Infectious Diseases Society of America 2016; (63(1)):33-8 doi:10.1093/cid/ciw237.

    PMID: 27114380
  11. 11

    Outcome of fetuses with congenital cytomegalovirus infection and normal ultrasound at diagnosis: systematic review and meta-analysis.

    Buca D, Di Mascio D, Rizzo G, et al.

    Ultrasound in obstetrics & gynecology : the official journal of the International Society of Ultrasound in Obstetrics and Gynecology 2021; (57(4)):551-559 doi:10.1002/uog.23143.

    PMID: 33030767
  12. 12

    Diagnostic challenges in congenital cytomegalovirus infection in pregnancy: A case report.

    Mathias CR, Joung SJS

    Case reports in women's health 2019; (22()):e00119 doi:10.1016/j.crwh.2019.e00119.

    PMID: 31192993
  13. 13

    Isolated Brain Cysts in Children Afflicted with Congenital Cytomegalovirus.

    Barnea-Melamed S, Sever A, Shapira Rootman M, Bilavsky E

    The Pediatric infectious disease journal 2024; (43(3)):e104-e106 doi:10.1097/INF.0000000000004078.

    PMID: 37725803
  14. 14

    Cranial Ultrasound Findings in Infants With Congenital Cytomegalovirus Infection in a Universal Newborn Screening Study in Minnesota.

    Kruc RM, Osterholm EA, Holm T, et al.

    Journal of the Pediatric Infectious Diseases Society 2024; (13(8)):413-420 doi:10.1093/jpids/piae059.

    PMID: 38847778
  15. 15

    [Neurological sequelae in patients with congenital cytomegalovirus].

    de Juan Gallach A, Alemany Albert M, Marco Hernández AV, et al.

    Anales de pediatria 2020; (93(2)):111-117 doi:10.1016/j.anpedi.2019.12.021.

    PMID: 32111550
  16. 16

    Fetal Brain Magnetic Resonance Imaging Findings In Congenital Cytomegalovirus Infection With Postnatal Imaging Correlation.

    Averill LW, Kandula VV, Akyol Y, Epelman M

    Seminars in ultrasound, CT, and MR 2015; (36(6)):476-86.

    PMID: 26614131
  17. 17

    Prenatal predictors of adverse perinatal outcome in congenital cytomegalovirus infection: a retrospective multicenter study.

    Mappa I, De Vito M, Flacco ME, et al.

    Journal of perinatal medicine 2023; (51(1)):102-110 doi:10.1515/jpm-2022-0286.

    PMID: 35993868
  18. 18

    Blood Viral Load in Symptomatic Congenital Cytomegalovirus Infection.

    Marsico C, Aban I, Kuo H, et al.

    The Journal of infectious diseases 2019; (219(9)):1398-1406 doi:10.1093/infdis/jiy695.

    PMID: 30535363

This page provides educational information about diagnostic testing for fetal CMV during pregnancy. Always consult your maternal-fetal medicine specialist or obstetrician for advice regarding your specific test results and care plan.

Get notified when new evidence is published on Fetal cytomegalovirus syndrome.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.