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PubMed This is a summary of 5 peer-reviewed journal articles Updated
Medical Genetics · Trisomy 18

Why Do Parents Need Karyotype Testing After Trisomy 18?

At a Glance

Parents are offered karyotype testing after a baby has partial or translocation trisomy 18 to find out whether either parent carries a balanced chromosome rearrangement. The results help estimate recurrence risk and guide future pregnancy testing and choices.

When a baby is diagnosed with partial trisomy 18 (having extra material from only part of chromosome 18) or translocation trisomy 18 (where the extra chromosome 18 material is attached to a different chromosome), doctors usually recommend a blood test called a karyotype for both biological parents [1]. A karyotype examines the number and large-scale structure of your chromosomes [1]. Finding out whether the baby’s chromosomal change was inherited or happened entirely by chance helps your medical team provide accurate information about the chances of it happening again in future pregnancies.

It Is Not Your Fault

Being offered this test can sometimes feel like an accusation, especially when you are completely healthy. It is crucial to understand that no one does anything to cause a chromosome rearrangement. They happen naturally and are completely out of our control. If testing shows you do carry a translocation, it does not mean you are “at fault” or that you did something to cause your baby’s diagnosis.

What Is a Balanced Translocation?

Chromosomes are like instruction manuals for the body. A balanced translocation means that pieces of two different chromosomes broke off and traded places [2]. Because usually no large amount of DNA is lost or gained, a parent with a balanced translocation still has all the right genetic instructions—just in a different order [2]. As a result, carriers are usually healthy and have no symptoms [2].

However, when a parent’s body makes egg or sperm cells, chromosomes have to divide. Sometimes they divide unevenly. If an egg or sperm gets an “unbalanced” set of chromosomes, the baby might inherit an extra piece of chromosome 18 [2]. The exact clinical impact on the baby depends on the specific duplicated and deleted chromosome segments involved [2].

How Your Results Affect the Future

After reviewing the baby’s full chromosome report, testing both parents helps map out what to expect for the future:

  • If both parents have normal karyotypes: A normal result usually suggests the baby’s rearrangement was de novo (a new change that arose randomly in the egg, sperm, or early embryo) [3]. The chances of having another baby with trisomy 18 are generally low [3]. However, a normal blood karyotype cannot completely rule out very small, cryptic changes or mosaicism (where the change only exists in certain cells, like the reproductive organs), so your genetic counselor will still calculate an individualized recurrence estimate [3].
  • If a parent carries a balanced translocation: Your recurrence risk is higher. A genetics professional will review the exact chromosomes involved and provide a specific recurrence risk for your family [2].

Future Family Planning Options

If you are at an increased risk, genetic counseling can help you explore several reproductive choices. Reproductive decisions are deeply personal, and there is no single “right” choice. Options include natural conception, adoption, or using donor gametes. If you choose to pursue a biological pregnancy, options include:

  • Diagnostic Prenatal Testing: If you conceive naturally, you can choose to have chorionic villus sampling (CVS) (usually performed around 10 weeks) or amniocentesis (usually from 15 weeks) [4]. These are diagnostic procedures that can establish the baby’s chromosome complement [4]. However, CVS occasionally reflects confined placental mosaicism, and neither test rules out every possible genetic condition [4].
  • Preimplantation Genetic Testing for Structural Rearrangements (PGT-SR): This involves in vitro fertilization (IVF) to test embryos for the specific structural rearrangement before pregnancy [2]. PGT-SR requires IVF, has significant costs, may not produce a transferable embryo, and is generally followed by diagnostic testing during pregnancy [2].
  • Non-Invasive Prenatal Testing (NIPT): Standard NIPT is a maternal blood test that screens for whole-chromosome extra copies [5]. If there is a known familial rearrangement, standard NIPT may not reliably detect the specific partial duplication or translocation and should not be used as a substitute for diagnostic testing [5].

Common questions in this guide

Why are both biological parents offered karyotype testing after a baby has translocation trisomy 18?
A parental karyotype checks whether either parent carries a balanced translocation involving chromosome 18. A carrier is often healthy but can pass on an unbalanced chromosome set. The results help the genetics team determine whether the baby's change was inherited or new and estimate the chance in a future pregnancy.
Can a healthy parent have a balanced translocation?
Yes. In a balanced translocation, chromosome pieces have changed places without a large amount of genetic material being gained or lost, so carriers are usually healthy. It is not caused by anything the parent did.
What does a normal karyotype in both parents mean for another pregnancy?
Normal parental karyotypes usually suggest the baby's chromosome change happened randomly in the egg, sperm, or early embryo, and the chance of recurrence is generally low. A blood karyotype cannot completely exclude a very small chromosome change or mosaicism, so a genetic counselor should provide an individualized estimate.
What if one parent is found to carry a balanced translocation?
The chance of another pregnancy being affected is higher, but the exact risk depends on the chromosomes and segments involved. A genetics professional can explain possible unbalanced outcomes and discuss testing and reproductive options.
Which tests can check a baby's chromosomes in a future pregnancy?
Chorionic villus sampling, usually around 10 weeks, and amniocentesis, usually from 15 weeks, are diagnostic tests that can assess the baby's chromosome complement. IVF with PGT-SR can test embryos for a known structural rearrangement before pregnancy, but it requires IVF and is not guaranteed to produce a transferable embryo.
Can standard NIPT detect a familial translocation involving chromosome 18?
Standard NIPT is a screening test for extra copies of whole chromosomes and may not reliably detect a specific partial duplication or translocation. It should not replace diagnostic testing when a familial chromosome rearrangement is known.

Related questions

How Accurate Is a Positive NIPT for Trisomy 18? Next Steps What Is the Trisomy 18 Recurrence Risk in Pregnancy? What Is the Life Expectancy and Survival Rate for Trisomy 18? Prenatal Trisomy 18 Positive: Why Is Newborn Blood Negative? How Does CVS Compare With Amniocentesis for Trisomy 18?
Show more
How Are Weight Gain and Nutrition Managed in Trisomy 18? What Does Mosaic Trisomy 18 on CVS With Normal Amnio Mean? Can Prenatal Ultrasound Predict Trisomy 18 Outcomes? How Common Is Trisomy 18? Pregnancy and Birth Rates How Do I Understand My Baby's Trisomy 18 Genetic Report? What Does Comfort Care Include for Newborns With Trisomy 18? What Happens After a Trisomy 18 Diagnosis at Birth? What Treatments Are Available for Sleep Apnea in Trisomy 18? How Are Breathing Problems Evaluated in Trisomy 18? Can Blood Testing Miss Mosaic Trisomy 18? Other Tissue Tests Trisomy 18: How Are Swallowing and Aspiration Evaluated? What congenital heart defects occur in trisomy 18? Which Newborn Tests Help With Trisomy 18 Comfort Care? Where Is the Best Place to Deliver a Baby With Trisomy 18?

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References

References (5)
  1. 1

    Prenatal Diagnosis of Cleft Lip and Palate: A Retrospective Study.

    Brăila AD, Damian CM, Albu CC, et al.

    Journal of clinical medicine 2024; (13(16)) doi:10.3390/jcm13164804.

    PMID: 39200946
  2. 2

    A Case of a Derivative Chromosome: der(Y)t(Y;18)Pat with Congenital Abnormalities.

    Huang S, Xia Y, Ding H, et al.

    Fetal and pediatric pathology 2021; (40(3)):256-261 doi:10.1080/15513815.2019.1695297.

    PMID: 31805817
  3. 3

    Prenatal diagnosis and molecular genetic analysis of recurrent trisomy 18 of maternal origin in two consecutive pregnancies.

    Chen CP, Wu FT, Wong CH, et al.

    Taiwanese journal of obstetrics & gynecology 2023; (62(3)):444-447 doi:10.1016/j.tjog.2023.03.006.

    PMID: 37188451
  4. 4

    Detection of paternal origin of fetal trisomy 18 in a pregnancy conceived by assisted reproductive technology and in vitro fertilization.

    Chen CP, Chern SR, Wang LK, et al.

    Taiwanese journal of obstetrics & gynecology 2020; (59(4)):607-609 doi:10.1016/j.tjog.2020.05.024.

    PMID: 32653139
  5. 5

    The accuracy of cell-free fetal DNA-based non-invasive prenatal testing in singleton pregnancies: a systematic review and bivariate meta-analysis.

    Mackie FL, Hemming K, Allen S, et al.

    BJOG : an international journal of obstetrics and gynaecology 2017; (124(1)):32-46 doi:10.1111/1471-0528.14050.

    PMID: 27245374

This page explains why parental karyotype testing may be recommended after partial or translocation trisomy 18; it is for informational purposes only and does not replace advice from a genetic counselor or other healthcare professional.