What Is the Trisomy 18 Recurrence Risk in Pregnancy?
At a Glance
Trisomy 18 can recur, but the risk is low for most families and depends on whether the prior pregnancy had full, mosaic, or partial trisomy, the egg provider’s age, and parental chromosome results. Genetic counseling can provide a personalized estimate.
In this answer
4 sections
Thinking about a future pregnancy after experiencing a pregnancy affected by Trisomy 18 (Edwards syndrome) can be frightening and overwhelming. It is important to know that most cases of Trisomy 18 happen by chance, and nothing you or your partner did caused it.
Yes, Trisomy 18 can happen again in a future pregnancy, but for most families, the chance of recurrence is low. The exact risk depends heavily on the specific chromosomal makeup of the previous pregnancy, the pregnant person’s age, and whether a parent carries a genetic structural change. To understand your true risk, a genetic counselor or maternal-fetal medicine specialist (a doctor specializing in high-risk pregnancies) will need to review your specific test results.
Understanding the Types of Trisomy 18
The recurrence risk is directly tied to the type of Trisomy 18 that was diagnosed. “Free” versus “translocation” describes how the extra genetic material is arranged, while “full” versus “mosaic” describes how many cells are affected.
| Type of Trisomy 18 | What it Means | Cause & Recurrence Risk |
|---|---|---|
| Full (Free) Trisomy 18 | An extra, separate chromosome 18 is found in essentially all cells tested. | Usually caused by a random, one-time error during egg or sperm formation (meiotic nondisjunction) [1]. Recurrence is low, but not zero. |
| Mosaic Trisomy 18 | A mix of cells: some have the usual 46 chromosomes, and some have an extra chromosome 18 (47 chromosomes). | Usually caused by a random error during cell division after fertilization [2]. Recurrence risk is generally very low. |
| Partial / Translocation Trisomy 18 | Only a piece of chromosome 18 is extra, often attached to a different chromosome. | Can be random (de novo) or inherited from a parent with a “balanced translocation” [3]. If inherited, recurrence risk is higher. |
For a previous pregnancy with full (free) Trisomy 18, genetics professionals often cite a general rule of thumb for recurrence: approximately 1% or the maternal age-related risk, whichever is higher. However, because rare consecutive recurrences have been documented even when parents have normal chromosomes, your counselor will provide a personalized estimate rather than relying solely on a generic percentage [4].
If the previous pregnancy had partial Trisomy 18 due to an unbalanced translocation, the risk depends on whether a parent carries a balanced rearrangement. If it was a random (de novo) event in the baby, the recurrence risk is generally low [5][6]. If a parent carries a balanced translocation, the risk is higher and depends on the exact chromosomes involved [7][8].
The Role of Maternal Age
The risk of random chromosome separation errors (nondisjunction) increases continuously as the person providing the egg gets older [9]. While studies often highlight age 35 as a point where risk increases more noticeably, the change is gradual, not an abrupt jump [10][11]. Therefore, your age at the time of your next expected conception or delivery is a key factor in calculating your baseline risk for a future pregnancy.
Why Parental Testing (Karyotyping) Matters
If your previous pregnancy involved a partial trisomy or an unbalanced translocation, your doctor will likely recommend parental karyotyping [12]. This is a blood test that looks at the chromosomes of both biological parents to check for a balanced translocation.
If the parents’ blood karyotypes are normal, it significantly lowers the concern for an inherited structural rearrangement. However, a normal parental blood test does not guarantee zero risk. Age-related errors can still occur, and in rare instances, a parent might have gonadal mosaicism (where the genetic difference is only in the egg or sperm cells, not the blood) [5][6][4].
Planning a Future Pregnancy: Screening and Testing Options
If you decide to become pregnant again, there are several ways to monitor the pregnancy. It is crucial to understand the difference between screening (which estimates risk but is not diagnostic) and diagnostic testing (which analyzes cells to provide a highly accurate, though not absolutely guaranteed, diagnosis).
Early Screening Options
- Non-Invasive Prenatal Testing (NIPT/cfDNA): This maternal blood test, done as early as 10 weeks, screens placental cell-free DNA floating in the pregnant person’s blood. While highly sensitive for Trisomy 18, it is only a screen [13][14]. A “low-risk” result is reassuring but does not definitively rule out Trisomy 18, as false negatives can occur [15]. A “high-risk” result is not a diagnosis and must be confirmed with diagnostic testing before making irreversible pregnancy decisions [16][17].
- Detailed Ultrasound: Early ultrasounds (measuring nuchal translucency between 11–13 weeks) and the mid-pregnancy anatomy scan (around 18–22 weeks) can identify physical signs of Trisomy 18 [18]. However, an ultrasound alone cannot definitively diagnose or rule out a chromosomal condition.
Diagnostic Testing
Diagnostic tests are invasive procedures that sample cells directly from the pregnancy. Because they are invasive, they carry a small risk of procedure-related complications, including a slight risk of miscarriage.
- Chorionic Villus Sampling (CVS): Performed between 10 and 13 weeks, CVS takes a sample of the placenta [19][20]. While it provides early answers, placental cells can sometimes differ from the fetus’s cells (a condition called confined placental mosaicism). If CVS shows mosaicism, a follow-up amniocentesis may be recommended to determine the true fetal status [21][22].
- Amniocentesis: Usually performed after 15 weeks, this procedure takes a sample of the amniotic fluid containing cells shed directly by the fetus [23][24]. It is considered highly accurate for diagnosing Trisomy 18, though no medical test is a 100% absolute guarantee.
Note: If you have had a previous pregnancy with Trisomy 18, you can choose to bypass NIPT screening and proceed directly to diagnostic testing (CVS or amniocentesis) if you prefer a more definitive answer earlier in the pregnancy [25].
IVF and Embryo Testing
For families with a known inherited translocation, In Vitro Fertilization (IVF) paired with Preimplantation Genetic Testing for Structural Rearrangements (PGT-SR) might be an option [26]. PGT-SR allows doctors to test embryos for specific chromosomal imbalances before implantation. While this significantly reduces the chance of transferring an affected embryo, it does not guarantee a healthy pregnancy. Because PGT is a screening tool with technical limitations, prenatal diagnostic confirmation (like CVS or amniocentesis) is still strongly recommended during the pregnancy.
Common questions in this guide
How likely is Trisomy 18 to happen again?
Does age affect the chance of Trisomy 18 in a future pregnancy?
Should both parents have chromosome testing after a pregnancy with Trisomy 18?
What prenatal testing options are available in a future pregnancy?
Can a high-risk NIPT result confirm Trisomy 18?
Could IVF with PGT-SR reduce the chance of Trisomy 18?
Related questions
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References
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This page explains how Trisomy 18 recurrence risk is estimated for informational purposes only and does not replace medical advice. A genetic counselor or maternal-fetal medicine specialist can interpret your prior results and discuss testing options.