Genetics, Subtypes, and Your Test Results

To understand Trisomy 13, it helps to start with the basics of genetics. Typically, every cell in the body contains 23 pairs of chromosomes, for a total of 46. One pair—the 13th pair—usually has two copies. Trisomy 13 occurs when there is extra genetic material from the 13th chromosome ^[1].

However, it is crucial to understand that a positive screening test during pregnancy (such as NIPT or cell-free DNA) is not a confirmed diagnosis. Screening tests assess risk, but definitive answers require diagnostic testing, such as CVS or amniocentesis during pregnancy, or a blood karyotype or microarray after birth ^[2].

When diagnostic testing confirms Trisomy 13, the report will identify a specific “subtype.” The arrangement of the extra genetic material can significantly influence a child’s health and survival ^[3].

The Three Genetic Subtypes

1. Full Trisomy 13 (Free Trisomy): This is the most common form. It means that an entire extra 13th chromosome is present in every cell that the laboratory tested ^[3]. On a karyotype (a picture of the chromosomes), this is usually written as 47,XX,+13 (for a girl) or 47,XY,+13 (for a boy) ^[4].
2. Mosaic Trisomy 13: In this form, the extra chromosome is present in some cells, while other cells have the typical two copies ^[4]. The report might look like a fraction, such as mos 47,XX,+13[11]/46,XX[10], which tells you the ratio of cells found in the sample ^[5]. Note: The percentage of mosaic cells found in a blood sample does not perfectly predict the percentage in other tissues, like the heart or brain.
3. Translocation Trisomy 13: In this form, the extra genetic material is physically attached to another chromosome ^[2]. This can result in a whole extra 13th chromosome (full trisomy) or only a duplicated segment (Partial Trisomy) depending on the specific structural rearrangement. It is often written with terms like rob (for a Robertsonian translocation) or der (for a derivative chromosome) ^[6][2].

While Trisomy 13 is a life-limiting condition, survival outcomes vary. Research indicates that children with mosaic or partial variants often have different, sometimes longer, survival trajectories compared to those with full Trisomy 13, though individual outcomes depend heavily on which specific organs are affected ^[3][7].

Decoding Your Lab Reports

You may see two types of diagnostic tests on your child’s chart:

• Karyotype: A visual map of the physical chromosomes. It is the best test for seeing how chromosomes are arranged and is necessary to identify a structural translocation ^[2].
• Microarray (CMA): This test is like a high-powered zoom lens. It can detect very tiny extra or missing pieces of DNA (copy-number gains and losses), but it generally cannot show whether that extra material is physically attached to another chromosome (a translocation) ^[8][4].

Genetic Counseling and Recurrence Risk

It is highly recommended that every family meet with a genetic counselor to discuss their individualized recurrence risk.

If your child’s report shows a Translocation, it is standard medical practice for both biological parents to undergo their own karyotyping ^[6]. In some translocation cases, a parent may be a “balanced carrier,” meaning they have all the right genetic material but it is shuffled. This significantly increases the risk of passing on an unbalanced translocation in future pregnancies ^[9][10]. Knowing your own carrier status allows your medical team to give you accurate information about the risk of Trisomy 13 occurring again ^[11][6].

Even if the test shows “Full” or “Mosaic” Trisomy 13, which are usually random events during cell division, the recurrence risk for future pregnancies is not literally zero. A genetic counselor will help you understand residual risks, including maternal age-related factors and the rare possibility of parental mosaicism ^[4].