The Path to Diagnosis: Genetics and Testing

Understanding the technical side of a Trisomy 18 diagnosis is a vital step in advocating for your child. The path to a diagnosis often begins with a screening test that raises a “red flag,” followed by more definitive testing to confirm what is happening at the chromosomal level.

The Three Subtypes of Trisomy 18

The impact of Trisomy 18 depends significantly on how many cells are affected and which parts of the chromosome are duplicated. While every child is unique, research highlights distinct differences between the three forms:

• Full (Free) Trisomy 18: The most common form, where an extra chromosome 18 is present in nearly all cells tested [F073]. This typically results in the most severe multisystem challenges, including heart defects (found in about 80% of cases) and significant developmental delays [F073][F004]. Because this usually results from a random chromosomal separation event, it is almost never inherited.
• Mosaic Trisomy 18: The extra chromosome is only in some cells [F074]. The clinical “picture” varies widely—some children may have signs very similar to full Trisomy 18, while others may have much milder symptoms [F074]^[1]. Importantly, the percentage of affected cells found in a blood test does not always predict how a child will grow or develop, as different organs may have different levels of mosaicism [F075]^[2].
• Partial Trisomy 18: Only a specific segment of chromosome 18 is tripled, often from an unbalanced chromosomal rearrangement [F078]. Because the entire chromosome isn’t involved, the outlook and survival can differ significantly from the full form, often depending on exactly which part of the chromosome is extra [F078]^[3]. In these cases, a genetics referral is vital for discussing recurrence counseling.

Screening vs. Diagnosis: Understanding NIPT

Many families first hear about Trisomy 18 through Non-Invasive Prenatal Testing (NIPT), also called cell-free DNA (cfDNA) screening. It is crucial to understand that NIPT is a screening test, not a diagnostic one [F083]^[4].

NIPT looks at fragments of DNA in the mother’s blood that come primarily from the placenta [F083]. While NIPT has a high sensitivity for Trisomy 18 (up to 97.7% in some studies), it can produce “false positives” [F079]^[5]. In real-world studies, the Positive Predictive Value (PPV)—the chance that a “high risk” result actually means the baby has the condition—was approximately 56% to 80% [F079][F080]^[6]. These PPV numbers are context-dependent ranges that change substantially with pretest probability, maternal age, baseline prevalence, and the specific laboratory method used. Because of this, medical guidelines state that a positive NIPT should always be confirmed with diagnostic testing before making irreversible care decisions whenever a family wants confirmation [F084]^[7].

Confirmatory Diagnostic Testing

To get a definitive answer, doctors use tests that look directly at the baby’s cells or the placenta’s core:

1. Chorionic Villus Sampling (CVS): Performed between 10 and 13 weeks of pregnancy, this test samples the placenta [F086]. Because it samples placental tissue, it can occasionally be “discordant” (different) from the baby’s actual DNA due to a rare event called confined placental mosaicism [F087]^[8].
2. Amniocentesis: Usually performed after 15 weeks, this test samples the fluid around the baby, which contains the baby’s own skin and bladder cells [F086]^[9]. It is generally considered the most accurate prenatal diagnostic tool [F085]^[10].
3. Postnatal Karyotype: After birth, a blood sample is taken from the baby to confirm the diagnosis [F011]^[11].

The Role of Ultrasound

Ultrasound findings can raise a strong suspicion of Trisomy 18, but they cannot provide a genetic diagnosis on their own [F010]^[12]. Common markers seen on ultrasound include:

• Fetal growth restriction: The baby is smaller than expected for their age [F037].
• Clenched hands: A characteristic “overlapping finger” position [F037]^[13].
• Choroid plexus cysts: Small fluid-filled spaces in the brain (though these can also occur in healthy babies) [F037].
• Heart defects: Visible structural issues in the heart [F037]^[14].
• Micrognathia: A small or recessed chin [F037].

Completeness Checklist: Your Cytogenetic or Genetic Report

When you receive a cytogenetic or genetic report, it should be detailed enough to provide a clear picture. You can use this checklist to review the report with your genetics team:

• [ ] Specimen Source: Does the report clearly state if the cells came from blood, the placenta (CVS), amniotic fluid, or another tissue like a cheek (buccal) swab? [F091][F093]^[15]
• [ ] Subtype Identified: Does it explicitly name Full, Mosaic, or Partial Trisomy 18? [F091]^[1]
• [ ] Cell Count (Metaphases): For a standard karyotype, the report should state how many cells were analyzed (often 20 or more). If mosaicism is suspected, analyzing a higher number of cells is often required to be accurate, though a blood result alone may miss tissue-specific mosaicism [F092][F041]^[16].
• [ ] ISCN Notation: Does the report include the official “shorthand” code (e.g., 47,XY,+18) that specialists use to understand the exact chromosomal arrangement? [F091]
• [ ] Advanced Testing (If Needed): If the karyotype is unclear, does the report mention FISH (a faster, targeted test for chromosome 18) or Chromosomal Microarray (a high-resolution scan that can find tiny extra pieces of DNA)? [F011][F039]^[17]