Prenatal Trisomy 18 Positive: Why Is Newborn Blood Negative?

Conflicting genetic results are incredibly stressful. When a prenatal test suggests trisomy 18 but a newborn genetic blood test (such as a karyotype, FISH, or microarray—not the routine heel-prick metabolic screen) is negative, it does not automatically mean the baby is clear. Discordance can happen for several reasons: the prenatal result may have been a “false positive” (due to a vanishing twin, maternal chromosomal differences, or an abnormality confined only to the placenta), or the baby may have mosaic trisomy 18 with abnormal cells that are absent or undetectable in their blood ^[1][2][3]. To understand what the results mean for your baby, genetics specialists will review exactly which tests were performed.

1. What Kind of Prenatal Test Was Positive?

The explanation for the mismatch heavily depends on what was tested before birth.

• Screening tests (NIPT/cfDNA): Non-Invasive Prenatal Testing analyzes fragments of DNA floating in the mother’s blood, which mostly come from the placenta, not the baby ^[1][4]. NIPT is a screen, not a diagnosis. False positives can occur because of a “vanishing twin,” maternal chromosome variations, or confined placental mosaicism ^[2].
• Chorionic Villus Sampling (CVS): CVS tests a sample of the placenta. Because it tests the placenta, it can sometimes show an extra chromosome that is not present in the baby (confined placental mosaicism) ^[5].
• Amniocentesis: This tests cells floating in the amniotic fluid, which come directly from the fetus. If an amniocentesis showed trisomy 18, it is generally much more representative of the baby ^[6]. A negative newborn blood test after a positive amniocentesis is more concerning for true fetal mosaicism and requires careful review by a genetics team ^[3].

2. Confined Placental Mosaicism

If your positive prenatal result came from NIPT or CVS, an important possible explanation is confined placental mosaicism (CPM).

During early pregnancy, the cells that form the placenta separate from the cells that form the baby. In CPM, an error during cell division causes some or all of the placenta’s cells to have three copies of chromosome 18, while the baby’s cells develop typically ^[6][7]. Because NIPT and CVS test the placenta, they flag a high risk for trisomy 18 ^[1]. The newborn blood test might be negative because the extra chromosome was never in the baby’s body ^[8].

3. Low-Level or Tissue-Specific Mosaicism

If a baby has mosaic trisomy 18—meaning some cells have the extra chromosome and some do not—a newborn blood test can still be negative.

• Tissue-specific differences: Mosaicism does not spread evenly through the body. A baby might have the extra chromosome in their skin, internal organs, or cheek cells, but have completely typical chromosomes in their blood ^[9][10].
• Declining blood levels: Research shows that in some cases of mosaic trisomy 18, the percentage of abnormal cells in the blood can naturally decline over time, making them harder to detect ^[3].

4. The Limits of Newborn Blood Tests

A “negative” blood result only means the lab did not find the extra chromosome in the specific sample they analyzed. The testing method matters:

• Karyotyping: This traditional test looks at the chromosomes in a set number of cultured cells (often 20 to 50) ^[3]. If a baby only has trisomy 18 in 2% of their cells, karyotyping might miss it if those specific cells aren’t randomly selected for analysis ^[11]. Furthermore, growing (culturing) the cells in a lab can sometimes favor the growth of typical cells, masking the mosaicism ^[9].
• FISH and Microarray: Other tests like FISH (Fluorescence In Situ Hybridization, which can look at hundreds of uncultured cells) or chromosomal microarray (CMA) have different strengths, but neither is an absolute rule-out test for low-level mosaicism if the abnormal cells simply aren’t in the blood ^[12][13].

5. Next Steps and Further Evaluation

A normal physical examination and a single negative blood test do not completely rule out mosaic trisomy 18. Your genetics team will evaluate the baby’s newborn exam, prenatal growth patterns, and any congenital anomalies to help interpret the lab results ^[14][15].

If the clinical picture remains unclear, or if the prenatal test was an amniocentesis, specialists might consider testing another tissue, such as a cheek swab (buccal cells) or a small skin biopsy (fibroblasts) ^[16][11]. However, testing other tissues is highly individualized: no single test guarantees 100% certainty, and skin biopsies are invasive ^[10]. Your medical genetics team or genetic counselor will help you weigh whether further testing will change your baby’s medical care and follow-up.