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Maternal-Fetal Medicine · Patient Counseling

Trisomy 13

Patau Syndrome — Understanding Your Screening Results

Prepared by Your MFM Care Team

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Genetics

What is Trisomy 13?

Definition

Trisomy 13, or Patau syndrome, occurs when a baby has three copies of chromosome 13 instead of the usual two.

Cause

This extra chromosome arises from an error in cell division — typically a random event, not inherited.

Prevalence

Trisomy 13 affects approximately 1 in 5,000–12,000 live births.

Chromosome 13 Pair

Normal (2)

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Trisomy 13 (3)

The extra chromosome (red) disrupts normal fetal development.

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Screening

How NIPT Works

Cell-Free DNA

NIPT analyzes fragments of cell-free DNA (cfDNA) from the placenta that circulate in maternal blood.

Key Point

NIPT is a screening test, not a diagnostic test. A positive result requires confirmation.

🩸 Maternal Blood Draw

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🧬 cfDNA Extraction & Sequencing

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💻 Bioinformatic Algorithm

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📋 Risk Result (High / Low)

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Test Performance

NIPT Performance for Trisomy 13

Sensitivity

91–100%

Good at detecting true cases

Specificity

99.97%

Very low false alarm rate

Positive Predictive Value (PPV)

18–33%

In general (low-risk) populations

PPV with Abnormal Ultrasound

~100%

When major anomalies are present

What PPV Means for You

A PPV of 18–33% means that in a general population, up to 2 out of 3 positive NIPT results for Trisomy 13 may be false positives.

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Biology of Discordance

Why False Positives Happen

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Confined Placental Mosaicism (CPM)
The extra chromosome 13 is only in the placenta, not the baby. NIPT reads placental DNA, so it may flag a healthy fetus.
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Vanishing Twin
DNA from an early twin that did not develop can persist in maternal blood, triggering a positive result for the surviving healthy baby.
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Low-Level Fetal Mosaicism
A small proportion of cells carry the extra chromosome, creating discordant results between different tests.
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Maternal Factors
Advanced maternal age and diminished ovarian reserve are associated with a higher frequency of aneuploidy detection.

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Ultrasound Integration

The Role of Ultrasound

Critical Next Step

A detailed anatomy ultrasound is performed after a positive NIPT to look for structural differences associated with Trisomy 13.

Normal Ultrasound

If the detailed ultrasound is entirely normal, the likelihood of a true positive decreases significantly — suggesting a possible false positive or CPM.

PPV by Ultrasound Findings

NIPT alone ~30%

NIPT + Abnormal Ultrasound ~100%

NIPT + Normal Ultrasound Low

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Ultrasound Findings

Sonographic Findings of Trisomy 13

System	Finding
Brain	Holoprosencephaly
Face	Median cleft lip/palate, cyclopia
Heart	Cardiac defects (VSD, ASD)
Kidneys	Renal cysts, dysplasia
Abdomen	Omphalocele
Limbs	Postaxial polydactyly
Growth	Intrauterine growth restriction

Key Marker

Holoprosencephaly (failure of the brain to divide) is one of the most characteristic findings of Trisomy 13.

Multiple Anomalies

When two or more major anomalies are identified together, the probability of a true trisomy increases substantially.

Not all findings are present in every case. Your physician will review your specific ultrasound results with you.

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Diagnosis

Confirming the Diagnosis

Why Confirmation is Required

NIPT is a screening test. A diagnostic procedure tests the baby's actual chromosomes and provides a definitive answer.

Clinical Guidance (ACOG/SMFM)

Invasive diagnostic testing is recommended before any irreversible clinical decision is made.

Option 1 · CVS

Chorionic Villus Sampling

Performed at 10–13 weeks. Samples placental tissue. Earlier result, but may reflect CPM rather than fetal status.

Option 2 · Amniocentesis (Preferred)

Amniocentesis

Performed after 15 weeks. Samples fetal cells (amniocytes) directly — the gold standard for fetal diagnosis.

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Prognosis

Survival in Trisomy 13

Stillbirth Rate

~30%

Of diagnosed pregnancies

Survival to 1 Year

~10%

Of liveborn infants

Variability

Outcomes vary by the specific anomalies present and the extent of chromosomal involvement (full trisomy vs. mosaicism).

Mosaic Trisomy 13

A small subset of patients have mosaic Trisomy 13, where only some cells carry the extra chromosome. These individuals may have milder features and longer survival.

Important Note

Prognosis discussions are individualized. Your physician will review your specific findings and what they mean for your pregnancy.

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Your Next Steps

We will guide you through each step with clear information and compassionate care.

1

Detailed Anatomy Ultrasound

A comprehensive scan to evaluate fetal anatomy and look for structural differences associated with Trisomy 13.

2

Genetic Counseling

A dedicated session to review your specific PPV, risk factors, and all available options — without pressure.

3

Diagnostic Testing (if desired)

Amniocentesis or CVS to obtain a definitive chromosomal diagnosis from fetal cells.

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Shared Decision-Making

We review all results together. Your values and choices guide every decision we make as a team.

🏥 Atlanta Perinatal Associates

📞 Ask your care team any time

🤝 You are not alone

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