Factors Associated with Low Fetal Fraction in Non-Invasive Prenatal Testing

Maternal Factors

• Maternal high body mass index (BMI) is strongly associated with low fetal fraction, with studies showing that NIPT may fail to provide results in 20% of women with high BMI and 50% of women with very high BMI 1, 2
• The American College of Obstetricians and Gynecologists recommends considering alternative screening methods for women with high BMI or other factors predisposing to low fetal fraction 1, 2
• Use of certain anticoagulants can interfere with cell-free DNA analysis, affecting fetal fraction 3, 4
• Maternal autoimmune conditions have been linked to higher rates of no-call results 3, 4

Gestational Age Factors

• Fetal fraction is typically lower at early gestational ages, making NIPT less likely to provide a result at very early gestational ages 1, 2

Fetal and Placental Factors

• Low fetal fraction has been associated with a higher than expected rate of certain fetal chromosome disorders, particularly trisomies 13 and 18, and triploidy 3, 4, 1
• The presence of multiple fetuses can affect fetal fraction measurements 1
• The presence of a second gestational sac has been associated with false-positive NIPT results 2

Technical Factors

• Different NIPT platforms have varying minimum fetal fraction requirements and technical approaches to analyzing cell-free DNA 1

Clinical Implications of Low Fetal Fraction

• Repeat testing provides a result in approximately 75-80% of cases with insufficient fetal fraction 3, 4
• The American College of Obstetricians and Gynecologists recommends offering diagnostic testing to individuals with a no-call result 3
• Alternative screening methods may be more appropriate for women with high BMI or other factors predisposing to low fetal fraction 1, 2

Understanding Fetal Fraction 8 on NIPT

Interpretation of Fetal Fraction

• A fetal fraction of 8% is considered adequate for reliable NIPT analysis, and with this value, the test is unlikely to result in a "no-call" result, which occurs in approximately 1% of NIPT samples 5, 6
• The American College of Medical Genetics and Genomics recommends that a minimum fetal fraction of approximately 4% is required to provide NIPT results, and a fetal fraction of 8% is well above this threshold 5

Clinical Implications

• A fetal fraction of 8% suggests that the NIPT result is likely to be reliable, with higher fetal fraction associated with greater test accuracy and lower likelihood of test failure 5

Diagnostic Considerations

• If NIPT results are positive despite adequate fetal fraction, diagnostic testing (amniocentesis or CVS) is recommended for confirmation, as NIPT remains a screening test, not a diagnostic test, with a positive predictive value of 50-95% for trisomy 21 5, 7, 8

Important Considerations

• NIPT with adequate fetal fraction significantly reduces the need for invasive diagnostic procedures compared to traditional screening methods, and pre-test and post-test genetic counseling are integral aspects of NIPT, regardless of fetal fraction 7, 8, 9

Diagnostic and Counseling Recommendations for Persistent No‑Call NIPT Results

Preferred Diagnostic Testing

• Amniocentesis is the preferred diagnostic test after two failed NIPT attempts, performed in the second trimester. The American College of Obstetricians and Gynecologists recommends offering diagnostic testing to individuals with persistent no‑call results. Guideline recommendation 10

Comprehensive Chromosomal Evaluation

• Amniocentesis provides definitive karyotype analysis and can be supplemented with chromosomal microarray (CMA) when indicated. This enables detection of both whole‑chromosome aneuploidies and sub‑microscopic copy‑number variants. 10

Timeliness of Diagnosis

• Proceeding directly to amniocentesis avoids further delays and yields conclusive information about fetal chromosomal status. Rapid definitive results facilitate timely clinical decision‑making. 10

Genetic Counseling Requirement

• Patients with screen‑positive or no‑call NIPT results should receive genetic counseling from a qualified genetics professional. Counseling ensures informed understanding of test limitations, possible outcomes, and next steps. 10

Limitations of NIPT Interpretation

• NIPT analyzes placental‑derived cell‑free DNA, which may not fully represent the fetal genome due to confined placental mosaicism. This biological limitation can lead to discordant results between NIPT and fetal karyotype. 10

Causes of Discordant NIPT Results

• Potential sources of discordant NIPT findings include: