Understanding FET (S): Normal vs Optimal Ranges

?What is FET (S)?

FET (S) measures the proportion of fetal cell-free DNA (cfDNA) present in maternal blood during pregnancy. This metric is essential for assessing the quality and reliability of noninvasive prenatal testing (NIPT) results. A sufficient fetal fraction ensures accurate detection of chromosomal abnormalities such as trisomy 21, 18, and 13.

!Why It Matters

The fetal fraction directly impacts the sensitivity and specificity of prenatal genetic screening. Low fetal fractions may suggest an increased risk of test failure or inconclusive results, which can be associated with maternal obesity, early gestational age, or certain placental conditions. Higher fetal fractions generally improve diagnostic confidence in detecting chromosomal abnormalities.

Reference Ranges

Lab normal ranges may vary between laboratories. Optimal and longevity targets are based on research literature and should be interpreted with your physician.

Symptoms of Imbalance

• Inconclusive or failed NIPT results
• Need for test retesting or alternative screening methods
• Concern about accuracy of prenatal genetic screening
• Maternal factors affecting fetal DNA detection

How to Improve Your Levels

• 1Ensure accurate gestational age dating to optimize fetal fraction at time of testing
• 2Maintain healthy weight management before and during pregnancy to improve cell-free DNA detection
• 3Schedule NIPT testing at recommended gestational age (typically 10 weeks or later) when fetal fraction is more reliable
• 4Discuss timing and test quality with healthcare provider if initial results show low fetal fraction
• 5Avoid excessive stress and maintain good overall health during pregnancy
• 6Consider repeat testing if fetal fraction is borderline, as it may improve with advancing pregnancy

When to Test

NIPT is typically performed between 10–14 weeks of gestation, though it can be done as early as 9 weeks with adequate fetal fraction. Testing may be repeated if initial fetal fraction is too low for reliable results.

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