Non-Invasive Prenatal Testing (NIPT) screens for the genetic health of an unborn child by analyzing DNA fragments in a pregnant person’s bloodstream. A key aspect of this test is the “fetal fraction,” which is the proportion of this circulating DNA originating from the placenta. This placental DNA serves as a proxy for the fetus’s genetic material and is important for NIPT’s accuracy and reliability.
Understanding Fetal Fraction
Fetal fraction is the percentage of cell-free DNA (cfDNA) in maternal blood originating from the placenta. During pregnancy, placental cells release their DNA into the pregnant person’s bloodstream. This cfDNA includes both maternal and placental components. Sufficient placental cfDNA is needed for reliable NIPT results.
Measuring fetal fraction ensures enough fetal DNA is present for accurate analysis. Laboratories distinguish between maternal and placental DNA to determine this percentage. The accuracy of NIPT in detecting chromosomal conditions and fetal sex relies on an adequate fetal fraction.
Interpreting a 6% Fetal Fraction
Most laboratories and medical guidelines require a minimum fetal fraction of 2% to 4% for reliable NIPT results. This threshold is adequate for the statistical algorithms used in NIPT. It allows for accurate distinction between normal and abnormal chromosomal patterns.
A 6% fetal fraction is considered a good and sufficient level for NIPT. This percentage falls within the acceptable range for accurate screening by most NIPT providers. It allows for robust analysis, reducing the likelihood of an inconclusive result from insufficient fetal DNA. Therefore, a 6% fetal fraction indicates the test has enough genetic material for meaningful information.
Factors Affecting Fetal Fraction
Several biological and technical factors influence the fetal fraction detected in a NIPT sample. Gestational age is a primary factor, as fetal fraction increases as pregnancy progresses. For example, it might be 4-6% at 10 weeks, rising to over 10% by 20 weeks. Maternal weight also plays a role; higher maternal body mass index (BMI) often correlates with a lower fetal fraction due to a dilution effect.
Other factors can affect fetal fraction, including:
- Multiple pregnancies, where fetal fraction may be distributed among fetuses.
- Certain medical conditions or medications.
- The health of the placenta, the source of cfDNA.
- Technical aspects like blood sample timing and handling.
What Happens with Low Fetal Fraction
When fetal fraction is too low, NIPT may yield an “inconclusive” or “no-call” result. This means insufficient fetal DNA was present for a reliable assessment of chromosomal conditions. Healthcare providers recommend repeating the NIPT after a few weeks. This allows more time for the fetal fraction to increase, improving the chances of a clear result.
An inconclusive result from low fetal fraction does not necessarily indicate a pregnancy problem. It primarily reflects a test limitation in that instance. If repeat testing also yields a low fetal fraction, alternative screening or diagnostic options, such as amniocentesis or chorionic villus sampling, may be discussed. While low fetal fraction can sometimes be associated with a higher risk of certain conditions, it most often means the test lacked sufficient information.