Non-Invasive Prenatal Testing (NIPT) screens for the risk of certain fetal chromosomal conditions by analyzing cell-free DNA in a pregnant person’s blood. The “fetal fraction,” the proportion of fetal DNA in the sample, is important for accurate NIPT results. This article explains what a low fetal fraction NIPT result means.
Understanding Fetal Fraction
Fetal fraction is the percentage of cell-free DNA in a pregnant person’s blood that comes from the placenta and fetus. This DNA is analyzed to screen for chromosomal conditions like Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13). Sufficient fetal DNA is needed for NIPT to accurately distinguish between maternal and fetal genetic material.
A minimum amount of fetal DNA is required for a reliable result. If the fetal fraction is too low, the lab may not have enough information for an accurate risk assessment. Fetal fraction naturally increases as pregnancy progresses, usually becoming sufficient for testing around 10 weeks of gestation.
Interpreting Low Fetal Fraction
A low fetal fraction means the fetal DNA in the blood sample is below the threshold for a reliable NIPT result. While thresholds vary, a fetal fraction below 4% is often considered low. Insufficient fetal fraction means NIPT cannot provide an accurate risk assessment, leading to an “inconclusive” or “no result.”
A low fetal fraction does not indicate a problem with the pregnancy or fetus. It means the NIPT could not be reliably interpreted due to insufficient fetal genetic material. This outcome requires discussion with a healthcare provider to determine next steps. An inconclusive result does not mean there is an underlying issue with the baby.
Factors Contributing to Low Fetal Fraction
Several factors can lead to a low fetal fraction. Early gestational age is common, especially if the blood sample is drawn before 10 weeks, as fetal fraction naturally increases with advancing gestation. At 10 weeks, the fetal fraction is typically around 10-15%.
Maternal body mass index (BMI) is another factor. Higher BMI often means a larger blood volume, diluting cell-free fetal DNA and making it harder to detect. This can result in a lower fetal fraction even if the absolute amount of fetal DNA is typical. Fetal fraction decreases with increased maternal weight or BMI.
Certain maternal medical conditions, like autoimmune disorders or those affecting placental function, can also impact fetal fraction. Less commonly, some medications or technical issues with sample collection or lab processing can contribute. However, early gestational age and maternal weight are the most common factors.
Navigating Next Steps
A low fetal fraction NIPT result requires discussion with a healthcare provider to determine the next steps. Often, repeating the NIPT is recommended, especially if the initial test was early in pregnancy or if maternal weight was a factor. Repeating the test later in gestation usually allows for a higher fetal fraction, increasing the chance of a conclusive result.
If a repeat NIPT is also inconclusive, or if there is a higher suspicion of a chromosomal condition, alternative screening or diagnostic tests may be considered. These can include a nuchal translucency (NT) scan with maternal serum screening (MSS). For a definitive diagnosis, invasive procedures like chorionic villus sampling (CVS) or amniocentesis might be discussed, as these analyze fetal cells directly. A low fetal fraction result means the NIPT could not be reliably interpreted, not necessarily that there is an underlying issue with the pregnancy. Further guidance from medical professionals is important.