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When you are pregnant your blood contains tiny pieces of DNA from the placenta. The placenta is an organ which only develops when you become pregnant. It connects your developing baby to you and releases its DNA into your blood. 99% of the time this DNA is identical to the baby. This is called “circulating cell-free DNA” (ccfDNA), as it is not found inside the cells, where our DNA is usually stored, but rather floating freely in the blood. Levels of this DNA increase as the pregnancy progresses. When the amount of baby DNA is compared to the total amount of DNA in your blood, the portion belonging to the baby is referred to as the foetal fraction.
Non-invasive prenatal testing (NIPT) has transformed prenatal screening during pregnancy, giving expectant parents valuable information about their baby’s genetic health through a simple blood test. While the focus is on the result, which is reported as “low risk” or “high risk,” for a chromosomal problem, it is the foetal fraction that directly impacts how accurately the test can detect chromosomal conditions such as Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18) and Patau syndrome (Trisomy 13).
If an NIPT report says the foetal fraction is 6%, this means 6% of the DNA analysed in the blood sample belongs to the baby.
Why is foetal fraction important?
In simple terms the higher the amount of foetal DNA in your blood, (ie a higher foetal fraction) the more reliable results.
If the foetal fraction is too low the test may not be able to provide a clear result. This happens in 1% to 8% of pregnancies. More often than not, a repeat test a few weeks a later provides a higher foetal fraction, and a result can be provided.
What is a normal foetal fraction?
Most NIPT labs require a minimum foetal fraction of between 2–4%, to issue results. In the majority of pregnancies, foetal fraction levels range between 4% and 15%.
For example:
What affects foetal fraction levels?
Several factors can influence how much foetal DNA is present in a mother’s blood;
Maternal factors
A mother’s body plays an important role in determining foetal fraction. Influencing factors include:
Pregnancy and placental factors
Foetal fraction is also influenced by the age of the pregnancy and placental health.
What to do after an inconclusive result
An inconclusive NIPT result does not mean there is a problem with the baby. It simply indicates that there wasn’t enough foetal DNA to provide a clear answer.
Common next steps include:
Healthcare providers can guide anxious parents through the best option for the situation.
In summary
When receiving NIPT results, it is important not to just focus on the “high” or “low” risk categories. Take time to check the foetal fraction mentioned in your report. It’s a key indicator of how robust and reliable your results are.
If you get a “no-call” due to low fetal fraction, try not to panic. It’s often just a matter of waiting a couple of weeks for a re-draw. Your healthcare provider will be able to explain your specific results and guide you through any necessary next steps.
To learn more about NIPT testing visit:
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Learn more about non-invasive prenatal testing, the various options available, and how to select the best test for your needs as expectant parents. It is not “just another” blood test. The implications of the result are huge, and accuracy and reliability are paramount.