Fetal DNA fraction is the proportion of a baby’s DNA circulating in the mother’s bloodstream during pregnancy. This measurement is a component of non-invasive prenatal testing (NIPT), a screening method used to assess the likelihood of certain genetic conditions in the developing fetus. NIPT analyzes these small DNA fragments to provide information about the baby’s genetic health without invasive procedures.
The Origin and Role of Fetal Fraction
During pregnancy, a woman’s blood contains a mixture of her own genetic material and small, free-floating pieces of DNA from the pregnancy, known as cell-free DNA (cfDNA). These fragments are released into the bloodstream when cells die and break down. While most of the cfDNA in an expectant mother’s blood is her own, a portion originates from the placenta.
The placenta is an organ that provides oxygen and nutrients to the baby. Its cells undergo a natural turnover, releasing their DNA, which is identical to the fetus’s DNA, into the maternal circulation. The fetal fraction is the percentage of this placental cfDNA out of the total cfDNA in the mother’s blood.
A sufficient fetal fraction is necessary for NIPT to yield a reliable result. Laboratories analyze the cfDNA mixture to count fragments from specific chromosomes. If the fetal fraction is too low, the lab cannot easily distinguish the fetal DNA from the maternal DNA. This makes it difficult to accurately assess the risk for chromosomal abnormalities, such as Down syndrome.
Factors That Affect Fetal Fraction
Several factors influence the percentage of fetal DNA. The most significant is gestational age, as the fetal fraction increases as a pregnancy progresses. For example, the fetal fraction is around 13% at 10 weeks of gestation. This is why NIPT is performed after the 10th week of pregnancy.
Maternal body mass index (BMI) also plays a role, as women with a higher BMI often have a lower fetal fraction. This occurs because a higher body weight corresponds to a larger maternal blood volume, which dilutes the fetal cfDNA. A higher BMI is also associated with increased shedding of the mother’s own cfDNA, further reducing the fetal proportion.
Certain pregnancy-related conditions can also have an impact. For instance, placental issues can affect the amount of DNA it releases. A very low fetal fraction can also be associated with specific fetal chromosomal conditions, like trisomy 13 or 18. However, a low reading is not a diagnosis and is more often linked to factors like gestational age or maternal weight.
Understanding Your Fetal Fraction Result
When you receive NIPT results, the fetal fraction is reported as a percentage. Laboratories require this value to be above a certain threshold, often around 4%, to analyze the fetal chromosomes. A result above this cutoff indicates the sample contained enough fetal DNA for a reliable screening.
A low fetal fraction is a measure of test quality, not a direct indicator of a problem with the baby. If the fetal fraction is below the lab’s threshold, the result is reported as “inconclusive” or “no-call.” This means the lab could not gather enough genetic information to produce a reliable outcome.
An inconclusive result from a low fetal fraction is different from a “high-risk” result. A high-risk result indicates an increased chance for a specific chromosomal condition and prompts discussion about diagnostic testing. An inconclusive result means the test could not be completed, and the next steps involve addressing the sample quality.
Navigating a Low Fetal Fraction Result
Receiving an inconclusive NIPT result due to a low fetal fraction can be unsettling, but there are clear next steps. The most common recommendation is to repeat the blood test. Waiting a week or two before the redraw allows more time for the fetal fraction to increase, often leading to a successful result.
Your provider may also recommend a detailed ultrasound to assess the baby’s growth, anatomy, and placental health. This imaging provides another layer of information about the pregnancy’s well-being. It can be reassuring when NIPT results are inconclusive and helps confirm that development is on track.
A conversation with a genetic counselor or your physician is an important step. They can help you understand potential reasons for a low fetal fraction based on your circumstances. This consultation allows you to review all available options and decide how to proceed with prenatal screening.
If an inconclusive result persists or other risk factors are present, diagnostic tests like chorionic villus sampling (CVS) or amniocentesis may be considered. These tests analyze cells taken directly from the placenta or amniotic fluid and provide definitive answers about fetal chromosomes. Because these procedures are invasive, a thorough discussion with a healthcare professional is important.