Updated February 2026

What Is Fetal Fraction and What Does It Mean?

Fetal fraction is the percentage of cell-free DNA (cfDNA) in a pregnant person's blood that comes from the placenta and fetus. This fraction is important for non-invasive prenatal testing (NIPT) because a sufficient amount of fetal DNA is needed to accurately screen for chromosomal abnormalities, such as trisomy 21.

Fetal fraction meaning specifically describes the percentage of cell-free DNA in the mother’s bloodstream that originates from the fetus via the placenta. A low fetal fraction can result in an invalid test or a false negative, and can also be associated with adverse pregnancy outcomes like preeclampsia or small for gestational age infants.

Before our daughter was born we had our standard health service prenatal tests. These showed I was at higher risk for some abnormalities. Due to this my husband and I decided to have non-invasive prenatal tests privately. This was to give us some peace of mind and get a fuller picture of potential issues. From our experience we completely understand why you may be thinking the same or similarly to us. So if you are hoping to get some answers to what all this means, read on.

Non-invasive prenatal testing (NIPT) or sometimes called noninvasive prenatal screening (NIPS), is a screening test used to detect the risk that a fetus will be born with certain genetic abnormalities. It can be performed as early as 10 weeks in pregnancy and only requires a blood sample from the mother. NIPT analyzes small pieces of DNA, called cell-free DNA (cfDNA), found in the mother’s bloodstream.

To know whether NIPT may already be facilitated, determine first the gestational age of the fetus. The gestational age is the period from conception to birth. A due date calculator may help determine if the fetus is at least 10 weeks before undergoing the NIPT.

Kristie holds her baby on the sofa — Me holidng my daughter.

What is Cell-Free Fetal DNA?

This test examines small fragments of DNA circulating in a pregnant woman's blood. Unlike the majority of DNA, which resides inside a cell's nucleus, these fragments are free-floating outside of cells and are referred to as cell-free DNA (cfDNA).

Most DNA in the body is found inside cells in a part called the nucleus. Cell-free DNA is DNA that is not found inside a cell and is floating freely in the blood. Cell-free DNA (cfDNA) is released into the bloodstream when cells die and break apart, as part of their life cycle.

During pregnancy, the mother has cfDNA belonging to cells found in the placenta circulating through her bloodstream. The cfDNA from the placental cells is identical to the fetus’ DNA or the DNA in placental cells typically matches the DNA of the fetus. By analyzing cfDNA from the placenta, healthcare providers can detect certain genetic abnormalities early, without posing any risk to the fetus. This placental DNA is called cell-free fetal DNA (cffDNA) and is what laboratories analyze during NIPT.

What Does Fetal Fraction Mean in Simple Terms?

Fetal fraction is calculated as the proportion of fetal (placental) DNA compared to the total amount of DNA in a blood sample.

In simple terms, fetal fraction means: “How much of the DNA in this blood sample belongs to the baby?”

For example, if 10 out of 100 DNA fragments are fetal in origin, the fetal fraction is 10%.

How Does Fetal Fraction Affect NIPT Results?

In order to analyze the placental cfDNA, there must be at least four percent fetal fraction. The average woman will have between 10 and 15 percent fetal fraction between 10 and 20 weeks gestation. Fetal fraction measuring less than three to four percent is considered unsuitable to obtain accurate test results.

What Is a ‘Normal’ or Good Fetal Fraction?

Most laboratories require a minimum fetal fraction of about 2–4% to issue a reportable NIPT result]

Typical fetal fraction ranges include:

Below 4% – often considered low and may result in no result
4%–15% – common range for most pregnancies
Above 15% – higher fetal fraction, generally associated with strong test confidence

The average fetal fraction between 10 and 20 weeks of pregnancy is around 10–15%.

What Determines Fetal Fraction?

Fetal fraction varies for each woman and is determined by multiple factors, including:

Weight: Women who are overweight or obese have lower fetal fraction
Gestation: Fetal fraction is at its highest between 10 and 21 weeks gestation
Chromosomal abnormalities in the fetus: For example, maternal blood tends to have a higher than average fetal fraction when the fetus has trisomy 21, also called Down syndrome.
Whether the pregnancy is considered a singleton (one fetus) or a twin (two fetuses): Fetal fraction has been shown to increase by 1.6 fold for twin fetus pregnancies compared to singleton pregnancies.

Several biological and technical factors can affect fetal fraction levels:

Biological factors

Gestational age
Maternal body mass index (BMI)
Singleton vs. twin or multiple pregnancy
Placental size and health

Higher maternal BMI is associated with lower fetal fraction due to increased maternal cfDNA diluting the fetal signal.

Technical factors

Blood sample handling and storage
Delays in processing the sample

Improper handling can cause maternal cells to break down, increasing maternal DNA and lowering the relative fetal fraction.

How Does Gestational Age Affect Fetal Fraction?

Fetal fraction increases as pregnancy progresses.

At earlier gestational ages (before 10 weeks), fetal fraction is often lower, which is why many providers recommend waiting until at least 10 weeks for NIPT.

As the placenta grows, it releases more fetal DNA into the maternal bloodstream, increasing the fetal fraction.

What if my NIPT Results Indicate Low Fetal Fraction?

If your NIPT results indicate that there was a low fetal fraction, this does not mean that there is a health concern for the fetus. A low fetal fraction measurement in NIPT testing simply means that there was not enough placental cfDNA to obtain accurate results. In this case, your doctor may recommend that you repeat the test a little later on in your pregnancy when fetal fraction is more likely to be higher. If you receive a low fetal fraction measurement in your second blood draw, your doctor may recommend alternative methods of prenatal screening.

What Does Low Fetal Fraction Mean?

A low fetal fraction does not automatically mean there is a problem with the baby.

Low fetal fraction most commonly occurs because the test was done too early, maternal BMI is higher, or there are placental variations.

When fetal fraction is too low, providers may recommend:

Repeating the test at a later gestational age
Using alternative screening methods
Considering diagnostic testing if clinically indicated

Can High Fetal Fraction Mean Anything?

In most cases, a higher fetal fraction is beneficial because it improves test reliability.

Some research suggests unusually high fetal fraction levels may be associated with certain placental conditions, but this is still an area of ongoing study and not typically a cause for concern on its own.

Have Questions About NIPT and Fetal Fraction?

As mentioned above Fetal fraction means the percentage of fetal (placental) DNA present in a pregnant person’s blood sample.

If you have questions regarding fetal fraction and how it affects NIPT screening, consider speaking with a genetic counselor.

In general, anyone considering prenatal screening should consider working with a genetic counselor. Genetic counselors are medical professionals who have completed specialty training in medical genetics and counseling. A prenatal and preconception genetic counselor assists expecting mothers and couples through the process of prenatal screening and prenatal diagnostic testing. They support their patients by interpreting genetic testing results, providing information on testing options, and emotionally supporting patients undergoing the prenatal genetic testing process.

If you need legal advice search in your area for a professional such as ‘Atlanta Birth Injury Lawyer’. It’s important to find help from those experienced in handling these types of issues who can provide you with the legal guidance and support you need during this challenging time.

SOURCES:

*Please consult a licensed medical practitioner before choosing any medication or health plan.

Scientific journal articles for further reading

Committee Opinion No. 640: Cell-Free DNA Screening For Fetal Aneuploidy. Obstet Gynecol. 2015 Sep;126(3):e31-7. doi: 10.1097/AOG.0000000000001051. PubMed: 26287791.

Goldwaser T, Klugman S. Cell-free DNA for the detection of fetal aneuploidy. Fertil Steril. 2018 Feb;109(2):195-200. doi: 10.1016/j.fertnstert.2017.12.019. PubMed: 29447662.

Rose NC, Kaimal AJ, Dugoff L, Norton ME; American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics; Committee on Genetics; Society for Maternal-Fetal Medicine. Screening for Fetal Chromosomal Abnormalities: ACOG Practice Bulletin, Number 226. Obstet Gynecol. 2020 Oct;136(4):e48-e69. doi: 10.1097/AOG.0000000000004084. PubMed: 32804883.

PIN FOR LATER