NIPT, or noninvasive prenatal testing, screens for the most common chromosomal conditions in a developing baby, primarily Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). It can also reveal the baby’s sex and detect sex chromosome differences. The test works by analyzing tiny fragments of DNA floating in the pregnant person’s blood, and it can be done as early as 10 weeks into pregnancy.
How the Test Works
Throughout pregnancy, the placenta sheds cells into the mother’s bloodstream. As those cells break down, small fragments of DNA (usually fewer than 200 building blocks long) are released and circulate alongside the mother’s own DNA fragments. A standard blood draw captures this mix, and a lab then sorts out which fragments come from the placenta, which shares nearly all of its genetic material with the baby.
The proportion of DNA in the blood sample that originates from the placenta is called the fetal fraction. It generally needs to be above 4% for the test to produce reliable results, and that threshold is typically reached around 10 weeks of pregnancy. Several factors can lower the fetal fraction: testing too early, a higher maternal body weight, sampling errors, or occasionally a fetal abnormality itself. When the fetal fraction is too low, the lab may report that no result could be generated and recommend a repeat draw.
The Three Main Trisomies
NIPT’s core purpose is screening for three extra-chromosome conditions, each caused by having three copies of a particular chromosome instead of the usual two.
- Down syndrome (trisomy 21) is the most common. NIPT detects it with roughly 99% sensitivity, meaning it catches nearly every affected pregnancy. When a result comes back positive in a general-risk population, there is about an 80% chance the baby truly has the condition.
- Edwards syndrome (trisomy 18) causes severe developmental differences, and most affected pregnancies do not survive to term. NIPT sensitivity is around 98%, but because the condition is rarer, a positive result has about a 41% chance of being a true positive.
- Patau syndrome (trisomy 13) is the rarest of the three and also the hardest to detect. Sensitivity drops to around 80%, and a positive result carries only about an 11% chance of reflecting a true diagnosis.
That gap between detection rate and the chance a positive result is real is important. It illustrates why NIPT is a screening test, not a final answer. A “high risk” result means more information is needed, not that a diagnosis has been made.
Sex Chromosome Conditions
Most NIPT panels also screen for differences in the X and Y chromosomes. These conditions are generally less severe than the three main trisomies but can affect growth, fertility, and development in varying degrees.
- Turner syndrome (45,X) affects females who have one X chromosome instead of two. It is the most frequently detected sex chromosome difference and can present in mosaic forms where only some cells are affected.
- Klinefelter syndrome (47,XXY) affects about 1 in 660 males and involves an extra X chromosome. Many people with Klinefelter syndrome are not diagnosed until adulthood, as symptoms can be subtle.
- Triple X syndrome (47,XXX) involves an extra X chromosome in females and often causes few noticeable symptoms.
- 47,XYY syndrome involves an extra Y chromosome in males and is similarly mild in most cases.
Because NIPT reads DNA from the placenta, and sex chromosome conditions can exist in mosaic patterns, accuracy for these conditions is lower than for the three main trisomies. False positives and false negatives are both more common here.
Fetal Sex
Since the test analyzes DNA that includes sex chromosomes, NIPT can tell you the baby’s sex as early as 10 weeks, well before an anatomy ultrasound (typically done around 18 to 20 weeks). Most labs report this automatically, though you can usually ask not to be told if you prefer to wait.
Expanded Panels and Microdeletions
Some labs offer expanded NIPT panels that look beyond the standard conditions. These may include microdeletion syndromes, where a small piece of a chromosome is missing rather than an entire extra chromosome being present. Common additions include 22q11.2 deletion (associated with DiGeorge syndrome), Prader-Willi syndrome, and Cri-du-chat syndrome. Some panels also screen for rare autosomal trisomies involving chromosomes other than 13, 18, and 21.
The reliability of these expanded screens is substantially lower. Rare autosomal trisomies show up in only about 0.2% of pregnancies, and the positive predictive value is just 4% to 6%, meaning the vast majority of positive results turn out to be false alarms. For microdeletions, the numbers are similarly limited. Both the Society of Obstetricians and Gynecologists of Canada and the International Society for Prenatal Diagnosis currently recommend against routine screening for microdeletions and rare trisomies through NIPT.
That said, a positive result for a rare autosomal trisomy is not meaningless. Even when the baby itself does not carry the trisomy, the result may reflect mosaicism in the placenta, which has been linked to a higher rate of pregnancy complications. In one study, 26% of pregnancies with false-positive results originating from placental mosaicism experienced adverse outcomes.
NIPT in Twin Pregnancies
NIPT can be performed with twins, but accuracy drops. While the total amount of placental DNA in a twin pregnancy is about 1.6 times higher than in a singleton pregnancy, the amount per individual baby is actually lower (roughly 0.8 times the singleton level for non-identical twins). This makes it harder for the test to pick up a condition affecting just one twin.
One meta-analysis found that sensitivity for Down syndrome in twins was reduced by about 9%, and for Edwards syndrome by about 22%. Test failure rates are also higher: up to 13% of twin pregnancies fail to produce a result on the first blood draw, and in IVF-conceived twins, that failure rate can climb above 50%. If one twin has been lost earlier in the pregnancy (sometimes called a “vanishing twin”), NIPT is generally not recommended at all. The demised twin’s placenta continues to release DNA, which can trigger false positives or report the wrong sex.
For triplet pregnancies, NIPT availability is limited. Some specialized labs offer it beginning at 12 weeks rather than 10.
Screening, Not Diagnosis
NIPT predicts whether a baby is at high or low risk for a chromosomal condition. It does not diagnose anything. The DNA being analyzed comes from the placenta, which usually matches the baby’s genetics but not always. Differences between placental and fetal chromosomes (confined placental mosaicism) are one of the main reasons false positives occur.
If your NIPT result comes back high risk, the typical next steps include genetic counseling, a detailed ultrasound, and a diagnostic procedure: either chorionic villus sampling (CVS), performed between 10 and 13 weeks, or amniocentesis, done after 15 weeks. These tests analyze cells directly from the placenta or amniotic fluid and provide a definitive answer. A low-risk NIPT result, on the other hand, is highly reassuring for the three main trisomies, though no screening test can guarantee a completely typical pregnancy.
Factors That Affect Results
Several things can influence whether NIPT produces accurate, usable results. Maternal obesity is the most common cause of low fetal fraction, because a higher volume of maternal DNA in the blood dilutes the placental signal. Testing before 10 weeks carries the same dilution problem from the other direction: there simply has not been enough time for placental DNA to accumulate. In rare cases, a maternal chromosomal condition the pregnant person did not know about can interfere with results, as can certain tumors that shed abnormal DNA into the bloodstream.
When the lab cannot generate a result due to low fetal fraction, a second blood draw a week or two later often solves the problem. However, repeated test failures have themselves been associated with a slightly higher risk of chromosomal abnormalities, so your provider may recommend diagnostic testing even without a clear NIPT result.