NIPT is typically done at 10 weeks of pregnancy or later. Before that point, the amount of fetal DNA circulating in your bloodstream is too low to produce reliable results. Most providers order the test between weeks 10 and 13, though it can be performed at any point after week 10 through the remainder of pregnancy.
Why 10 Weeks Is the Earliest
NIPT works by analyzing tiny fragments of DNA from the placenta that float freely in your blood. These fragments, called cell-free DNA, start appearing as early as five weeks of pregnancy, but at that stage there’s simply not enough of them. The fetal DNA needs to make up at least 2% to 4% of the total cell-free DNA in your blood for the test to work. By week 10, fetal DNA accounts for roughly 10% of the total, which gives the lab enough material to produce an accurate reading.
When fetal fraction falls below that threshold, two things can happen: the lab either can’t generate a result at all, or the test returns a false negative, meaning it misses a chromosomal condition that’s actually present. This is why your provider will confirm your gestational age with an ultrasound before ordering the test.
The Ideal Testing Window
Most people have NIPT drawn between 10 and 13 weeks, during the first trimester. Testing in this window gives you results early enough to plan next steps if anything comes back high-risk, while also ensuring enough fetal DNA is present for accuracy. Results usually come back within one to two weeks, so if you test at week 10, you’ll likely have answers by week 12.
There’s no upper limit on when NIPT can be done. If you missed the first-trimester window or switched providers, the test still works in the second or third trimester. Fetal fraction actually increases as pregnancy progresses, so later testing tends to be equally or even more reliable. That said, earlier results give you more time and more options for follow-up.
What NIPT Screens For
NIPT is the most sensitive and specific screening test available for the three most common chromosomal conditions: trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). In a study of nearly 147,000 pregnancies, the detection rate was 99.2% for trisomy 21, 98.2% for trisomy 18, and 100% for trisomy 13, with specificity above 99.9% for all three.
The American College of Obstetricians and Gynecologists now recommends that cell-free DNA screening be routinely available to all pregnant patients, not just those considered high-risk. You can also opt in to screening for sex chromosome conditions. However, ACOG does not recommend routine screening for microdeletions (small missing pieces of chromosomes) in the general population, noting that patients interested in that information should consider diagnostic testing instead.
Screening vs. Diagnostic Testing
NIPT is a screening test, not a diagnostic one. A high-risk result means the probability of a chromosomal condition is elevated, but it doesn’t confirm the diagnosis. If your NIPT comes back positive for any trisomy, you’ll be offered a diagnostic procedure, either chorionic villus sampling (which can be done as early as 10 to 13 weeks) or amniocentesis (typically available from 15 weeks onward). These tests analyze fetal cells directly and give a definitive yes or no answer.
The key advantage of NIPT over older screening methods like the first-trimester blood panel is that it catches more true positives and produces far fewer false alarms. That means fewer people end up needing invasive testing, which carries a small risk of complications including miscarriage.
Factors That Can Delay Testing
Certain factors reduce fetal fraction in your blood, which can lead to a failed test or a “no call” result where the lab can’t provide an answer. The most significant is high BMI. Women with obesity have higher turnover of their own cells, which dilutes the relative proportion of fetal DNA. Test failure rates can reach as high as 24% in women with significant obesity, compared to a few percent in the general population. If you have a high BMI, your provider may suggest waiting a week or two beyond the 10-week mark to allow fetal fraction to rise, or they may recommend an alternative screening approach.
Early gestational age is the other common reason for a no-call result. If dating is off by even a week, what you think is 10 weeks might actually be 9, and that single week can make the difference between having enough fetal DNA and falling short. A dating ultrasound before the blood draw helps avoid this.
NIPT With Twins
NIPT works for twin pregnancies and is available from late in the first trimester, similar to singletons. For identical twins, accuracy is comparable to or better than singleton testing because both babies share the same DNA and contribute to a higher combined fetal fraction. For fraternal twins, the test is slightly more complex since each baby contributes its own DNA, but screening for trisomy 21 in twins still shows sensitivity around 99%. The main difference is that twin pregnancies have a somewhat higher rate of inconclusive results due to lower per-fetus fetal fraction.
What to Expect on Test Day
NIPT requires a simple blood draw from your arm. No fasting, no special preparation, no ultrasound gel. The sample is sent to an external lab, and results typically arrive within 7 to 14 days. Your provider receives the results first and then contacts you, often with a phone call or portal message. If everything is low-risk, that’s usually the end of the screening process. If a result comes back high-risk, your provider will walk you through diagnostic options and connect you with a genetic counselor if one isn’t already involved.