NIPT (Cell-Free DNA Test)

What is NIPT, and how and when is it done?

NIPT (non-invasive prenatal test, that is, a prenatal test that does not involve entering the womb) is carried out on the cell-free DNA (small pieces of DNA floating freely in the blood, outside cells) circulating in the mother's blood. In pregnancy, some of these fragments in the mother's blood belong to the baby; most of them pass into the blood from the placenta (the afterbirth). In other words, the test largely reflects the genetic make-up of the placenta; this is why, in the rare situations where the chromosomes of the placenta and the baby differ, the result can be misleading.

• How is it done? A single tube of blood taken from the mother's arm is enough. The baby and the pregnancy sac are not touched; for this reason the test does not carry a risk of miscarriage.
• When is it done? Once enough of the baby's DNA has built up in the mother's blood, it can be done from the start of week 10 of pregnancy.

What does it screen for, and how reliable is it for each?

NIPT mainly screens for the three most common trisomies (having three copies of a chromosome instead of two). The detection rates (the percentage of affected babies the test picks up) reported in the guidelines are shown below:

The sex chromosome differences in the table are: Turner syndrome (one of the two X chromosomes missing in a girl), Klinefelter syndrome (an extra X chromosome in a boy), triple X and XYY (an extra sex chromosome). In this group the accuracy of the test is lower than for the three trisomies.

A long list of conditions screened for does not mean the test is more reliable. Microdeletions in particular are very rare in the population, so a considerable share of positive results for these conditions may be false alarms.

This table shows how well the test picks conditions up. But what it means when your result comes back "positive" is a separate question — it is explained below.

What does a "positive" (high-risk) result mean?

The laboratory accuracy of NIPT is very high. Even so, the chance that a positive result really shows the condition is present in the baby — the positive predictive value (the proportion of positive results that turn out to be correct) — depends on how common the condition is in that group of pregnant women.

• In pregnant women aged 35 and over, Down syndrome is more common, so the chance that a positive NIPT result is correct can reach 80–90% according to the sources.
• In young pregnant women found to be at low risk in other tests, the condition is rare, so a considerable share of positive results may be false positives (the test showing high risk although the baby does not have the condition). In this group the chance that a positive result is correct can fall to 40–50%, or even lower.

You can think of it this way: the rarer a condition is, the larger the test's very small margin of false alarms becomes compared with the real cases. This is why the same test result carries different meanings in different age and risk groups. When the result is interpreted, your age and your previous screening results are considered together.

The known causes of a false positive result are:

• Confined placental mosaicism (the chromosome difference being present only in the placenta, not in the baby)
• Vanishing twin (the DNA of a twin that stopped developing early remaining in the mother's blood)
• Unrecognised chromosome differences in the mother herself, or certain health conditions

NIPT is not a diagnostic test: how is the result confirmed?

Confirmation is done, depending on the week of pregnancy, with one of two invasive diagnostic tests (tests in which a sample is taken and the chromosomes are examined directly):

• Chorionic villus sampling — CVS (taking a small tissue sample from the placenta): in weeks 11–14 of pregnancy.
• Amniocentesis (taking a sample of the fluid around the baby): after week 15.

In the sample taken, the baby's chromosomes are assessed by karyotyping (counting the chromosomes and examining their structure) or by microarray (a detailed genetic test that also shows small missing or extra pieces of chromosomes), and the result is settled in this way.

What does "no result" (no-call) mean?

For the test to work reliably, a sufficient part of the cell-free DNA in the mother's blood has to belong to the baby. This proportion is called the fetal fraction (the share of DNA in the mother's blood coming from the baby and the placenta) and it generally needs to be above 4%. In about 1–5% of pregnant women this proportion is too low, and a "no result" report may be issued. Possible causes:

• The mother's weight being high (the baby's DNA being diluted because the blood volume is larger)
• The blood being taken too early, before week 10
• The mother taking anticoagulant (blood-thinning) medicine
• Certain situations in which the placenta develops poorly — such as trisomy 13, trisomy 18 and triploidy (three copies of every chromosome)

Because of the last point, in pregnancies where no result is obtained the risk of these conditions may be slightly increased. For this reason the doctor explains the options of a detailed ultrasound examination, repeating the blood sample, or an invasive diagnostic test.

NIPT in twin pregnancies and IVF pregnancies

• Twin pregnancy: whether the twins have separate placentas or share one, the detection rate of NIPT for Down syndrome is high (98–99%). ISUOG and the UK National Screening Committee accept the use of NIPT for screening for trisomies 21, 18 and 13 in twin pregnancies.
• Vanishing twin: the DNA of a twin lost early can stay in the mother's blood for months and can lead to a false positive result. This information should be shared with the doctor before the test.
• IVF pregnancies: NIPT can be used in IVF (in vitro fertilisation) pregnancies, including those achieved with donated eggs. However, in twin pregnancies conceived by IVF and when the mother's weight is high, the chance of no result because of a low fetal fraction is somewhat higher than in single pregnancies.

NIPT, ultrasound and combined/triple screening

The detailed ultrasound done in weeks 11–14 of pregnancy is needed to examine the nuchal translucency (measurement of the fluid layer at the back of the baby's neck), the nasal bone and structures such as the head, abdomen and heart, as well as to assess major structural differences and the risk of early pre-eclampsia (a pregnancy condition with high blood pressure). What is looked at in this examination is described in detail on the Screening Tests in Pregnancy page.

• If the nuchal translucency measures 3.5 mm or more on ultrasound, or a clear structural difference is seen, the guidelines recommend going directly to an invasive diagnostic test (CVS or amniocentesis with microarray), which allows a wider genetic examination, rather than waiting for NIPT.
• Depending on the result of combined screening (a risk calculation in the first three months that assesses the nuchal translucency measurement together with two hormone values in the mother's blood), some guidelines recommend NIPT as a second-step test (see below).

Who is NIPT recommended for?

The guidelines take two different approaches on this:

• ACOG and ACMG (USA): recommend that NIPT be offered to all pregnant women at a suitable week of pregnancy, regardless of age or personal risk, as one of the screening options, based on informed choice.
• RCOG, NICE and many European guidelines: offer NIPT mainly as a second-step screening test to pregnant women found to be at intermediate or high risk in first-trimester combined screening.

The priority groups are: pregnant women aged 35 and over, those found to have an increased risk of chromosome differences on ultrasound or blood tests, those with a previous pregnancy affected by a trisomy, and those with a family risk of chromosome differences. Which test suits you is decided together with your doctor, according to your age, your previous test results and your preferences. For the other check-ups and tests in pregnancy, see the Pregnancy Care page.

At the practice in Muratpaşa, Antalya, information is given about the screening options in pregnancy.