From the moment they hear the news "you are pregnant," what most people wonder about first is "whether the baby is healthy." In the clinic, because there are so many kinds of prenatal tests, there are really many people who get confused about what NIPT is and what amniocentesis is, and when to undergo which test. The test that confirms pregnancy itself and the test that examines fetal chromosomal abnormalities differ in both purpose and timing. In this article, dividing prenatal tests into the two branches of screening tests and diagnostic tests, I will outline in what order they proceed according to gestational week.
Screening tests and diagnostic tests differ in purpose
The first button to understanding prenatal tests is to distinguish the difference between screening tests and diagnostic tests. The two may look similar but their roles are completely different.
A screening test estimates how high the possibility is that the fetus has a chromosomal abnormality such as Down syndrome. Because it is done with only the mother's blood or ultrasound, there is almost no risk to the fetus, but a positive result does not mean the abnormality is confirmed. The American College of Obstetricians and Gynecologists (ACOG, 2020) likewise states that cell-free DNA screening is not equivalent to a diagnostic test and has the possibility of false positives and false negatives.
A diagnostic test, by contrast, directly collects fetal cells and checks the chromosomes, so a confirmed diagnosis is possible. Instead, because a needle accesses inside the uterus, although at a low frequency, procedure-related risks are reported. So in the clinic, we usually guide a flow of first gauging the risk with a screening test, then recommending a diagnostic test only to those who need it.
A screening test "looks at the possibility," a diagnostic test "confirms the diagnosis" — if you remember just this one line, the explanations that follow will read much more easily.
The first-trimester fetal anomaly test is done in early pregnancy
The first-trimester fetal anomaly test is an early screening test received around weeks 11 to 13 of pregnancy. Commonly called "early detailed" or "first fetal anomaly test," this test looks at two things together.
The first is the nuchal translucency measurement, which measures the clear space at the back of the fetus's neck by ultrasound. The thicker this is, the more an association with chromosomal abnormality is reported. The second is a serum test measuring marker substances such as PAPP-A and free beta hCG in the mother's blood. According to ACOG (2020), first-trimester combined screening, performed with ultrasound and serum testing together, is recommended between weeks 10 and 13 weeks 6 days of pregnancy.
If this window is missed, the first-trimester test is hard to perform. So it is important to have the first prenatal exam not too late. If you are curious about the early-pregnancy exam and checkup schedule, referring to the pre-pregnancy test guide also helps.
We look again with the second-trimester fetal anomaly test
The second-trimester fetal anomaly test is a screening test done with the mother's blood around weeks 15 to 20 of pregnancy. Commonly called the "quad test," it measures four marker substances in the blood to assess the risk of Down syndrome, Edwards syndrome (trisomy 18), and neural tube defects.
The second-trimester test plays a role of complementing the first-trimester test. It is especially meaningful for those who started their first prenatal exam in the second trimester or missed the timing of the first-trimester test. There is also an integrated/sequential screening method that integrates the results of the two periods to calculate the risk, so it is combined to fit the mother's situation.
However, the point that both the first- and second-trimester tests are screening tests after all is the same. If the risk comes out high, you consider a more accurate test as the next step, and here NIPT and diagnostic tests come in.
If you are curious about the prenatal test schedule, get a consultationNIPT is a high-accuracy screening test
NIPT (non-invasive prenatal testing) is a screening test that analyzes fragments of fetal-derived DNA floating in the mother's blood. In clinical experience, it is also the test for which inquiries have increased the most over the past few years.
NIPT is done with only the mother's blood draw, so there is almost no risk to the fetus. ACOG (2020) explains that NIPT is the screening test with the highest sensitivity and specificity for common chromosomal abnormalities such as trisomy 21, 18, and 13, and can be performed any time after weeks 9 to 10 of pregnancy. Also, through the 2020 recommendation revision, it broadened its position so that NIPT can be guided and recommended to all pregnant people regardless of the mother's age or risk factors.
The points of caution are also clear. NIPT has high accuracy but is still a screening test, and cannot replace a diagnostic test. If a high-risk result comes from NIPT, a diagnostic test such as amniocentesis is needed to confirm it. Those curious about tests that handle genetic information in general are recommended to also look at information that genetic testing can reveal.
Amniocentesis is a diagnostic test done around weeks 16 to 20 of pregnancy
Amniocentesis is a diagnostic test that collects amniotic fluid with a thin needle and directly analyzes fetal cells. If a screening test looks at the possibility, amniocentesis is a test that checks the chromosomes to confirm the diagnosis.
As explained in the original article, amniocentesis performs the puncture around weeks 16 to 20 of pregnancy to obtain enough culturable cells. After culturing a small amount of collected amniotic fluid, chromosomal and biochemical tests are carried out with the cultured cells. ACOG (2020) recommends performing amniocentesis after week 15 of pregnancy, and does not recommend early performance before week 15.
The procedure proceeds in the following order.
- The position of the fetus and placenta is checked first by ultrasound
- The abdominal skin is locally anesthetized
- Amniotic fluid is slowly aspirated with a thin needle
- The entire process is carried out through sterile disinfection
If there are few fetal-derived cells in the collected amniotic fluid, or maternal blood is mixed in, re-collection may be needed. As it is a diagnostic test, although at a low frequency, a procedure-related miscarriage risk is reported, so it is best to decide whether to test after thorough consultation with the attending doctor.
If a diagnostic test is needed in early pregnancy, consider chorionic villus sampling
Chorionic villus sampling (CVS) is another diagnostic test that collects part of the placental tissue and checks the chromosomes. Like amniocentesis, a confirmed diagnosis is possible, but the timing of performance differs.
The biggest characteristic of CVS is that it can be performed around weeks 10 to 13 of pregnancy, that is, at an earlier time than amniocentesis (ACOG, 2020). So it is meaningful for those who need an early confirmed diagnosis due to family history or early screening test results. However, neural tube defects are not confirmed by CVS, so a separate serum test or ultrasound is also needed.
Organizing it at a glance by week gives the following.
| Test | Classification | Approximate timing | Method |
|---|---|---|---|
| First fetal anomaly test | Screening | Weeks 11–13 | Nuchal translucency ultrasound + serum |
| NIPT | Screening | After week 10 | Mother's blood draw |
| Chorionic villus sampling | Diagnostic | Weeks 10–13 | Placental tissue collection |
| Second fetal anomaly test | Screening | Weeks 15–20 | Mother's blood draw (quad) |
| Amniocentesis | Diagnostic | Weeks 16–20 | Amniotic fluid puncture |
To whom do we end up recommending a diagnostic test?
Diagnostic tests such as amniocentesis or chorionic villus sampling are not tests recommended uniformly to all pregnant people. In the clinic, we consider a diagnostic test together in the following cases.
- When the mother's age at delivery is 35 or older (31 or older for twin pregnancy)
- When she has previously been pregnant with a fetus with a chromosomal abnormality
- When one or more of the parents has a chromosomal abnormality
- When a close relative has a history of chromosomal abnormality or Down syndrome
- When there is a history or family history of a Mendelian or multifactorial genetic disease
- When there is a history of recurrent (habitual) miscarriage
- When a screening test shows a positive finding for chromosomal abnormality
- When there is an abnormal ultrasound finding
However, ACOG (2020) holds that a diagnostic test, too, should be able to be guided to all pregnant people who want it, regardless of the mother's age or risk factors. In other words, even if you do not fall under the above items, you can choose the test if you wish after thorough consultation. Which test to undergo at which time differs from mother to mother, so I recommend examining your individual situation at the pregnancy/contraception clinic.
I cheer on all the mothers in the world. Testing is not to amplify anxiety but a process to help you know and prepare in advance. If you are at a loss about which test you need, try organizing your prenatal test plan together via chat consultation. I will guide you so that you can proceed while fully understanding the test you undergo and its meaning.
Written by: Lee Dong-hee Director · Obstetrics and Gynecology Specialist · View medical staff profile
First published June 3, 2024 · Last reviewed May 30, 2026
References: American College of Obstetricians and Gynecologists, Practice Bulletin No. 226 Screening for Fetal Chromosomal Abnormalities (2020), ACOG Prenatal Genetic Diagnostic Tests FAQ (2020)
This article is intended to provide general health information and does not replace individual diagnosis or treatment. If you have symptoms, please consult through an examination.