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What Is Sequential Screening Test: Why It Is Required And How It Works

6 min read
What Is Sequential Screening Test: Why It Is Required And How It Works

What Is Sequential Screening Test: Why It Is Required And How It Works

Pregnancy is a precarious time when many tests must be conducted for the excellent health and development of the mother and the baby. Some of the screening tests conducted during pregnancy are:

Not everybody has to undergo screening tests. One of  such screening test is “sequential screening.” This screening is an added test done for a diagnosis that is optional and suggested by the doctor. It is a test which most of the expectant mothers are not comfortable, but since it is crucial for the mother, we will highlight everything related to this test.
Going forward, we will be discussing what this test all about, the cost involved is, the reasons why it needs to be done, its duration, procedure, efficacy, etc.

What Is Sequential Screening?

This screening consists of a sequence of tests done in the prenatal stage like ultrasound and blood test for checking the defects in the neural tube and genetic aberrations. This test is done in two steps to assess the probability of the fetus with Down syndrome, anencephaly, Trisomy 18 Spina Bifida, etc.
The process of screening assimilates both the findings of the blood tests and the report of the ultrasound from the first trimester. This test is not mandatory and can be opted if one wants. A medical practitioner helps in deciding if the mother should undertake this test.
These tests cannot help in diagnosing a specific situation but helps in comprehending any threats which are there.

How Does Sequential Screening Test Work?

How Does Sequential Screening Test Work

How Does Sequential Screening Test Work

The screening process is carried in two-steps, the first being the ultrasound and the blood test that is carried within ten to fourteen weeks and another stage where a single blood test is performed at fifteen to twenty-one weeks.

1. Sequential Screening Stage One:


NT refers to the accumulation of the fluid at the rear portion of the neck of the baby. The ultrasounds also assist in assessing the fluid in the baby’s body. All babies who get diagnosed with trisomy 18, Down syndrome, or other anomalies are disposed to an additional fluid as well as higher NT.
Apart from the NT magnitude, the ultrasound done during the first trimester helps in determining the baby’s development, gender, and gestational age.

Blood testing:

The placenta is known to produce proteins at the time of pregnancy, which can be transmitted to the mother’s blood as well. The babies who are diagnosed with any anomalies, Down syndrome, trisomy 18 will exhibit higher protein levels. The blood tests also help to assess the following threats, as well.

  • PAPP-A

The lower protein levels increase the chances of threats of trisomy 18 and down syndrome. It is related to other pregnancy threats comprising of high BP ratings, low weight at birth, miscarriage, etc. but all are not necessarily a sign of unhealthful pregnancy.

  • HCG:

The increased levels of HCG are a sign of Down’s syndrome and trisomy 18. Different types of HCG screening are done for detecting protein levels.

2. Sequential Screening Stage Two:

Quad screen, as it is also commonly known as comprises of the blood test that assists in calculating the Alpha-fetoprotein, dimeric inhibin, HCG free estriol.
The placenta produces the increased number of proteins that get transmitted to the mother’s blood, and all babies with anomalies show either a too high or too low level of proteins.
The typical proteins that can be calculated in stage two of the blood test are as follows:


The liver of the baby produces a protein in the second half of the pregnancy called Alfa Feto Protein. The level of this protein eventually increases as pregnancy advances. However, the level of AFP will be on the higher side in case the baby has any threats of a defect that happens when the spinal cord and the spine do not develop properly.
Thus, increasing the AFP levels in mothers’ blood indicates neural tube defects such a Spina bifida or chromosomal issues such as Down syndrome in the developing baby. Whereas, AFP levels are found significantly reduced if the baby has any risks of Edward’s syndrome.


The placenta produces HCG protein soon after the implantation of the fertilized egg. However, its levels start diminishing during the second trimester. While increased levels are related to Down Syndrome, declining levels are linked with trisomy 18.

Unconjugated estriol (UE3):

Both the liver and the placenta produce this protein, and this stays at increased levels all through the pregnancy. Decreased levels in protein indicate the baby having chances of Edward’s Syndrome.

Dimeric inhibin A (DIA):

Placenta produces this protein, and this protein level lies steady among the 15th as well as the 18th week. The increased protein levels are connected with Trisomy 21, while the decreased levels with Edward’s Syndrome.

How Accurate Is the Sequential Screening Test?

How Accurate Is the Sequential Screening Test

How Accurate Is the Sequential Screening Test

The sequential screening test cannot reveal any genetic anomalies. The decisiveness is dependent on the individual’s skills and the test results, as well. This screening, on the other hand, helps to find the following:

  • 90% of the infants with Down syndrome
  • 80% of the newborn with trisomy 18
  • 80% of the newborn baby with Spina bifida

Trisomy 21, also referred to as Down syndrome, is produced by an additional chromosome, and one in about 600-900 babies is born with it. It also causes cardiovascular disease and mental disorders amongst all. The possibility of Down syndrome in a baby multiplies with the age of the mother.
The growing baby has an additional chromosome that leads to Trisomy 18. One in 6000-8000 babies is born with it. It can lead to mental retardation, cardiovascular disease, weak growth, etc. Alike Trisomy 21, the possibility of Edward’s syndrome also increases with the age of the mother.
ONTD or Open neural tube defects take place if the spine is not formed appropriately, exposing and damaging the spinal cord. One out of 1,000 infants has the chance to foster these anomalies.
The results of this test are an indication to show the baby having genetic anomalies, but other tests are recommended as well.

How Much Time Do the Results Take?

The results of the sequential screening can be expected in a week’s or less after submitting the samples of the blood.

Sequential Screening Test Results:

The very first round of the ultrasound assesses the levels of NT. In case they are too high, the doctor will recommend more tests. To evaluate the cell-free fetal DNA (cf DNA), chorionic villus sampling may conduct. In case the NT or the nuchal translucency fluid is not normal, there are more chances of the baby to be detected with chromosome related anomalies.

What to Do If Sequential Screening Shows High Risk or Abnormal Results?

In a case where the sequential screening shows high risks or not typical results, the doctor will most probably refer the mother to a specialist who deals with cases of high risks in pregnancy like a perinatologist, maternal medicine fetal specialist, etc. These specific doctors can thus recommend additional tests or even refer to other counselors who can describe the detailed results, resolving all queries, and make the ideal decision.
Advanced testing options comprise of prenatal screening. An ultrasound will be performed. Amniocentesis, as well as a test that helps to assess if the lady has higher chances of having a baby with trisomy 21, will also be performed (all of them not compulsory though).

What to Do If Sequential Screening Shows Low Risk or Normal Results?

In case the screening results of stage one are on the normal side, the doctor will ask for a revert at 15-22 weeks for screening at stage 2, and the findings at this stage will further help in assessing the trisomy 18, down syndrome and other anomalies. In case the results are not abnormal and negative, and the baby has fewer chances to have genetic defects. The medical practitioner will persist in keeping monitoring the baby all through the prenatal checks.

Are Sequential Screening Procedures Standard for All Pregnant Women?

Sequential screening tests are recommended to all the expectant women, yet more importantly, to expecting mothers having a high-risk pregnancy. Sequential screening tests are conducted for women who are:

  • Above 35 years in age
  • Those who have had congenital disabilities in the family history
  • Those who underwent radiation treatments
  • Those who contracted diabetes
  • Those who had massive exposure to medicines.

Does Sequential Screening Also Work Accurately for Twins?

Twin detection is also done by sequential screening; however, the evaluation is a tricky one, and its accurateness is lower as well. Distinct laboratories have several approaches to this screening. Few medical experts separately evaluate every fetus, while others assess them as a unit as a whole. The doctor can throw more light with details related to the screening.
Thus, sequential screening helps to figure the risk that the baby has concerning anomalies in chromosomes and offer the next due step. It is even essential to let a genetic/ medical practitioner analyze the results for better clarity and vision.

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