Second Trimester Prenatal Screening Test

Second Trimester Prenatal Screening Test And Birth Defects Covered

The second-trimester prenatal screening test is also a routine blood test, called multiple markers blood test. These multiple markers provide information about a fetus’s (baby inside the womb) risk with certain genetic diseases or birth defects. Screening is usually done with an expectant mother’s venous or peripheral blood sample between the 15th and 20th weeks + 6 days of pregnancy.

If any pregnant woman has missed the first-trimester prenatal screening. Then she does need not to worry. For this purpose only, the second-trimester screening is done between the 15th and 20th weeks of pregnancy, and it is the same as the first-trimester screening only.

It is strongly recommended :

  • for pregnant women age 35 and older; 
  • Family history of birth defects.

What are birth defects?

Birth Defects are chromosomal health conditions that can be presented from birth. These birth defects can alter the shape or function of one or more parts of the body and can also cause common health problems. These depend on how the body function and develops.

What are the birth defects covered in the second-trimester screening test?

This blood test covers chromosomal defects/abnormalities such as :

  • Down’s Syndrome
  • Edward Syndrome
  • Patau Syndrome
  • Neural tube Defects

Down’s syndrome is the chromosomal condition when there are three copies of chromosome 21,  as usual of two copies.

Edward syndrome is the presence of three copies of chromosome 18.

Patau syndrome is the condition of three copies of chromosome 13.

Neural tube defect is birth defects of the brain and spinal cord also called Spina Bifida or NTD.

What are the blood markers measured in the second-trimester screening test?

The blood markers measured in this test include are :

  1. Alpha-fetoprotein (AFP): AFP is a protein normally produced by the liver of the fetus and is present in the fluid that surrounds the fetus (amniotic fluid) and crosses the placenta in the mother’s blood. The AFP blood test is also known as MSAFP (AFP maternal serum).

Abnormal levels of AFP can indicate:

  • ONTD: Open neural tube defects, such as spina bifida
  • Down syndrome
  • Other chromosomal abnormalities
  • Defects in the fetal abdominal wall
  • More than one embryo is producing proteins that are – Twin
  • An incorrectly calculated date, as levels, vary during pregnancy.

2. hCG: Human Chorionic Gonadotropin, a protein secreted by the placenta.

3.   Estriol: A hormone secreted by the placenta.

4. Inhibin: A Harmon secreted by the placenta.

Is it a confirmatory test for the above birth defects?

Screening for multiple markers is not diagnostic. This means that it is not 100% accurate and is just a screening test to determine who should be offered more pregnancy tests in the population. Incorrect positive results can occur – indicating a problem when the fetus is actually healthy or an incorrect negative result – indicating a normal outcome when the fetus is having health problems.

“When a woman takes a first and second-trimester screening test, the ability of tests to detect an abnormality is greater than using only one screening independently. Most cases of Down syndrome can be detected when the first and second trimester exams are used.”

When it is called the Triple and Quad Test?

This is only a screening test for MSAFP – Spina Bifida, this can be confirmed by USG and amniocentesis.

What is the importance of the Second Trimester Screening Test?

As the pregnant couple has visited the healthcare provider or Gynecologist in the first trimester. They must continue to do the same in the second trimester too.

These tests and checkups will help monitor the development of fetus and pregnant mothers as well.

Only frequent visits to the healthcare provider can tell if the pregnancy is in high-risk conditions. 

What parameters are included in the second-trimester screening test for risk calculations?

The parameters included in the second trimester for risk calculation include :

  1. Age, Date of Birth
  2. Weight ( Latest)
  3. LMP Date
  4. Some soft markers from ultrasound report such as :

5. Blood sample – In the laboratory where blood samples have been sent they check for hormones : 

  • hCG level
  • Estriol
  • Inhibin
  • Alfa Fetoprotein

If already done the first-trimester screening test, then the second-trimester screening test is required?

If the screening test for birth defects or chromosomal abnormalities is already done in the First Trimester then it is not required to do the Second Trimester Screening Test. Still, you must consult the healthcare provider or gynecologist.

(Disclaimer: Information provided in this piece of article is purely for educational purposes only. All results must be clinically correlated with the patient’s data to make an accurate diagnosis.) 


References :

  1. 2020. Quad Screen – Mayo Clinic. [online]
  2. Common Tests During Pregnancy. [online]


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