First Trimester Screening Tests to detect Fetal Anomalies


The incidence of chromosomal abnormalities in live babies is approximately 0.5-1%. Down syndrome (Trisomy21) is the most common chromosomal disorder among live births. The others are Patau’s syndrome (Trisomy13), Edward’s syndrome (Trisomy18) and Turner’s syndrome (Monosomy45X0).

Most of these Chromosomal abnormalities and structural defects can be identified with variable accuracy at the end of first trimester with the help of Ultrasonography and Double marker tests from the mother’s blood. They are together called as ‘Combined Tests’. They are routine prenatal screening tests and poses no risk of miscarriage or other pregnancy complications.

First Trimester Scan popularly called as NT Scan-

It is recommended between 11-13+6 weeks of Gestational Age. It is prudent to get this test done by well trained and experienced maternal fetal medicine specialists to avoid false results.

Its main objectives are

  • To measure the number of fetus and their size.
  • To calculate the expected date of delivery.
  • To detect abnormalities markers like –
  • Nuchal Thickness.

1- Nuchal Fold Transluscency Thickness.

Nuchal fold is the normal fluid filled clear space in the skin tissue at the back of the baby’s neck.

Its thickness>3 mm may indicate high risk of Chromosomal abnormalities especially Down’s syndrome, Congenital heart disease or other structural abnormalities like diaphragmatic hernia or exomphalos etc.

2- Nasal bone.

Around 70% of fetuses with the chromosomal abnormalities especially Down’s syndrome have absent or underdeveloped nasal bone against only about 1% of normal fetuses.

first trimester marker2

3- Ductus Venosus.

It is a slender trumpet like blood vessel that shunts the blood from the portion of umbilical vein in the liver to the inferior Venacava just before it enters the heart. This is to ensure that predominant portion of well oxygenated blood received from placenta reaches the fetal heart and from there to rest of the body.

Absence or reversal of blood flow in ductus venosus may indicate chromosomal abnormalities associated with major cardiac defects, fetal heart failure, immune or non immune fetal hydrops, academia or increased risk of fetal demise.

ductus venosus

4- Tricuspid Valve.

It is a tri-leaf valve on Right side of Heart between Right Atria and Right Ventricle. It permits one way blood flow from Atria to Ventricle during Ventricular relaxation but prevents back flow when the Ventricles contract favoring forward flow to lungs through Pulmonary artery.

The malfunction of tricuspid valve may result into backflow of blood into Right Atria. This may be associated with Chromosomal abnormalities with major heart defects, abnormal heart rhythm, Heart failure etc.

The isolated Tricuspid regurgitation in absence of other defects can be due to large heart size in 1-2% of normal babies.

5- Other miscellaneous abnormalities like neural tube defects etc can also be diagnosed.

Double Marker Test-

The levels of fetus & placenta related hormones in mother’s blood may help to screen chromosomal abnormalities in baby.

The Double marker tests include measurement of

  • Pregnancy associated Plasma Protein-A (PAPP-A) and
  • Human Chorionic Gonadotropin (HCG) hormones.

The level of these two hormones are adjusted according to various parameters like age of the mother, ethnicity, life style, smoking etc and expressed in Multiples of Median (MOM). The final risk assessment is given as low, moderate or high risk probability against age-adjusted risk for Trisomy 21, 13 and 18.

First Trimester Screening-English

In case of moderate to high risk, the patients are offered invasive definitive tests like chorionic villous sampling, amniocentesis or percutaneous fetal blood sampling, through which fetal cells are retrieved from chorionic tissue, amniotic fluid or fetal blood respectively and karyotyped for confirmation of chromosomal aneuploidies.


Recently a Non-invasive Prenatal Diagnostic tests (NIPDT) has been in clinical use that segregates cell-free fetal DNA from the mother’s blood for karyotype. It has a high specificity for chromosomal abnormalities and no risk for miscarriage as against invasive tests. But, it is very costly.


Limitations of Screening Tests:

  • First trimester screening correctly identifies about 85 percent of women who are carrying a baby with Down syndrome.
  • About 5 percent of women have a false-positive result, meaning that the test result is positive but the baby doesn’t actually have Down syndrome.
  • A low-risk result doesn’t guarantee that your baby won’t have one of these conditions. Likewise, a high-risk result doesn’t guarantee that your baby will be born with one of these conditions.

Things to know before undergoing Screening tests:

  • The screening tests are routine prenatal tests and pose no risk of miscarriage or pregnancy complications.
  • They are optional but it is advisable to undergo to rule out chromosomal aneuploidies in baby.
  • Discuss with your spouse and Doctor about what the results will mean to you? Whether the screening will be worth any anxiety it might cause in your case? How will it influence the management of your Pregnancy? What level of risk would be enough for you to choose a more invasive follow-up test?
  • Some couple may elect not to pursue screening or additional tests due to
  • They are comfortable with the results no matter what the outcome
  • Because of personal, moral, or religious reasons, making a decision about carrying the child to term is not an option
  • Some parents choose not to allow any testing that poses any risk of harming the developing baby

It is important to discuss the risks and benefits of testing thoroughly with your healthcare provider. Your health care provider or a genetic counselor will help you understand your test results and what the results mean for your pregnancy.


The information is shared to create awareness towards Pregnancy and Childcare to reduce maternal and child deaths. At most care has been taken by the author to include the verified information from authentic sources. However, kindly discuss the same with your health care provider before implementation.