First Trimester Screening

Dr Babina Thangjam *

It is now possible during the early stages of pregnancy to estimate the possibility of a mother having a baby affected by Down Syndrome (Trisomy-21) using biochemical tests, in combination with ultrasound measurements. This type of screening may also indicate the possibility of other chromosomal disorders, such as Edwards Syndrome (Trisomy-18).

The screening provides an opportunity to assess the possibility of these conditions being present without performing more invasive procedures. Only those women whose screening results indicate that they are in a higher risk group are counselled about further tests.

Like other screening tests, the First Trimester Screening do not look only at the results from the blood tests and the ultrasound measurements. They compare a number of factors such as, age, ethnicity, family history, diabetic status, etc. and then estimate what a person's chances are of having a baby with chromosomal abnormality. These tests DO NOT diagnose a problem; they only signal the need for further testing.

WHAT DOES THE SCREENING TEST INVOLVE?

A sample of blood is taken between 11 and 14 weeks of pregnancy. At the same time, an ultrasound scan is performed. The substances in the blood that are markers of Down Syndrome are:

i) pregnancy-associated plasma protein A (PAPP-A); and
ii) free b-hcg (human chorionic gonadotropin)

The specific ultrasound marker for the screening test is
iii) nuchal translucency (NT) thickness of the foetus.

WHEN IS THE FIRST TRIMESTER SCREEN PERFORMED?

The First Trimester Screen is performed between the 11th and 14th week of pregnancy. Because the test is performed so early, it is often used to determine whether a mother should consider undergoing an early diagnostic test such as chorionic villus sampling, or second trimester amniocentesis.

WHAT DOES THE FIRST TRIMESTER SCREEN LOOK FOR?

In babies who are at an increased risk for chromosomal abnormalities, increased fluid is seen in the nuchal translucency. This would be evident from the ultrasound scan. Abnormally high or low free b-hcg and PAPP-A levels are also found in such babies. The first trimester screen combines the results from these three measurements (free b-hcg, PAPP-A, nuchal translucency) with maternal age risk and determines an overall risk factor for chromosomal abnormalities.

RISKS AND SIDE EFFECTS TO THE MOTHER OR BABY

Except for the slight discomfort during the drawing of blood, there are no known risks or side effects associated with the First Trimester Screen. Though considered highly accurate, there is still a 5% false-positive rate for the test.

WHAT DO THE RESULTS MEAN?

It is important to remember that the First Trimester Screen is only a screening test and not a diagnostic test. This test only notes that a mother is at a risk of carrying a baby with a genetic disorder. Instead of separate quantitative values for the tests, only the risk levels are given. Abnormal test results warrant further testing for making a diagnosis.

HOW ACCURATE IS FIRST TRIMESTER SCREENING?

Because this is just a screening test, a positive result (showing an increased risk) does not mean that the baby has a problem. It only indicates that further testing are options that one may consider. Also, a negative or normal result (one that shows a decreased risk) does not mean that the baby will not have a chromosome abnormality. The first trimester screening's detection rate is 85-95% for Down Syndrome.

A nuchal translucency sonogram can be performed without measuring the free b-hcg and PAPP-A levels; however, the detection rate is reduced to about 70% in these instances. Finally, this screen is not designed to provide information about the possibility of other chromosome conditions, nor about many other genetic conditions, genetic disorders, birth defects or causes of mental retardation.

SHOULD ONE STILL GO FOR SECOND TRIMESTER SCREENING AFTER THE FIRST TRIMESTER SCREENING?

The second trimester maternal serum screening test, also known as the "triple screen" or "quad screen" is performed between 16-20 weeks of pregnancy. Both measure chemicals in the mother's blood. Like the first trimester screening, results from the second trimester "triple screen" or "quad screen" can be used to statistically adjust a woman's age-related risk for Down Syndrome and Trisomy-18.

In addition, the Alpha Fetoprotein (AFP) portion of the screen can identify pregnancies at an increased risk for neural tube defects such as spina bifida, which the first trimester screening does not include.

While the second trimester screen is certainly an additional screening option after the first trimester screening, results from the second trimester screen cannot be interpreted in the light of the first trimester screen. Currently, the results from the first trimester screening and the second trimester screening cannot be combined to give an overall adjusted risk figure.