First trimester screen in pregnancy
First trimester screening is a
new, non-invasive, optional assessment that combines maternal blood screening
with fetal ultrasound to identify risk for specific chromosomal abnormalities,
including trisomy 21 and trisomy 18, Down syndrome.
In addition to screening for
these abnormalities, part of the test (known as nuchal translucency) can help
identify other serious fetal abnormalities, such as heart problems. The
screening test does not detect neural tube defects. The overall screening
accuracy rate for the aforementioned chromosomal abnormalities is approximately
85% with a false positive rate of 5%.
It means that:
- About 85 out of 100 infants suffering from abnormalities covered by screening will be identified.
- About 5% of all normal pregnancies will get a positive result or an abnormal level.
- A positive test means you have a 1 in 100 to 1 in 300 chance of having one of the anomalies.
It is important to understand
that a positive result does not equate to an abnormality, but rather serves as
an incentive to discuss further testing. Screening should not be confused with
second trimester screening.
These screening methods are
less accurate and are performed between 15 and 20 weeks. Blood screening
measures two hormones associated with pregnancy: hCG and PAPP-A. An ultrasound
assessment measures the transparency of the collar space (fluid under the skin
behind the child's neck). This non-invasive procedure combines blood and
ultrasound findings, as well as the mother's age, to determine risk factors.
What is a screening test?
It is very important to
remember what a screening test is before you do it. This will help reduce the
anxiety that may accompany the test results. Screening tests are not limited to
blood test results. They compare a number of different factors (including age,
ethnicity, blood test results, etc.) and then evaluate the likelihood of a
person having an abnormality.
These tests do NOT diagnose
the problem; they only signal that further testing is needed.
How is first trimester screening done?
Blood screening involves
drawing blood from the mother, which takes about 5-10 minutes. The blood sample
is then sent to a laboratory for analysis. Ultrasound is performed by a
sonographer or perinatologist and lasts from 20 to 40 minutes. The results are
evaluated within a week after the test.
What are the risks and side effects for the mother or baby?
Other than the discomfort
associated with drawing blood, there are no known risks or side effects
associated with first trimester screening. A false positive test result is 5%.
Parents should be aware of the possibility of abnormal results and then
discover, after further testing, that the child is healthy.
When is the first trimester screening done?
First trimester screening is done
between 11 and 13 weeks of pregnancy. Because the test is performed very early,
it is often used to determine whether a mother should consider an early (first
trimester) diagnostic test, such as a chorionic villus sampling or second
trimester amniocentesis.
What does first trimester screening look like?
In children at increased risk
of chromosomal abnormalities, there is often an increase in fluid in the nuchal
space. Abnormally high or low levels of hCG and PAPP-A are also often found.
First trimester screening combines these three measurements (nuchal opacity,
hCG, and PAPP-A) with maternal age-related risk factors to determine an overall
risk factor for chromosomal abnormalities.
What do first trimester screening results mean?
It is important to remember
that first trimester screening is a screening test and not a diagnostic test.
This test only notes that the mother is at risk of carrying a child with a
genetic disease. Many women who have an abnormal test find out later that the
test was incorrect.
You will not get specific
quantitative values for different parts of the Q1 screen. Instead, you will
be told if your results are "normal or abnormal" and your genetic
counselor will advise you of the level of risk. Your consultant will tell you
your risk factor for chromosomal abnormalities based on test results (eg 1/250,
1/1300).
Abnormal test results require
further testing to establish a diagnosis. Your genetic counselor will discuss
the results with you and help you decide on diagnostic tests such as chorionic
villus sampling or amniocentesis. These invasive procedures should be carefully
discussed with your doctor and between you and your partner. Additional tips
may be helpful.
What are the reasons for additional testing?
First trimester screening is a
routine screening that is not invasive and does not pose a known risk to the
mother or baby. Screening results in the first trimester may require further
testing. The reasons why further testing should be done vary from person to
person and couple to couple.
Performing additional tests
allows you to confirm the diagnosis, and then opens up certain possibilities
for you:
- Pursue potential interventions that may exist (fetal surgery for spina bifida)
- Begin planning for a child with special needs
- Start addressing anticipated lifestyle changes
- Identify support groups and resources
- Make a decision about carrying the child to term
Some individuals or couples
may refuse to be tested or additional tested for a number of reasons:
- They are satisfied with the results, regardless of the outcome
- For personal, moral or religious reasons, deciding to carry a child to term is not an option.
- Some parents choose not to allow tests that could harm a developing child.
It is important to carefully discuss the risks and benefits of testing with your doctor. Your doctor will help you evaluate whether the benefits of the results may outweigh the risks of the procedure.