Question:
I
am 40 and expecting my second child. Our doctor has recommended the maternal fetal
screening test because of my age. What does the test screen for and do I have
to have the test?
Answer:
Generally
I encourage my patients over 35 to have a meeting with a genetic counsellor to
understand the different screening options that are available. However, the
decision to do any screening test is yours to make. While there is quite a high
accuracy with these tests, results cannot be guaranteed. Basically there are
three types of screening tests.
The First Trimester Combined Screening
Measures the “nuchal translucency” of
the baby through an ultrasound done between week 11 and two days to week 13 and
three days. A blood test is also drawn the same day. By adjusting for the women’s
age, it will give a numerical estimate of the chances your baby will have Down
syndrome.
Maternal Serum Screening
This test is done at 15 to 20 weeks and measures specific
proteins and hormones in your blood at that time. It screens for Down syndrome
and a rarer genetic disorder known as Trisomy 18. About 85 percent of
pregnancies with Down syndrome and 60 percent of pregnancies with Trisomy 18
will be detected by this test. Results are available within one week. If the
test is positive you will be offered a detailed ultrasound, which can also reveal
spina bifida. You might subsequently be offered an amniocentesis to obtain more
detailed information.
The Integrated Prenatal Screening
This test combines the measurements from the nuchal
translucency and the two different blood tests described above. It is the most thorough
and therefore most reliable. The advantages of the Integrated Prenatal
Screening is that there is a lower false positive rate and can include
screening for spina bifida as part of the second blood test. If the screen is
positive, further testing and genetic counseling will be offered. It should be
noted that in the two to three percent of women who screen positive, the
majority will not be carrying a child with Down syndrome. This is known as a
false positive result. Close to 90 percent of pregnancies affected with Down
syndrome will screen positive.
