In the first trimester, screening involves blood work and an ultrasound. This may be done alone or in combination with further testing at a later date. It is usually performed at 11–14 weeks of pregnancy. Measurement of chemical markers in the mother’s blood combined with an ultrasound measurement of the thickness of the back of the fetal neck (“nuchal translucency”) help detect those pregnancies at increased risk for Down syndrome or other chromosomal problems. This test will detect the majority (82–87%) of Down syndrome but may not detect all cases.
In the second trimester, the level of four substances in your blood may be measured to screen for Down syndrome, trisomy 18, and neural tube defects. This test is typically performed between 15–20 weeks of pregnancy. This test is less sensitive than the first-trimester screen, detecting 70–81% of Down syndrome. A part of this test can be performed to look only for neural tube defects (problems with the incomplete closure of the fetal spine), known more commonly as “spina bifida.” It can detect around 80% of these abnormalities.
The sequential screening test (first-trimester screening plus second-trimester screening) is the recommended screening test for Down syndrome and other chromosomal aneuploidies for women under age 35. It allows us to identify 95% of infants with Down syndrome with a 5% false positive rate.
This test looks at DNA from the pregnancy that crosses into the mother’s blood. This test is usually performed at 10 weeks or beyond. It is to screen for genetic disorders such as Down syndrome and is also the test that can screen for gender. This test can detect about 99% of Down syndrome cases with the lowest percentage of false positive results. This is the most appropriate test for women who will be 35 years or older by their due date but is available to women of all ages.
A CVS allows you to know the exact number of chromosomes for the fetus, which will provide information about specific genetic disorders such as Down syndrome. A CVS also looks at the number of sex chromosomes and can confirm gender.
This diagnostic test is similar to a CVS but done at a later point of time in a pregnancy. It is typically performed between 15 and 20 weeks of gestation by taking a sample of amniotic fluid through a needle into the uterus. It is done under ultrasound guidance. Because this test involves invasive methods, it does carry a potential risk of miscarriage.