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 pregnancies 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 pregnancies. 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 screen plus second trimester screen) is the recommended screening test for Down syndrome and other chromosomal aneuploidies for women under age 35. It allows us to identify 95% of Down syndrome pregnancies 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 the most sensitive screening option for Down syndrome and other chromosomal aneuploidies, and is also the test that can detect fetal gender. This test can detect at least 99% of Down syndrome pregnancies with the lowest percentage of false positive results (less than 0.1%). 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.
This diagnostic test is typically performed between 10 and 13 weeks. If you want to pursue this type of testing, we will refer you to a highly trained specialist. 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 test is performed by taking a sample of a few cells from part of the developing placenta under ultrasound guidance, using a thin needle through the abdomen or in the vagina through the cervix. Because this test involves invasive methods, it does carry a potential risk of miscarriage.
This diagnostic test is typically performed between 15 and 20 weeks. If you want to pursue this type of testing, we will refer you to a highly trained specialist. An amniocentesis allows you to know the exact number of chromosomes for the fetus, which will provide information about specific genetic disorders such as Down syndrome. An amniocentesis also looks at the number of sex chromosomes and can confirm gender. This test is performed by taking a sample of amniotic fluid under ultrasound guidance, using a thin needle through the abdomen. Because this test involves invasive methods, it does carry a potential risk of miscarriage.
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