Down syndrome occurrence is mostly related to advancing maternal age.
Overall, about 1 in 700 babies will have Down syndrome. A patient’s specific risk of having a baby with this condition is directly related to her age, family history, and other factors. Many women choose to have testing during their pregnancy to find out their chance to have a baby with Down syndrome. There are both non-invasive and invasive ways to test for this condition.
The non-invasive tests include non-invasive prenatal testing (NIPT) and serum screening. Both tests screen for Down syndrome (trisomy 21) and two other chromosome conditions called trisomy 18 and trisomy 13. NIPT may also look for fetal sex and differences in the number of sex chromosomes. NIPT is done via a single blood draw any time after 10 weeks.
Serum screening may involve an ultrasound and one or two blood draws. The first part of this test is usually performed between 10 to 14 weeks gestation and involves a blood test and an ultrasound to measure the size of the fluid pocket at the back of the baby’s neck called the nuchal translucency (NT). If necessary, the second blood draw will be done at 15 to 20 weeks gestation. Some results may be available in the first trimester, while others are reported in the second trimester.
If you choose to have a non-invasive screening test, the results may help you decide if additional testing is necessary. Most women choose non-invasive testing initially and only move on to invasive testing if the results indicate an increased risk.
Invasive tests include Chorionic Villus Sampling (CVS), which can be done between 10 and 13 weeks, and Amniocentesis, which can be done beginning at the start of the second trimester (>16 weeks). Additional information about these testing options can be found under the “Diagnostic Testing” topic.
The timing of the above tests varies depending upon where you live. Do talk to your health care provider about the options you have as well as the timing of the tests.