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Demystifying Down Syndrome, Part 3

A blog by Regine Lim, August 22, 2014

Note: This is a three part blog series on screening and diagnostic testing for Down Syndrome. Read Part 1 and Part 2.

Finally, we’ve reached diagnostic testing – the only way to know for sure if a pregnancy is affected with Down syndrome or countless other genetic conditions. Do not forget that the choice to complete a diagnostic test is entirely yours.

Unlike the screening tests mentioned in the previous post, diagnostic testing provides a definite yes or no that your pregnancy is affected. Diagnostic tests are considered invasive, meaning there is a risk of miscarriage associated with each procedure.


Chorionic villus sampling (CVS) is the earliest possible diagnostic procedure available during pregnancy.

It requires that a needle be placed transabdominally (through the abdomen) or transcervically (through the cervix), depending on the orientation of the placenta.
Transcervical CVS is often described as feeling like a pap smear followed by a bit of cramping. Transabdominal CVS can be described as feeling like a prick (like a blood draw) followed by pressure and a bit of cramping.

Here’s what the procedure is targeting: little projections at the surface of the placenta called chorionic villi..

The DNA in the chorionic villi should reflect the DNA present in the baby. The ball of cells that forms when egg and sperm initially combine eventually splits, becoming the basis for the baby and the placenta. In rare cases, the test will reveal mixed results, suggesting that the DNA in the placenta may not be exactly like the DNA seen in the baby – if this happens, you’ll need a follow-up amniocentesis, a procedure I describe below. This is one of the limitations of CVS.

With the CVS sample, all the chromosomes can be seen to make sure there are two of each, and special DNA tests can be ordered on the same sample in addition to the regular chromosome analysis.

This test does not identify risk of open neural tube defects (ONTDs) like spina bifida, so it is usually paired with a simple blood test for mom to screen for ONTDs in the second trimester.

Timing: Usually between 10 and 13 weeks gestation, though different providers may have a preferential time range within that period.

Major risk: The risk of miscarriage following a CVS is approximately 1/200.


Amniocentesis is the second available diagnostic test during pregnancy. Some women miss the time cut-off for CVS or are not interested in CVS and opt to complete an amniocentesis instead.

Amniocentesis requires that a needle be placed transabdominally under ultrasound guidance to remove some of the amniotic fluid that surrounds the baby.

The amniotic fluid, mostly fetal urine, contains the baby’s cells that have sloughed off his/her skin. The amniotic fluid can undergo all the genetic testing available in the CVS in addition to testing alpha-fetoprotein (AFP) for ONTDs.

Amniocentesis is often described as feeling like a prick (like a blood draw) followed by pressure and perhaps some cramping.

Timing: Usually performed between 16 and 22 weeks of pregnancy.

Major risk: The risk of miscarriage following an amniocentesis is approximately 1/300-1/500.

There are of course nuances to both procedures and a very wide range of testing available for each procedure, so discussing these options with a healthcare provider is the best way to help you make your decisions regarding diagnostic testing.


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