Prenatal detection of Down syndrome

Risk factors of having a baby with Down syndrome
No one knows all the factors associated with the occurrence of Down syndrome. The following have been shown to increase the incidence of giving birth to a child with Down syndrome:

Advancing maternal age. Women who are 35 years or older when they get pregnant are at increased risk of giving birth to a child with Down syndrome, although this can also occur to younger pregnant women.

Being carriers of the genetic translocation for Down syndrome. Both men and women can pass the genetic translocation for Down syndrome on to their children. If a parent has balanced translocation, there is an up to 15% chance of having another child with Down Syndrome.

Having had one child with Down syndrome. Parents who have one child with Down syndrome and parents who have a translocation themselves are at an increased risk of having another child with Down syndrome.
https://www.cdc.gov/ncbddd/birthdefects/downsyndrome.html

https://www.mayoclinic.org/diseases-conditions/down-syndrome/diagnosis-treatment/drc-20355983

A routine part of prenatal care includes screening for Down Syndrome. That screening involves optional tests, some of which are more conclusive than others for the presence of Down Syndrome.

Prenatal tests for Down Syndrome

These are the tests often carried out to detect Down Syndrome in an infant described by the Mayo Clinic:
The first trimester combined test, which is done in two steps, includes:

• Blood test. This blood test measures the levels of pregnancy-associated plasma protein-A (PAPP-A) and the pregnancy hormone known as human chorionic gonadotropin (HCG). Abnormal levels of PAPP-A and HCG may indicate a problem with the baby.
• Nuchal translucency test. During this test, an ultrasound is used to measure a specific area on the back of your baby's neck. This is known as a nuchal translucency screening test. When abnormalities are present, more fluid than usual tends to collect in this neck tissue.

Information from these tests and consideration of the maternal age is used to estimate the risk of the infant having Down syndrome.

New blood test

The NHS has been in consultation on a new, simple blood test – noninvasive prenatal testing (NIPT) – which measures fetal DNA in the mother's blood as early as 10 weeks. It has a 98%-plus accuracy rate for detecting Trisomy 21 or Down's syndrome. Scientists discovered fragments of fetal DNA circulating in maternal plasma, called cell-free DNA. NIPT is often used to look for chromosomal disorders in the cell-free DNA caused by an extra copy of a chromosome, such as Down syndrome (Goldwaser & Klugman, 2018). https://medlineplus.gov/genetics/understanding/testing/nipt/

The integrated screening test is done in two parts during the first and second trimester of pregnancy. The results are combined to estimate the risk that a baby has Down syndrome.

• First trimester. Part one includes a blood test to measure PAPP-A and an ultrasound to measure nuchal translucency.
• Second trimester. The quad screen measures your blood level of four pregnancy-associated substances: alpha-fetoprotein, estriol, HCG, and inhibin A.

Additional more conclusive tests can be done, but they are done at the end of the first trimester and the beginning of the second trimester of pregnancy with a low risk of miscarriage.

Diagnostic tests that can identify Down syndrome include:

• Chorionic villus sampling (CVS). In CVS, cells are taken from the placenta and used to analyze the fetal chromosomes. This test is typically performed in the first trimester, between 10 and 13 weeks of pregnancy. The risk of pregnancy loss (miscarriage) from a CVS is very low.
• Amniocentesis. A sample of the amniotic fluid surrounding the fetus is withdrawn through a needle inserted into the mother's uterus. This sample is then used to analyze the chromosomes of the fetus. Doctors usually perform this test in the second trimester, after 15 weeks of pregnancy. This test also carries a very low risk of miscarriage.

References

Mai CT, Kucik JE, Isenburg J, Feldkamp ML, Marengo LK, Bugenske EM, Thorpe PG, Jackson JM, Correa A, Rickard R, Alverson CJ, Kirby RS; National Birth Defects Prevention Network. Selected birth defects data from population-based birth defects surveillance programs in the United States, 2006 to 2010: featuring trisomy conditions. Birth Defects Res A Clin Mol Teratol. 2013 Nov;97(11):709-25. doi: 10.1002/bdra.23198. PMID: 24265125; PMCID: PMC4636004.

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