Genetic Testing

What is Down syndrome?

Down syndrome is one of many chromosome disorders. Chromosomes are the genetic blueprints that determine how a baby will develop. Chromosome disorders occur by chance when a baby receives too few or too many chromosomes at conception. Down syndrome occurs when a baby receives an extra chromosome 21. This disrupts development and causes birth defects and mental retardation.

What is genetic screening?

Screening tests are blood and or ultrasound tests that estimate the likelihood for the fetus to have certain types of birth defects, including Down syndrome and Trisomy 18. A normal result does not necessarily mean that no abnormalities are present. They do not guarantee a normal baby. Screening tests always have a certain amount of false positives (an abnormal test in a baby with no birth defects) and false negatives (a normal test in a baby that has birth defects).

Who should have genetic screening?

Screening is offered to all pregnant women who are between 11 to 18 weeks pregnant. Because they are at a higher risk of having a baby with chromosome abnormalities, the American College of Obstetricians and Gynecologists recommends that all women over age 34 or with a family history of chromosome abnormalities be offered chorionic villi sampling or an amniocentesis for chromosome testing. These tests detect 99.9% of all chromosome abnormalities. In contrast, screening detects 60 to 95% of Down syndrome, Trisomy 18, and Trisomy 13. Five to ten percent of babies with these chromosomal problems will not be detected with screening alone.

How and when is genetic screening done?

There are two ways to do genetic testing. If you choose to undergo genetic screening, you will only need to have one of these tests performed:

First trimester ultrascreen: This test is done between 11 and 13 6/7 weeks and consists of 1) a special sonogram to measure the nuchal translucency (NT) – the amount of fluid behind the neck of the baby and 2) blood testing to measure protein leels (B-HCG and PAPP-A). This test detects more than 90% of Down syndrome, Trisomy 18, and Trisomy 13. This test does not detect spinal defects. Therefore, further testing with a sonogram will be done at 18-20 weeks to detect spinal defects.

Triple Screen: This test is done between 16 and 18 weeks of pregnancy. It measures three different chemicals in the blood. No sonogram is necessary. It will detect 60 to 70% of babies with Down’s syndrome and 80% of babies with spinal defects.

Am I required to have genetic testing?

NO. The decision whether to have the screening tests performed is your personal decision. If these tests show that your baby is at an increased risk for a chromosome abnormality, it does not mean a problem has been diagnosed, only that further evaluation, usually with an amniocentesis, is indicated. This can cause a lot of anxiety as well as some extra expense. These are reasons why the decision to have genetic screening is left to the patient, rather that just doing it on all patients.

CVS Amino Term

Age	DS	ALL	DS	All	DS	All
20			1/1230		1/1923
25			890		1205
30			690		885
31			650		826
32			560		725
33			450	1/200	592	1/333
34			350	167	465	250
35	1/238	1/110	270	125	365	200
36	175	83	210	100	287	167
37	133	64	166	83	225	125
38	100	48	129	67	177	100
39	75	37	100	53	139	83
40	56	28	73	40	109	67
41	42	25	61	33	85	50
42	36	16	47	25	67	40
43	24	12	37	20	53	33
44	18	9	29	17	41	25
45	13	7		13	32	20
46	10	6		10	25	15
47	7	4		8	20	12
48	6	3		6	16	10
49				5	12	7

Last modified: April 01, 2011