Screening

Prenatal
Maternal Serum Screening
15th to 18th week with a detection rate of 95% and 5% false positives.
Blood sample is collected and levels of hormone will be analyzed to determine risk. Sometimes called the triple test or the quadruplet screening, depending on the hormone test available.

Alpha-fetoprotein is made in the part of the womb called the yolk sac and in the fetal liver, and some amount of AFP gets into the mother's blood. In neural tube defects, the skin of the fetus is not intact and so larger amounts of AFP is measured in the mother's blood. In Down syndrome, the AFP is decreased in the mother's blood, presumably because the yolk sac and fetus are smaller than usual.

Estriol is a hormone produced by the placenta, using ingredients made by the fetal liver and adrenal gland. Estriol is decreased in the Down syndrome pregnancy.

Human chorionic gonadotropin hormone is produced by the placenta, and is used to test for the presence of pregnancy. A specific smaller part of the hormone, called the beta subunit, is increased in Down syndrome pregnancies.

Inhibin A is a protein secreted by the ovary, and is designed to inhibit the production of the hormone FSH by the pituitary gland. The level of inhibin A is increased in the blood of mothers of fetuses with Down syndrome.

PAPP-A, which stands for pregnancy-associated plasma protein A, is produced by the covering of the newly fertilized egg. In the first trimester, low levels of this protein are seen in Down syndrome pregnancies.

Once the blood test results are determined, a risk factor is calculated based on the "normal" blood tests for the testing laboratory. The average of normals is called the "population median." Test results are sometimes reported to doctors as "Multiples of the Median (MoM)." The "average" value is therefore called 1.0 MoM. Down syndrome pregnancies have lower levels of AFP and estriol, so their levels would be below the average, and therefore less than 1.0 MOM.

Ultrasound Screening
Also known as nuchal translucency, because the ultrasound is used to measure the width of the nuchal area (fluid compartment behind the neck)

Women need to have a full bladder for a transabdominal ultrasound (an ultrasound of the belly) to be performed in the early months — you may be asked to drink a lot of water and not urinate. You'll lie on an examining table and your abdomen will be coated with a special ultrasound gel. A technician will pass a wand-like instrument called a transducer back and forth over your abdomen. You may feel some pressure as the technician presses on the bladder. High-frequency sound waves "echo" off your body (and the fetus) and create a picture of the fetus inside on a computer screen.

Sometimes, if the technician isn't able to see a good enough image from the ultrasound, he or she will determine that a transvaginal ultrasound is necessary. This is especially common in early pregnancy. For this procedure, your bladder should be empty. Instead of a transducer being moved over your abdomen, a slender probe called an endovaginal transducer is placed inside your vagina.

The following tests:
Your health care provider may recommend them if you:
• are older than age 35
• have a family history of genetic disorders (or a partner who does)
• have a previous child with a genetic disorder or had a previous pregnancy with a chromosomal abnormality
• have had an earlier screening test that indicates that there may be a concern

Amniocentesis
This procedure is used to collect amniotic fluid, the liquid that is in the womb (13th to 18th week). A needle is inserted through the mother's abdominal wall into the uterus, using ultrasound to guide the needle. Approximately one ounce of fluid is taken for testing. This fluid contains fetal cells that can be examined for chromosome tests. It takes about 2 weeks to determine if the fetus has Down syndrome or not. Risk of miscarriage is 1%.

It carries a low risk of uterine infection, leakage of amniotic fluid, and injury to the fetus.

Chorionic Villus Sampling (CVS) (10th to 12th week)
In this procedure, instead of amniotic fluid being taken, a small amount of tissue is taken from the young placenta (also called the chorionic layer). These cells contain the fetal chromosomes that can be tested for Down syndrome. The cells can be collected the same way as the amniocentesis, but another method is to insert a tube into the uterus through the vagina. Risk of miscarriage is 2%. Early on in the use of CVS, a number of babies were identified with missing or shortened fingers or toes. However, that has been connected to the use of CVS before the 10th week of pregnancy.
Its advantage over an amniocentesis is that it can be performed earlier, allowing more time for expectant parents to receive counseling and make decisions.

Transcervical. Using ultrasound as a guide, a thin tube is passed from the vagina into the cervix. Gentle suction removes a sample of tissue from the chorionic villi. Some women experience cramping with the removal.
Transabdominal. A needle is inserted through the abdominal wall with ultrasound guidance, and a sample of the chorionic villi is removed. Cramping may be felt with this approach as well.
Results available in 2 weeks.

Percutaneous Umbilical Blood Sampling (PUBS) (after 18 weeks)
This test obtains fetal blood by guiding a needle into the umbilical cord. It's primarily used in addition to an ultrasound and amniocentesis if your health care provider needs to quickly check your baby's chromosomes for defects or disorders.

Nonstress Test (26 to 28 weeks)
A nonstress test (NST) can determine if the baby is responding normally to a stimulus. Used mostly in high-risk pregnancies or when a health care provider is uncertain of fetal movement, an NST can be performed at any point in the pregnancy after the 26th to 28th week when fetal heart rate can appropriately respond by accelerating and decelerating.

The health care provider will measure the response of the fetus' heart rate to each movement the fetus makes as reported by the mother or observed by the doctor on an ultrasound screen. If the fetus doesn't move during the test, he or she may be asleep and the health care provider may use a buzzer to wake the baby. You also may be asked to drink or eat to try to stimulate the baby more.

http://www.diagnosticultrasound.org.uk/nuchal_translucency.htm
http://kidshealth.org/parent/system/medical/prenatal_tests.html
http://www.ds-health.com/prenatal.htm

please visit
http://www.babycentre.co.uk/pregnancy/antenatalhealth/scans/nuchalscan/
or
http://www.babycenter.ca/pregnancy/antenatalhealth/scans/nuchalscan/
to see the differences of the NT scan for a normal baby and a down syndrome baby.

Postnatal
After the patient is diagnosed using the signs and symptoms, a blood sample is taken to make a karyotype. This will show any abnormalities in the genes.


Trisomy 21 :)