Screening Tests for Down syndrome

Aim of screening test: to predict the chance the child has of having a particular disease

ULTRASOUND
-Checking the foetus
-10-14 weeks of pregnancy
-detects 60% of Down Syndrome pregnancies
-strong association between the size of a collection of fluid at the back of the foetal neck, called nuchal translucency, and the risk of Down syndrome.
-echogenic bowel, echogenic intracardiac focus (small bright spot seen in the baby’s heart on an ultrasound exam. This is thought to represent mineralization, or small deposits of calcium, in the muscle of the heart), and dilitation of the kidneys (pyelctasis).
-In early 2001, a study (Smith-Bindman, 2001) was published that looked at all of the previous studies on this topic. The authors concluded that "[these markers] could not discriminate well between unaffected foetuses and those with Down syndrome."
-A more specific marker that is currently under investigation is the measurement of the foetal nose; foetuses with Down syndrome appear to have smaller noses on ultrasound than foetuses without chromosomal abnormalities. However, there is still no standardized technique to measuring the nasal bone and it is considered strictly investigational at this time.

MATERNAL SERUM SCREENING
-blood sample from mother
-The triple screen (also called the multiple marker test) and the alpha fetoprotein plus. These tests measure the quantities of various substances in the mother's blood, and together with the woman's age, estimate the likelihood that her baby has Down syndrome.
available 14-20 weeks of pregnancy
-detects two-thirds (60-70%) of Down syndrome
-Triple screen: alpha-fetoprotein (AFP), unconjugated estriol (uE3), and human chorionic gonadotropin (hCG)
-Sometimes a marker called inhibin A is added, making the "quadruple screen."
-Another marker called PAPP-A was found to be of use for earlier in pregnancy (1st trimester)
-Alpha-fetoprotein is made in the part of the womb called the yolk sac and in the foetal liver, and some amount of AFP gets into the mother's blood. In neural tube defects, the skin of the foetus is not intact and so large amounts of AFP is measured in the mother's blood. In Down syndrome, the AFP is lower in the mother's blood, because the yolk sac and foetus are smaller than usual.
-Estriol is a hormone produced by the placenta, using ingredients made by the foetal 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 foetuses 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.

SOURCES
http://www.dsav.asn.au/
http://www.ds-health.com/prenatal.htm
http://www.kidshealth.org/parent/medical/genetic/down_syndrome.html