Prenatal screening and diagnosis

Prenatal screening refers to testing for diseases or conditions in a fetus before it is born. The aim is to detect birth defects
A variety of tests can be used to detect birth defects, including Down syndrome (trisomy 21) and trisomy 18. Down syndrome and trisomy 18 are chromosome abnormalities that cause birth defects and mental retardation. In any pregnancy there is a small chance that the baby has one of these abnormalities. This risk increases as the woman gets older and women over 35 are usually offered amniocentesis, which is the test that can tell definitively whether the baby has a chromosome abnormality.
Different tests can be performed depending on which trimester the mother is in.
First trimester screening, performed between 11.5 and 13.5 weeks of pregnancy involves two parts; a specialized ultrasound scan (a nuchal translucency scan) and a blood test. The combination of nuchal translucency measurement and the first trimester blood test detects about 85% of fetuses with Down syndrome or trisomy 18.
A nuchal translucency scan measures the fluid filled area at the back of the neck between 11.5 to 13.5 weeks. The measurement can be used to calculate the chance that the fetus has Down syndrome or trisomy 18.
First trimester biochemical measurement of free beta hCG (human chorionic gonadotrophin) and PAPP-A (pregnancy associated plasma protein A) maximizes the detection of Down syndrome and trisomy 18 fetuses.
If the early screen is positive, this does not mean that the fetus has a chromosome problem. Approximately 5-7% of all patients having these tests receive an increased risk and will be offered further testing such as early (earlier than 14 weeks) or standard amniocentesis
The first trimester screen results in a high detection rate (85%) for Down syndrome and trisomy 18. This detection rate is better than second trimester triple marker screening (MSAFP Triple Screen), especially for women under 35.
In the second trimester, one can perform the MSAFP blood test. This is performed at 15 to 20 weeks and measures three substances; AFP, hCG, and estriol.
Prenatal diagnosis refers to tests that are performed during pregnancy to diagnose certain birth defects or genetic disorders. These include 3 primary modalitiesàamniocentesis, chorionic villus sampling and ultrasound.
Amniocentesis is the removal of a small amount of fluid that surrounds the fetus. This procedure is usually performed around 16 to 17 weeks of pregnancy. Side effects include cramping and fatigue. Other possible but rare complications include leakage of fluid, bleeding, and infection , as well as a small risk of miscarriage (1/400).  Amniocentesis can routinely detect major chromosome abnormalities (like Down syndrome) and neural tube defects (spina bifida and related defects).
CVS is the removal of a small sample of tissue from the placenta, using a thin catheter inserted through the cervix (entrance to the uterus) or inserted abdominally. This procedure is usually done at 10 to 12 weeks of pregnancy. CVS can detect major chromosome abnormalities and some genetic disorders. However, it involves a slightly higher risk for miscarriage than amniocentesis
The best time to perform an ultrasound is around 20 weeks, which is three to five weeks after the usual time for amniocentesis. Typically, the purpose of ultrasound is to evaluate the brain and heart anatomy movements and fetal face.
– Kian Hong (Pre and post natal course)

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