You will be offered some screening tests during pregnancy to try to find any health problems that could affect you or your baby, such as infectious diseases, Down’s syndrome, or physical abnormalities. The tests can help you make choices about care or treatment during your pregnancy or after your baby is born.

All screening tests offered by the NHS are free. Screening tests are used to find people at higher risk of a health problem. This means they can get earlier, potentially more effective, treatment or make informed decisions about their health. Screening tests are not perfect. Some people will be told that they or their baby are at high risk of having a health problem when in fact they do not have the problem. Also, a few people will be told that they or their baby are at low risk of having a health problem when in fact they do have the problem.

Different screening tests are offered at different times during pregnancy. The screening test for sickle cell and thalassaemia should be offered before 10 weeks. You will be offered screening for Down’s syndrome around the time of your dating scan, which happens when you are around 11 to 14 weeks pregnant. You will be offered screening for abnormalities at a mid-pregnancy scan when you are around 18-21 weeks pregnant. The test include the following :

 

1. First trimester screening:

is a prenatal test that offers early information about a baby’s risk of certain chromosomal conditions — Down syndrome (trisomy 21) and extra sequences of chromosome 18 (trisomy 18).

First trimester screening, also called the first trimester combined test, has two steps:

A blood test to measure levels of two pregnancy-specific substances in the mother’s blood — pregnancy-associated plasma protein-A and human chorionic gonadotropin (HCG)

An ultrasound exam to measure the size of the clear space in the tissue at the back of the baby’s neck (nuchal translucency or NT) and nasal bone (NB). Typically, first trimester screening is done between weeks 11 and 14 of pregnancy. Using your age and the results of the blood test and the ultrasound, your health care provider can gauge your risk of carrying a baby with Down syndrome or trisomy 18. If results show that your risk level is moderate or high, you might choose to follow first trimester screening with another test that’s more definitive. In babies who are at an increased risk for chromosomal abnormalities, increased fluid is often found in the nuchal translucency. Abnormally high or low hCG and PAPP-A levels are also often found. First trimester screening doesn’t evaluate the risk of neural tube defects, such as spina bifida.

Result :

Your health care provider will use your age and the results of the blood test and ultrasound exam to gauge your risk of carrying a baby with Down syndrome or trisomy 18. Other factors — such as a prior Down syndrome pregnancy — also might affect your risk. First trimester screening results are given as positive or negative and also as a probability, such as a 1 in 250 risk of carrying a baby with Down syndrome. First trimester screening correctly identifies about 85 percent of women who are carrying a baby with Down syndrome. About 5 percent of women have a false-positive result, meaning that the test result is positive but the baby doesn’t actually have Down syndrome.

• Increase level of Free BHCG / Decrease level of Papp-a : positive Screening for Trisomy 21
• Decrease level of Papp-a and Free BHCG : positive Screening for Trisomy 13/18
• Measure of NT is more than 2.5 MM and Absent or Notreport NB : positive Screening for Trisomy 21 .

2. Second Trimester Screening :

The quad screen — also known as the quadruple marker test, the second trimester screen or simply the quad test — is a prenatal test that measures levels of four substances in a pregnant woman’s blood:

• Alpha-fetoprotein (AFP), a protein made by the developing baby
• Human chorionic gonadotropin (HCG), a hormone made by the placenta
• Estriol, a hormone made by the placenta and the baby’s liver
• Inhibin A, another hormone made by the placenta

Ideally, the quad screen is done between weeks 15 and 18 of pregnancy — the second trimester. However, the rocedure can be done up to week 20. The quad screen is used to evaluate whether your pregnancy has an increased chance of being affected with certain chromosomal conditions, such as Down syndrome. The alpha-fetoprotein part of the test can help evaluate the chance for neural tube defects, such as spina bifida, and abdominal wall defects, such as omphalocele. If your risk level is low, the quad screen can offer reassurance that there is a decreased chance for Down syndrome, trisomy 18, neural tube defects and abdominal wall defects. If the quad screen indicates an increased chance of one of these conditions, you might consider additional screening or testing.

Result :

The quad screen measures levels of AFP, HCG, estriol and inhibin A in a pregnant woman’s blood. Your health care provider will use your age at the estimated time of delivery and the results of the quad screen to evaluate your chance of carrying a baby who has certain chromosomal conditions, neural tube defects or abdominal wall defects.Quad screen results are given as a probability, such as a 1 in 250 risk of carrying a baby who has Down syndrome. The quad screen correctly identifies about 80 percent of women who are carrying a baby who has Down syndrome. About 5 percent of women have a false-positive result, meaning that the test result is positive but the baby doesn’t actually have Down syndrome.

• Increase level of inhibin A and Tittre BHG / Decrease level of AFP And UE3 : positive Screening for Trisomy 21
• Increase level of AFP : positive Screening for NTD
• Increase level of inhibin / Decrease level of AFP And UE3 : positive Screening for SLOS or Trisomy 18/13

3. Prenatal cell-free DNA (cfDNA) screening:

This is a sophisticated blood test that examines fetal DNA in the maternal bloodstream to determine whether your baby is at risk of Down syndrome, extra sequences of chromosome 13 (trisomy 13) or extra sequences of chromosome 18 (trisomy 18). Some forms of cfDNA screening also screen for other chromosome problems and also provide  information about fetal gender. A normal result might eliminate the need for a more-invasive prenatal diagnostic test.

4. Combine Test :

۱) Integrated test    ۲) contingent test       ۳) sequential test

Contingent test :

This report combines the results of the first quarter and the second quarter of pregnancy. In this report a false positive, decreases. According to the report, patients are placed in the following three categories:

• Cutoff < 1:1500 = Negative Screen
• Cutoff > 1:100 = Positive Screen
• Cutoff : 1:100-1:1500 = Borderline Screen

 

After a positive test result, your health care provider and genetics professional will discuss your options, including additional testing. For example:

• Chorionic villus sampling (CVS):

CVS can be used to diagnose chromosomal conditions, such as Down syndrome. During CVS, which is usually done during the first trimester, a sample of tissue from the placenta is removed for testing. CVS poses a small risk of miscarriage.

• Amniocentesis:

Amniocentesis can be used to diagnose both chromosomal conditions, such as Down syndrome, and neural tube defects, such as spina bifida. During amniocentesis, which is usually done during the second trimester, a sample of amniotic fluid is removed from the uterus for testing. Like CVS, amniocentesis poses a small risk of miscarriage.

 

Effective factore of Screening for fetal abnormalities :

• A miscalculation of your baby’s age (how long you’ve been pregnant)
• Maternal race
• Maternal weight
• A multiple pregnancy
• Diabetes
• In vitro fertilization
• Smoking during pregnancy
• a prior Down syndrome pregnancy
• . . .

Will screening tests give me a definite answer?

No – a screening test does not usually say for certain if you or your baby have a health problem. It tells you if you or your baby are at a high or low risk of having the problem. Women or babies found to be at high risk of a problem will often be offered a diagnostic test. A diagnostic test gives a more definite “yes” or “no” answer.