women s health issues
Prenatal Diagnostic Testing
Screening of the Pregnant Woman
Measuring levels of certain substances (called markers) in blood can help identify women with an increased risk of problems, such as having a baby with a brain or spinal cord defect ( Neural Tube Defects and Spina Bifida ), Doctors usually offer to do blood tests to measure markers for chromosomal abnormalities as part of routine prenatal care. However, some couples decide not to have any testing done. Other couples, such as those with a high risk of certain disorders, may skip these blood tests and proceed directly to invasive prenatal genetic testing (such as Chorionic Villus Sampling or Markers are usually measured at 10 to 13 weeks of pregnancy (1st-trimester screening). Other markers are measured at 16 to 18 weeks of pregnancy (2nd-trimester screening). Screening during the first trimester usually consists of Blood tests to measure levels of pregnancy-associated placental protein A (produced by the placenta) and beta-human chorionic gonadotropin in the pregnant woman’s blood Ultrasonography to measure a fluid-filled space near the back of the fetus’s neck (called fetal nuchal translucency) The blood tests are done to estimate the risk of Down Syndrome (Trisomy 21) . They may be done at about 11 to 14 weeks of pregnancy. Ultrasonography can help estimate the risk of Down syndrome and certain other chromosomal abnormalities. It can show whether the space at the back of the fetus's neck is enlarged. If it is, risk of these abnormalities is increased. Alternatively, a blood test (called cell-free fetal nucleic acid [cfDNA] testing) may be done. For this test, small fragments of the fetus's DNA, which are present in the pregnant woman's blood in tiny amounts, are analyzed. This test can accurately determine the risk of Down syndrome and some other chromosomal abnormalities in couples with a high risk of having a fetus with a chromosomal abnormality. The test can be done as early as 10 weeks of pregnancy but can also be done later. Many doctors offer this test to all pregnant women. First-trimester screening provides results early. If results are abnormal and the couple wishes, Chorionic Villus Sampling can then be done early to determine whether Down syndrome is present. One advantage of 1st-trimester screening is that with earlier results, Abortion , if desired, can be done earlier, when it is safer. During the 2nd trimester, markers in the pregnant woman’s blood are measured and sometimes ultrasonography is done to evaluate the risk that the fetus will have certain abnormalities. Important markers include the following: Alpha-fetoprotein:A protein produced by the fetus Estriol:A hormone formed from substances produced by the fetus Human chorionic gonadotropin:A hormone produced by the placenta Inhibin A:A hormone produced by the placenta Thealpha-fetoproteinlevel in blood is usually measured in all women, even those who have had 1st-trimester screening or chorionic villus sampling. A high level may indicate an increased risk of having any of the following: A baby with a Neural Tube Defects and Spina Bifida of the brain (such as A baby with a birth defect of the abdominal wall Later pregnancy complications, such as Miscarriage , or a Ultrasonography None Confirming the length of the pregnancy Determining whether more than one fetus is present Determining whether the fetus has died Detecting many birth defects Targeted ultrasonography using high-resolution equipment can be done at some specialized centers. It provides more detail and may be more accurate than standard ultrasonography, particularly for small birth defects. Targeted ultrasonography done during the 2nd trimester can help estimate the risk of a chromosomal abnormality. Targeted ultrasonography aims to identify certain structural birth defects that indicate an increased risk of a chromosomal abnormality. This test can also detect certain variations in organs that do not affect function but may indicate an increased risk of a chromosomal abnormality. However, normal results do not necessarily mean that there is no risk of a chromosomal abnormality. If ultrasonography results are normal, a fetal problem is less likely, but certain conditions, such as neural tube defects, are still possible. Thus, whether ultrasonography results are normal or not, many doctors offer amniocentesis to all women. Amniocentesis None A high alpha-fetoprotein levelorthe presence of acetylcholinesterase in the amniotic fluid suggests A neural tube defect An abnormality in another structure, such as the esophagus, kidneys, or abdominal wall A high alpha-fetoprotein levelplusacetylcholinesterase in the amniotic fluid indicates a high risk of A neural tube defect, such as Neural Tube Defects and Spina Bifida or Sometimes the amniotic fluid sample is contaminated with blood from the fetus. This blood may increase the alpha-fetoprotein level even when the fetus does not have an abnormality, making the results hard to interpret. In such cases, the fetus may not have any abnormalities. Blood tests to measure other markers (estriol and beta-human chorionic gonadotropin) can help estimate the risk of Down Syndrome (Trisomy 21) and other chromosomal abnormalities. This testing may not be necessary for women who have had 1st-trimester screening. Measuring estriol and beta-human chorionic gonadotropin plus alpha-fetoprotein is called triple screening. Inhibin A may also be measured. Measuring these four markers is called quad screening. Triple or quad screening is done around 15 to 20 weeks of pregnancy. It can help estimate the risk of Down syndrome in the fetus. Quad screening results are abnormal (positive) in almost 80% of Down syndrome cases. Triple screening detects almost as many cases. If the risk of Down syndrome is high, Amniocentesis is considered. First-Trimester Screening (a blood test) may be done instead of triple or quad testing. For this test, small fragments of the fetus's DNA in the pregnant woman's blood are analyzed. For the most accurate results, both groups of tests—1st-trimester tests and 2nd-trimester tests—are done, and results from both are analyzed together. Results are not given to the couple until 2nd-trimester results are available. However, if couples want information sooner, they can request a type of screening that provides results during the 1st trimester. Then screening in the 2nd trimester depends on the level of risk indicated by the results of 1st-trimester screening: High risk: Invasive testing ( Chorionic Villus Sampling or Intermediate risk: Second-trimester screening is offered. Low risk: Second-trimester screening for Down syndrome is not offered because the 1st-trimester risk is so low. If results of 1st-trimester, 2nd-trimester, or combined screening are abnormal, amniocentesis or chorionic sampling may be offered to definitively confirm the diagnosis. Some couples may choose to have further testing with cell free DNA (cf DNA) analysis instead. However, results of cfDNA analysis are not definitive. Also, cfDNA testing may be very expensive and may delay definitive testing. Couples should remember that screening tests are not always accurate. They may miss abnormalities, or they may indicate abnormalities when none are present. 