Second-Trimester Prenatal Screening Tests
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 hormone produced by the placenta
The alpha-fetoprotein level 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 defectof the brain or spinal cord (spina bifida)
- A baby with a birth defect of the abdominal wall
- Later pregnancy complications, such as miscarriageor slowed growth or death of the fetus.
Ultrasonography is done if blood tests detect an abnormal alpha-fetoprotein level in a pregnant woman. It can help by doing the following:
- Confirming the length of the pregnancy
- Determining whether more than one fetus is present
- Determining whether the fetus has died
- Detecting many birth defects
High-resolution or targeted ultrasonography, which can be done at some specialized centers, provides more detail and may be more accurate than standard ultrasonography, particularly for small birth defects.
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 enables doctors to measure the alpha-fetoprotein level in the fluid that surrounds the fetus (amniotic fluid), to analyze the fetus’s chromosomes, and to determine whether the amniotic fluid contains an enzyme called acetylcholinesterase. Knowing what the alpha-fetoprotein level is and whether acetylcholinesterase is present helps doctors better assess risk.
A high alpha-fetoprotein level or the 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 level plusacetylcholinesterase in the amniotic fluid indicates a high risk of
- A neural tube defect, such as anencephaly or spina bifida.
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.