The first pregnancy blood test, often part of the first trimester screening, is primarily performed to assess the risk of certain chromosomal abnormalities in the fetus, specifically Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18).
Understanding the First Trimester Screening
The first trimester screening is not just one blood test, but a combination of two blood tests and a nuchal translucency ultrasound scan. These tests work together to provide a risk assessment, not a definitive diagnosis.
-
Blood Tests: The blood tests measure the levels of two substances in the mother's blood:
- Pregnancy-associated plasma protein-A (PAPP-A): This protein is produced by the placenta. Abnormal levels can indicate a higher risk of chromosomal abnormalities.
- Human chorionic gonadotropin (hCG): This hormone is produced during pregnancy. Abnormal levels can also indicate a higher risk of chromosomal abnormalities.
-
Nuchal Translucency Ultrasound: This ultrasound measures the clear (translucent) space at the back of the baby's neck. An increased thickness of this space can be associated with an increased risk of Down syndrome and other chromosomal abnormalities.
Why is this Screening Important?
The first trimester screening allows parents to understand the potential risks associated with their pregnancy early on. While the screening doesn't diagnose chromosomal abnormalities, it can help inform decisions about further diagnostic testing, such as chorionic villus sampling (CVS) or amniocentesis. These diagnostic tests provide a definitive diagnosis but carry a small risk of miscarriage. The screening provides valuable information for making informed choices about the next steps in prenatal care.
Interpretation of Results
It's crucial to understand that the first trimester screening provides a risk assessment, not a diagnosis. A "positive" or "high-risk" result does not mean the baby definitely has Down syndrome or Edwards syndrome. It simply means that the risk is higher than a certain threshold, and further testing is recommended to confirm or rule out these conditions. Conversely, a "negative" or "low-risk" result does not guarantee that the baby is free of chromosomal abnormalities, but it does significantly lower the likelihood.
