First trimester screening is a non-invasive, optional prenatal assessment performed early in pregnancy. Its purpose is to evaluate the risk of a fetus having certain chromosomal conditions, not to provide a definitive diagnosis. This screening combines different medical tests to offer a comprehensive risk assessment.
Components of First Trimester Screening
First trimester screening involves two distinct components: a specialized ultrasound and a maternal blood test. The nuchal translucency (NT) ultrasound measures the fluid-filled space at the back of the developing baby’s neck. This measurement is typically performed between 11 weeks and 13 weeks and 6 days of gestation. An increased NT measurement can indicate a higher chance of chromosomal abnormalities.
The maternal blood test analyzes specific substances in the mother’s blood. These include pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG). In pregnancies with certain chromosomal conditions, PAPP-A levels tend to be decreased, while hCG levels may be significantly increased. The NT measurement and the levels of these biochemical markers are combined to calculate a personalized risk assessment.
Conditions Detected by the Screening
First trimester screening aims to identify an increased risk for several chromosomal conditions. Down syndrome, also known as Trisomy 21, is one such condition, characterized by an extra copy of chromosome 21. Individuals with Down syndrome often exhibit distinctive facial features, mild to moderate intellectual disabilities, and may have heart or kidney issues.
Another condition screened for is Trisomy 18, or Edwards Syndrome, which results from an extra copy of chromosome 18. This condition is typically more severe, often leading to multiple birth defects and a significantly lower life expectancy. Trisomy 13, also called Patau Syndrome, involves an extra copy of chromosome 13. Babies with Trisomy 13 frequently have severe intellectual disabilities, major brain, eye, and heart defects, and a very low survival rate. This screening does not detect all possible genetic conditions.
Interpreting Your Screening Results
The results of first trimester screening provide a risk assessment. The outcome is presented as a probability, such as a 1 in 500 chance, indicating the likelihood of the baby having one of the screened conditions. A low-risk result suggests a very small chance of a chromosomal abnormality, and typically no further action is recommended.
A high-risk result signifies an increased chance, but not a certainty, that the baby may have a condition. Most high-risk screening results do not ultimately lead to a diagnosis of a chromosomal abnormality. Factors like maternal age are integrated into the risk calculation, as the likelihood of certain chromosomal conditions can increase with maternal age.
What Happens After a High-Risk Result
A high-risk result from first trimester screening prompts a recommendation for genetic counseling. A genetic counselor can explain the implications of the results, discuss the conditions, and outline available options. This counseling helps parents make informed decisions about next steps.
Further testing options are offered to provide a definitive diagnosis. Chorionic Villus Sampling (CVS) involves taking a small sample of placental tissue for analysis, typically performed between 10 and 13 weeks of pregnancy. Amniocentesis, another diagnostic procedure, involves sampling amniotic fluid, usually between 15 and 20 weeks of gestation. Both CVS and amniocentesis carry a small risk of complications, including miscarriage.
Non-Invasive Prenatal Testing (NIPT) may also be offered as an additional screening option. NIPT analyzes fetal DNA found in the mother’s blood, offering higher accuracy for certain conditions compared to the initial first trimester screening. While NIPT is more accurate, it is still a screening test, and a positive NIPT result would typically be followed by a diagnostic test for confirmation. The decision to pursue any of these further tests remains optional for the parents, made after careful consideration of the risks and benefits.