The First Trimester Screening is a non-invasive prenatal assessment offered to expectant parents early in pregnancy. It estimates the probability that the fetus may have certain common chromosomal conditions. The screening is typically performed between the 11th and 14th week of gestation. The goal is to provide a personalized risk score, helping providers determine whether further, definitive testing might be appropriate.
The Components of the Screening
The combined First Trimester Screening utilizes two distinct methods to gather data: a specialized ultrasound and a maternal blood test. Both parts are performed close together and the measurements are then mathematically integrated to produce a single risk estimate. Performing both the blood test and the ultrasound, along with factoring in the mother’s age, increases the overall accuracy of the screening results.
The biochemical analysis involves a maternal blood sample to measure the levels of two specific substances produced during pregnancy. These are Pregnancy-Associated Plasma Protein-A (PAPP-A) and free Beta-human Chorionic Gonadotropin (hCG). Both are produced by the placenta, and altered levels of these markers suggest certain chromosomal differences.
The second component is an ultrasound assessment focusing on the fetal neck area, known as the Nuchal Translucency (NT) measurement. A specially trained and accredited sonographer measures the collection of fluid located between the skin and the spine at the nape of the fetus’s neck. An increased NT measurement is associated with a higher likelihood of specific chromosomal abnormalities.
Conditions Targeted by the Screening
The First Trimester Screening is specifically designed to assess the risk for three of the most common chromosomal abnormalities, classified as aneuploidies. These conditions are caused by the presence of an extra copy of a chromosome. The screening focuses on the risk for Trisomy 21, Trisomy 18, and Trisomy 13.
Trisomy 21, or Down syndrome, results from an extra copy of the 21st chromosome and is the most prevalent condition identified by this screening. Trisomy 18 (Edwards syndrome) involves an extra copy of the 18th chromosome and is associated with severe intellectual disability and physical abnormalities.
Trisomy 13 (Patau syndrome) is caused by an extra copy of the 13th chromosome. Like Edwards syndrome, Patau syndrome is a severe condition that often involves multiple birth defects. While the screening cannot provide a definitive diagnosis, it offers an early indication of whether the risk for these conditions is elevated.
Understanding Risk Assessment Results
The First Trimester Screening is a risk assessment test, not a diagnostic one; it provides a probability, not a definitive answer. The results are presented as a specific risk ratio, such as “1 in 500” or “1 in 50.” This number represents the chance that the fetus has one of the targeted conditions, calculated by combining the mother’s age, the NT measurement, and the maternal serum marker levels.
A result is typically categorized as “high risk” or “screen positive” if the calculated probability exceeds a specific cut-off value, often set at a risk greater than 1 in 300. Conversely, a “low risk” or “screen negative” result means the chance is below this established threshold. It is crucial to recognize that a high-risk result does not confirm a diagnosis, but simply suggests an elevated probability.
For example, a high-risk score of 1 in 100 means there is a 1% chance the fetus has the condition, but also a 99% chance that the baby does not have the condition. The mother’s age is a significant factor in the initial calculation, as the background risk for these conditions naturally increases with advancing maternal age.
Follow-Up Testing Options
When a First Trimester Screening result indicates a high risk, healthcare providers will offer further testing to determine a definitive diagnosis. These follow-up options are invasive procedures that analyze fetal or placental cells to count the chromosomes precisely. The two main diagnostic procedures offered are Chorionic Villus Sampling (CVS) and Amniocentesis.
Chorionic Villus Sampling is performed earlier in the pregnancy, typically between 10 and 13 weeks of gestation. This procedure involves taking a sample of tissue from the placenta, which shares the baby’s genetic material. The sample is collected either through the abdomen or the cervix, guided by ultrasound imaging.
Amniocentesis is usually performed later in the second trimester, generally after 15 weeks of gestation. This procedure involves inserting a needle through the mother’s abdomen into the uterus to draw out a small amount of amniotic fluid. This fluid contains fetal cells that can be analyzed for chromosomal abnormalities. Both CVS and Amniocentesis carry a risk of miscarriage.