The First Trimester Screening (FTS) is an optional prenatal evaluation offered to pregnant individuals early in their journey. This non-invasive combination of tests is designed to assess the chance of a developing baby having certain chromosomal conditions. It is typically performed between the 10th and 14th week of pregnancy, offering information much earlier than many traditional screening methods. The general goal is to provide a risk estimate. This early information enables informed decision-making regarding potential follow-up diagnostic procedures if an increased probability is identified.
The Procedure and Timing
The First Trimester Screening integrates two distinct medical procedures: a specialized ultrasound and a maternal blood test. Both components are necessary to calculate the most accurate risk assessment for the pregnancy. The optimal window for performing this combined screening is between 11 weeks and 13 weeks and 6 days of gestation.
One part of the evaluation is the Nuchal Translucency (NT) scan. This scan focuses on measuring the clear, fluid-filled space located at the back of the developing baby’s neck. An increased measurement of this space is associated with a higher probability of certain chromosomal or structural issues. The sonographer also uses this ultrasound to confirm the gestational age and often to visualize other early anatomical features.
The second part is a maternal blood test, which analyzes the levels of two specific biochemical markers produced by the placenta and the developing baby. One marker is Pregnancy-Associated Plasma Protein-A (PAPP-A), a protein typically lower in pregnancies affected by Down syndrome. The other marker is free beta-human chorionic gonadotropin (hCG), a hormone often elevated in affected pregnancies. These two measurements, along with the NT measurement and the mother’s age, are mathematically combined to generate the final risk score. The timing of the blood draw is important for accurate analysis because the levels of these proteins change rapidly during the first trimester.
Conditions Identified by the Screening
The primary focus of the combined First Trimester Screening is to calculate the risk for three specific types of chromosomal abnormalities, known as trisomies. These conditions result from having an extra copy of a chromosome instead of the usual two. The most commonly known is Trisomy 21, which is the genetic cause of Down syndrome.
The screening also calculates the probability for Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome). Both Edwards and Patau syndromes are significantly more severe than Down syndrome and are often associated with severe intellectual disability and multiple physical birth defects.
While the blood markers focus on these genetic conditions, the Nuchal Translucency measurement offers additional, broader insights. An unusually thick NT measurement, even in the presence of a low-risk blood test, may suggest an increased probability of structural anomalies, such as congenital heart defects.
Understanding Risk Assessment Results
The First Trimester Screening is a risk assessment tool, not a definitive diagnosis. The result is presented as a ratio, such as “1 in 500” or “1 in 50,” which represents the statistical probability that the baby has the condition being screened for. For example, a result of 1 in 500 means that for every 500 pregnancies with this specific screening result, one baby is expected to have the condition.
The laboratory sets a threshold to classify results as either high-risk or low-risk. A common threshold is a risk greater than 1 in 300, which is often designated as a high-risk result. Receiving a high-risk result does not mean the baby definitely has the condition; rather, it indicates that the statistical probability is high enough to warrant consideration of further testing.
Conversely, a low-risk result, such as 1 in 1,500, suggests a significantly reduced chance, but it does not guarantee that the baby is unaffected. The FTS has a known false-positive rate, meaning some pregnancies are flagged as high-risk even though the baby is healthy. This is a common source of anxiety for parents, but genetic counselors are available to help interpret these probabilities and discuss the next steps.
Follow-Up Diagnostic Options
When a screening result indicates an increased probability of a chromosomal condition, there are clear pathways for follow-up testing. The first common step is often a Non-Invasive Prenatal Test (NIPT), which involves drawing maternal blood to analyze small fragments of the baby’s DNA circulating in the mother’s bloodstream. This test provides a much higher detection rate for the trisomies.
If the NIPT also returns a high-probability result, or if parents wish to bypass the NIPT for a definitive answer, they may consider invasive diagnostic procedures. These procedures are Chorionic Villus Sampling (CVS), performed earlier in the first trimester, or Amniocentesis, typically performed after 15 weeks. Both CVS and Amniocentesis involve obtaining a direct sample of placental tissue or amniotic fluid, respectively, which contain the baby’s cells. Analyzing these cells provides a definitive diagnosis, confirming or ruling out the chromosomal condition. These invasive procedures carry a small risk of miscarriage, which is an important factor in the decision-making process.