Pearls and Pitfalls of First-Trimester US Screening and Prenatal Testing: A Pictorial Review

First-trimester US is imperative in evaluation of early pregnancy to confirm pregnancy location and number and gestational age. The 2024 Society of Radiologists in Ultrasound consensus conference established a first-trimester lexicon to highlight the importance of clear and concise language, which is incorporated and featured by the authors. With improved technologies and understanding of fetal development, first-trimester anatomic studies, between 11 weeks and 13 weeks 6 days gestation, are becoming more frequently used. While not a replacement for the second-trimester anatomic study, systematic evaluation of fetal anatomy at this early gestational age allows detection of 40%-70% of anomalies, whether structural or related to aneuploidy. All patients, regardless of age or baseline risk, should be offered screening and diagnostic testing for chromosomal abnormalities. A variety of prenatal screening approaches are available, each with strengths and limitations. Noninvasive prenatal testing with detection of fetal cell-free DNA can be performed in the first trimester and is the most sensitive and specific screening for the common fetal aneuploidies, but is not equivalent to diagnostic testing. Alternatively, serum analytes for maternal biomarkers in conjunction with nuchal translucency (NT) measurement can be used to calculate a risk estimate for common trisomies. Increased NT is the most common abnormality seen in the first trimester. Positive screening results, increased NT, or other anomaly at US should prompt genetic counseling and be confirmed with diagnostic testing (chorionic villus sampling or amniocentesis). Early detection of aneuploidy and structural anomalies allows counseling and informs decisions for pregnancy management. ^©RSNA, 2025 Supplemental material is available for this article.