The first trimester encompasses the first 12 weeks of pregnancy, from conception through the end of week 12. Ultrasound during this period serves several important purposes: confirming an intrauterine pregnancy, establishing viability by demonstrating cardiac activity, dating the pregnancy accurately using crown-rump length measurement, determining the number of fetuses and the type of placentation in multiple pregnancies, and screening for chromosomal abnormalities through nuchal translucency measurement. First trimester ultrasound, particularly transvaginal ultrasound, provides the most accurate dating of a pregnancy, with an accuracy of plus or minus 5 to 7 days when performed before 14 weeks.
Sonography Term