Dating scan nuchal fold rule and exception dating
■ There is no epidemiologic support for a causal relationship between diagnostic ultrasound during pregnancy and adverse biologic effects to the fetus observed for outputs under a spatial-peak temporal-average intensity of 94 m W/cm From Fowlkes JB; Bioeffects Committee of the American Institute of Ultrasound in Medicine. National Council on Radiation Protection and Measurements.American Institute of Ultrasound in Medicine consensus report on potential bioeffects of diagnostic ultrasound. Exposure Criteria for Medical Ultrasound, II: Criteria Based on All Known Mechanisms.When an embryonic pole becomes identifiable, the best method of establishing the gestational age is measurement of the crown-rump length (CRL).Prior to the completion of the 7th gestational week, the anatomy of the embryonic pole is difficult to clearly delineate.The uterus and adjacent structures should be assessed in both longitudinal and axial sections, taking care to pass completely from side to side and from fundus to cervix to determine the number and location of gestational sacs and embryos.In the early first trimester, the transvaginal approach is ideal to detect any adnexal pathology or free fluid.Serial examinations may be needed to reach a diagnosis.
At this early stage, the gestational age is being estimated by determination of the mean sac diameter (MSD): the average of the sac length, width, and depth.In the 11th week of gestation, the fetus begins to flex and extend its body to a degree that may significantly affect CRL; therefore, CRL measurements need to be carefully standardized from this point on (Fig. Between 7 and 11 WEEKS’ GESTATION) The late first trimester scan is generally considered to be the first scheduled point for routine ultrasound assessment in pregnancy.It provides the same information as the early first trimester scan with a number of additional benefits.Bethesda, MD: National Council on Radiation Protection and Measurements; 2002. Effective prenatal diagnosis relies on a high standard of imaging.Several national and international bodies have described standards for imaging in the first, second, and third trimester of pregnancy.
Since it includes highly accurate estimation of gestational age, routine implementation of the late first trimester scan would lead to a significant reduction in postterm pregnancies.