At this stage, the CRL is defined as the longest dimension of the embryonic pole (Fig. Beginning with the 8th week of gestation, the embryonic head and torso become identifiable.
At this point, the CRL is defined as measurement between the top of the fetal head and the fetal rump along its longitudinal axis (Fig. Whereas some investigators advocate using the average of three CRL measurements to establish gestational age, most use the single best measurement. The fetus is in a neutral position and occupies the majority of the image. The accuracy of CRL measurement decreases with gestational age.
When performed, the examination is generally limited to determination of the location and number of gestations present, determination of chorionicity in cases of multiple gestations, assessment for viability, and estimation of gestational age.
Using transvaginal ultrasound, the presence of an intrauterine gestational sac can be consistently demonstrated by the completion of the 5th week of gestation.
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.
■ 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.
This chapter deals with normal fetal anatomy; however, frequent references to anomalies are made to underscore the pertinence of a good anatomic evaluation.
Each image used in this chapter was obtained using two-dimensional (2D) ultrasound.
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.
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.
A number of markers, the most important of which is the nuchal translucency measurement, can be employed to provide an accurate risk assessment for aneuploidy.