A finding that is commonly seen in the early first trimester and that deserves special mention is physiologic herniation of the midgut into the root of the abdominal cord insertion (Fig. Sagittal view of a 10- to 11-week fetus demonstrating a physiologic midgut herniation (arrow).Most examinations at this stage are performed for a specific clinical indication such as pain or vaginal bleeding associated with a positive pregnancy test.However, many experts in the field advocate the use of additional views to improve diagnostic performance.
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.
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.
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.