The crown–rump length is a more accurate indicator of gestational (menstrual) age than the mean gestational sac diameter.Mean sac diameter measurements are not recommended for estimating the due date ().This Practice Bulletin was developed by the American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins–Obstetrics and the American Institute of Ultrasound in Medicine in collaboration with Lynn L. The information is designed to aid practitioners in making decisions about appropriate obstetric and gynecologic care.
Sometimes structures can be difficult to visualize because of fetal size, position, and movement; maternal abdominal scars; increased maternal abdominal wall thickness; and reduced amniotic fluid volume.
During early pregnancy, an abdominal transducer with a frequency of 5 MHz or a transvaginal transducer with a frequency of 5–10 MHz or higher generally provides very good resolution while allowing adequate penetration.
A lower-frequency transducer may be needed to provide adequate penetration for abdominal imaging later in pregnancy or in an obese patient.
ACR-ACOG-AIUM-SRU Practice parameter for the performance of obstetrical ultrasound. An ultrasound examination may be performed either transabdominally or transvaginally.
ACR, Diagnostic Radiology: Ultrasonography Practice Parameters and Technical Standards, 2013. If a transabdominal examination is inconclusive, a transvaginal scan or transperineal scan is recommended.