Obstetric dating ultrasound
ACR, Diagnostic Radiology: Ultrasonography Practice Parameters and Technical Standards, 2013. A limited examination is performed when a specific question requires investigation. For example, a limited examination in the second trimester or the third trimester could be performed to confirm fetal heart activity in a patient experiencing vaginal bleeding or confirm placental loca- tion or to establish fetal presentation in a laboring patient.
These guidelines should not be construed as dictating an exclusive course of treatment or procedure.However, the mean gestational sac diameter may be recorded when an embryo is not identified.Caution should be used in presumptively diagnosing a gestational sac in the absence of a definite embryo or yolk sac.The components of the specialized examination are more extensive than for a standard ultrasound examination and are determined on a case-by-case basis.Also referred to as a “detailed,” “targeted,” or “76811” ultrasound examination, the specialized anatomic examination is performed when there is an increased risk of an anomaly based on the history, laboratory abnormalities, or the results of the limited examination or the standard examination ().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.
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 ().
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.
Other specialized examinations include fetal Doppler ultrasonography, biophysical profile, fetal echocardiography, or additional biometric measurements.