Obstetric dating ultrasound
The components of the specialized examination are more extensive than for a standard ultrasound examination and are determined on a case-by-case basis.
The purpose of this document is to present information and evidence regarding the methodology of, indications for, benefits of, and risks associated with obstetric ultrasonography in specific clinical situations.The necessary components of fetal anatomy in a standard examination are listed in Box 1 and commonly can be obtained after approximately 18 weeks of gestation, although it may be possible to document normal structures before this time.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.If a gestational sac is seen, its location should be documented.The gestational sac should be evaluated for the presence or absence of a yolk sac or embryo, and the crown–rump length of the embryo should be documented.Requests for authorization to make photocopies should be directed to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400. American College of Obstetricians and Gynecologists. Real-time ultrasonography is necessary to confirm fetal viability through observation of cardiac activity and active fetal movement.
The American College of Obstetricians and Gynecologists 409 12th Street, SW, PO Box 96920, Washington, DC 20090-6920 Ultrasound in pregnancy. The choice of transducer frequency is a trade-off between beam penetration and resolution.
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.
The American College of Obstetricians and Gynecologists, the American College of Radiology, the American Institute of Ultrasound in Medicine, the National Institute of Child Health and Human Development, the Society for Maternal–Fetal Medicine, and the Society of Radiologists in Ultrasound have adopted the following uniform terminology for the performance of ultrasonography in the second trimester and the third trimester: standard, limited, and specialized ( A standard obstetric ultrasound examination includes an evaluation of fetal presentation and number, amniotic fluid volume, cardiac activity, placental position, fetal biometry, and an anatomic survey.