Diagnostic ultrasound ( PDFDrive )

1022 PART IV Obstetric and Fetal Sonography
A
B
C
FIG. 28.8 Determination of Situs. (A) Scan plane and (B) transverse scan diagram. With fetus in cephalic position and spine on the maternal
right side, the left-sided stomach is “up” on the side closest to the transducer. (C) Scan plane and (D) with the fetus in breech position and spine
on the maternal right side, the left-sided stomach is “down” on the side farthest away from the transducer.
D
because of maternal medical or family history, or if abnormal
results are suspected on a routine scan. hese high-risk scans
include detailed views of fetal anatomy beyond those obtained
in the routine exam. Wax et al. 14 provide a more complete
description of these anomalies.
ROUTINE ULTRASOUND SCREENING
Estimation of Gestational Age
Determination of the expected date of delivery (EDD) is
especially important in obstetric practice because it is used to
intervene in pregnancies considered to be “growth restricted”
and in postterm pregnancies. Multiple studies have demonstrated
that routine use of ultrasound results in more accurate
assessment of the EDD than does last menstrual period (LMP)
dating or physical examination, even in women with regular
and certain menstrual dates. 15-18 Pregnancy dating is most
accurately performed in the irst half of pregnancy. Fetal
growth should be assessed by comparison to earlier scans in
pregnancy whenever possible. In a Cochrane review of nine
trials of routine ultrasound in early pregnancy, routine use of
early ultrasound and the subsequent adjustment of the EDD
led to a signiicant reduction in the number of postterm
pregnancies. 19
A rule of thumb is that in the irst trimester, LMP dating
should be maintained unless ultrasound yields an EDD more
than 7 days of; in the second trimester, ultrasound should be
used to change EDD if it is of by more than 2 weeks (and
follow-up is then needed to ensure appropriate interval growth);
and in the third trimester, a 3-week discrepancy between LMP
and ultrasound dating is allowed but needs to be taken into the
clinical context, with assessment for growth restriction or
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