Diagnostic ultrasound ( PDFDrive )

1056 PART IV Obstetric and Fetal Sonography
been measured using the irst International Reference Preparation
(IRP), the Second International Standard (IS), the hird IS, and
the Fourth IS. he Second IS produces values that are approximately
half that of the First IRP and hird and Fourth IS. he
Chorion laeve
Decidua vera
Decidua capsularis
A
Chorion frondosum
Decidua basalis
Amniotic sac
Chorionic cavity
Endometrial cavity
WHO recommends that the hird and Fourth IS should be used
for calibration of immunoassays. 37
In a literature review, Nyberg and Filly 6 noted the importance
of the appropriate use of a threshold level and a discriminatory
level of β-hCG for the appearance of a gestational sac. he
threshold level identiies the earliest one can expect to see a sac,
and the discriminatory level identiies when one should always
see the sac. Although the menstrual history provides useful
information early in a woman’s obstetric care, because of the
variability in the timing of ovulation and the unreliability of
menstrual history, discriminatory levels based on history are of
limited clinical use.
A gestational sac can oten be visualized sonographically at
low serum β-hCG levels. As TVS has improved, threshold values
B
FIG. 30.6 Double-Decidual Sign. (A) Diagram of anatomic basis
showing three layers of decidua and endometrial cavity. (B) Sagittal
TVS at 5 weeks 6 days shows the decidua capsularis around the gestational
sac (white arrow) and the maternal decidua vera (black arrow)
as separate echogenic bands.
FIG. 30.8 Echogenicity of Fluids. TVS of a 9-week sac with the
relatively echo-free in appearance of the amniotic cavity (AC) and mild
echogenicity in the chorionic cavity (CC).
A
B
FIG. 30.7 Early Gestational Sac. (A) Transverse and (B) sagittal TVS scan at 5 weeks 1 day. The sac is seen as a well-deined, smooth,
rounded luid collection with a thin surrounding echogenic chorion.