Diagnostic ultrasound ( PDFDrive )

CHAPTER 30 The First Trimester 1067
Yolk Sac Size and Shape
Studies have attempted to characterize the normal sonographic
appearance of the yolk sac and to identify abnormal morphologic
features that may predict poor fetal outcome. 80 In general, a yolk
sac diameter greater than 5.6 mm between 5 and 10 weeks is
associated with an abnormal outcome in singleton pregnancies. 43
However, a yolk sac greater than 5.6 mm may be seen normally
in MCMA twins. 44 Kucuk et al. 80 found that yolk sac shape alone
had a sensitivity of 29% and speciicity of 95% in predicting an
abnormal outcome. An abnormally large yolk sac is oten the
irst sonographic indicator of pathology and is oten associated
with subsequent embryonic demise. Even if the pregnancy survives
the irst trimester, the fetus may still be abnormal. Although the
number of cases is small, large yolk sacs have been associated
with abnormalities such as trisomy 21, partial molar pregnancy, 81
and omphalocele; thus follow-up anatomic surveys for the inding
of an abnormal appearing yolk sac are recommended.
A calciied yolk sac appears as a shadowing echogenic mass
in the absence of any other identiiable yolk sac. A calciied yolk
sac will usually be seen with a dead embryo and may calcify
within 36 hours ater demise (Fig. 30.26, Video 30.4).
he yolk sac can be illed with echogenic material and is not
the same as one that is calciied. his can be seen in live pregnancies.
Szabo et al. 82 followed such cases alone and in conjunction
with nuchal lucency in 3620 irst-trimester pregnancies. hey
found 39 cases (1.0%) of echogenic yolk sacs 1.8 to 4.0 mm in
diameter in pregnancies at 9 to 11 weeks’ menstrual age; 19 of
the 39 (49%) had both a nuchal lucency greater than 3 mm and
FIG. 30.22 Small Sac Size in Relationship to Embryo (MSD − CRL
= <5 mm). The gestational sac containing a yolk sac and embryonic
pole is smaller than expected. This pregnancy eventually failed.
FIG. 30.23 Empty Amnion. TVS at 6 weeks by dates shows
gestational sac (calipers) with an abnormally large empty amnion (arrows).
Early pregnancy failure was later conirmed.
A
B
FIG. 30.24 Expanded Amnion in Nonviable Pregnancy. Two different examples (A) and (B) show a small embryo (calipers) with amnion
(arrows in Figure B) that is too large for an embryo of this size. No cardiac activity was documented in either embryo. See also Video 30.3.