Diagnostic ultrasound ( PDFDrive )

CHAPTER 30 The First Trimester 1061
diameter and gestational age (r = 0.78; P < .001), CRL (r = 0.75;
P < .001), and biparietal diameter (r = 0.81; P < .001) but no
correlation with birth weight or placental weight. he cord
diameter was signiicantly smaller by at least 2 SD in patients
who developed preeclampsia or had a miscarriage.
Cysts and pseudocysts within the cord have been described
in the irst trimester. 52 Cysts are usually seen in the eighth week
and usually resolve by the 12th week. hey are singular, closer
to the embryo/fetus than the placenta, with a mean size of 5.2 mm
(Fig. 30.16). Cysts may originate from remnants of the allantois
or omphalomesenteric duct and characteristically have an epithelial
lining. 53 It is hypothesized that the cyst is an amnion
FIG. 30.16 Umbilical Cord Cyst at 10 Weeks. TVS shows a cyst
(small arrow) arising from the cord (large arrow). On subsequent examination
(not shown) the cyst was no longer seen.
inclusion cyst that occurs as the amnion was enveloping the
umbilical cord. In a series of 1159 consecutive patients scanned
between 7 and 14 weeks, Ghezzi et al. 54 found 24 cord cysts at
a prevalence of 2.1%. Single cysts in the irst trimester were
associated with a normal outcome and a healthy infant, whereas
multiple or complex cysts were associated with an increased risk
of miscarriage or aneuploidy. hus although umbilical cord cysts
have been associated with chromosomal abnormalities if seen
in the second and third trimesters, those seen in the irst trimester
typically resolve and are not associated with poor outcome.
ESTIMATION OF GESTATIONAL AGE
During the irst trimester, gestational age can be estimated
sonographically with greater accuracy than at any other stage
of pregnancy. As pregnancy progresses, biologic variation results
in wider variation around the mean for all sonographic parameters
at a given gestational age.
Gestational Sac Size
he MSD ofers an opportunity to date an early pregnancy before
the embryo can be visualized. he MSD is an average of the
diameter of the sac, obtained by adding the anteroposterior and
craniocaudad diameters on the sagittal view of the uterus to the
transverse diameter obtained on the transverse view and dividing
by three (Fig. 30.17). he gestational age can be predicted by
MSD using the following formula: menstrual age in days = MSD
in mm + 30. 55 he MSD increases in size at a rate of 1.1 mm per
day. 56 If MSD is very small, about 2 mm, gestational age is 4 to
4 1 2 weeks, and MSD of about 5 mm is 5 weeks. At 5 1 2 weeks, a
yolk sac appears (see Fig. 30.9A and B). At 6 weeks, an embryo
irst appears adjacent to the yolk sac (see Fig. 30.15A). When
the embryo is irst seen, cardiac activity is appreciated as a
consistent licker 40 (Fig. 30.15B and Video 30.1).
FIG. 30.17 Gestational Age Established by Mean Sac Diameter (MSD). Gestational age can be estimated measuring the sac in three
dimensions. Average of three measurements is used to correlate with gestational age prior to visualization of the embryonic pole. MSD of 12 mm
is consistent with gestational age of 6 weeks 0 days. However, these data are not used to formally establish pregnancy dating. Sonographic dating
of the pregnancy is done with the crown-rump length when cardiac activity is present.