Diagnostic ultrasound ( PDFDrive )

CHAPTER 42 Fetal Measurements 1445
FIG. 42.1 Gestational Sac. At 5.0 weeks’ gestation, gestational sac
(arrow) appears as a small, intrauterine luid collection.
FIG. 42.2 Yolk Sac. Gestational sac contains yolk sac (arrow) on
transvaginal sonogram at 5.5 weeks’ gestation. No embryo is seen.
SAG ML, Sagittal midline.
TABLE 42.1 Gestational Dating by
Mean Sac Diameter (MSD) in the Early
First Trimester
MSD (mm)
Gestational Age (Weeks)
2 5.0
3 5.1
4 5.2
5 5.4
6 5.5
7 5.6
8 5.7
9 5.9
10 6.0
95% Conidence interval = ±0.5 week.
Values from Daya S, Wood S, Ward S, et al. Early pregnancy
assessment with transvaginal ultrasound scanning. CMAJ. 1991;
144(4):441-446. 33
10 mm at 6 weeks, 33 a growth pattern that can be used to assign
gestational age during this period (Table 42.1).
he second method, based on the sonographic indings within
the gestational sac, is best done by transvaginal sonography and
relies on the observation that, on average, the gestational sac is
irst identiiable at 5.0 weeks, the yolk sac at 5.5 weeks (Fig.
42.2), and the embryo and embryonic heartbeat at 6.0 weeks 34
(Fig. 42.3, Video 42.1). he timing of these milestones is subject
to minimal variability, with a 95% conidence range of approximately
0.5 weeks. 31 Gestational age can be assigned based on
these milestones (Table 42.2).
From 6 weeks until the end of the irst trimester, gestational
age correlates closely with the crown-rump length (CRL) of the
embryo or fetus. 35,36 he term embryo is commonly used up to
10 weeks’ gestation, and the term fetus applies thereater. 37 he
CRL is the length of the embryo or fetus from the top of its head
to the bottom of its torso. It is measured as the longest dimension
of the embryo, excluding the yolk sac and extremities. From 9
or 10 weeks onward, the CRL measurements are most accurate
FIG. 42.3 Embryonic Heartbeat. Transvaginal sonogram and M-mode
at 6 weeks demonstrate cardiac activity originating from tiny embryo
(arrow) adjacent to the yolk sac. See also Video 42.1.
if taken with the fetus in neutral position 38 (Fig. 42.4). he CRL
can be used to assign gestational age accurately up to 14 weeks
because there is little biologic variability in fetal length up until
that age 39 (Table 42.3). Ater that point, the CRL of the longer,
more developed fetus becomes less reliable. At this later stage,
the CRL is afected by the fetal position, measuring shorter in
a fetus whose spine is lexed and longer in a fetus whose spine
is extended.