Diagnostic ultrasound ( PDFDrive )

1446 PART IV Obstetric and Fetal Sonography
TABLE 42.2 Gestational Dating by
Ultrasound in the First Trimester
Sonographic Finding
Gestational sac, no yolk sac,
embryo, or heartbeat
Gestational sac with yolk sac,
no embryo or heartbeat
Gestational sac with
heartbeat and embryo
<5 mm in length
Embryo/fetus ≥5 mm in
length
Gestational Age (Weeks)
FIG. 42.4 Crown-Rump Length Measurement. Cursors delineate
the length of the fetus from the top of its head to the bottom of its
torso.
5
5.5
6
Age based on crown-rump
length (see Table 42.3)
Second and Third Trimesters
Many sonographic parameters have been proposed for estimating
gestational age in the second and third trimesters. hese include
several fetal measurements: biparietal diameter (BPD), 40,41 head
circumference (HC), 42 abdominal circumference (AC), 43 femur
length (FL), 25,44-46 as well as combinations of two or more fetal
measurements: the corrected-BPD 41 and composite age formulas.
43,47 Measurements of structurally abnormal fetal body
parts should not be used in the assignment of gestational age.
Fetal Head Measurements
hree measurements or parameters involve the fetal head: BPD,
occipitofrontal diameter (OFD), and HC. All three measurements
are taken from transaxial sonograms of the fetal head at the level
of the paired thalami and cavum septi pellucidi 48 (Fig. 42.5).
hese head measurements are most accurate if the image is a
true axial plane demonstrating the head as oval with symmetry
of the thalami and cerebral hemispheres and without visualization
of the posterior fossa. 49 he BPD is measured from the outer
edge of the cranium nearest the transducer to the inner edge of
the cranium farthest from the transducer (“leading edge to leading
TABLE 42.3 Gestational Age Estimation
by Crown-Rump Length (CRL)
CRL
(mm)
Gestational
Age (Weeks)
CRL
(mm)
Values derived from formula in Robinson HP, Fleming JE. A critical
evaluation of sonar “crown-rump length” measurements. Br J Obstet
Gynaecol. 1975;82(9):702-710. 35
edge”) (Fig. 42.5). he OFD is obtained from the same transaxial
image as the BPD and is measured from mid-skull to mid-skull
along the long axis of the fetal head (Fig. 42.5). his latter
measurement is used in conjunction with the BPD to calculate
the corrected-BPD using the following formula 41 :
Corrected-BPD = ( BPD × OFD )/
1265 .
Gestational
Age (Weeks)
5 6.0 45 11.1
6 6.2 46 11.2
7 6.4 47 11.3
8 6.6 48 11.4
9 6.8 49 11.4
10 7.0 50 11.5
11 7.2 51 11.6
12 7.4 52 11.7
13 7.5 53 11.8
14 7.7 54 11.8
15 7.8 55 11.9
16 8.0 56 12.0
17 8.1 57 12.1
18 8.3 58 12.2
19 8.4 59 12.2
20 8.5 60 12.3
21 8.7 61 12.4
22 8.8 62 12.4
23 8.9 63 12.5
24 9.0 64 12.6
25 9.1 65 12.7
26 9.3 66 12.7
27 9.4 67 12.8
28 9.5 68 12.9
29 9.6 69 12.9
30 9.7 70 13.0
31 9.8 71 13.1
32 9.9 72 13.2
33 10.0 73 13.2
34 10.1 74 13.3
35 10.2 75 13.4
36 10.3 76 13.4
37 10.4 77 13.5
38 10.5 78 13.5
39 10.6 79 13.6
40 10.7 80 13.7
41 10.8
42 10.8
43 10.9
44 11.0