Diagnostic ultrasound ( PDFDrive )

CHAPTER 42 Fetal Measurements 1447
TABLE 42.4 Gestational Age Estimation
by Biparietal Diameter (BPD)
BPD OR
BPDc
(mm)
Gestational
Age (Weeks)
BPD OR
BPDc
(mm)
Gestational
Age (Weeks)
FIG. 42.5 Biparietal Diameter (BPD) and Occipitofrontal Diameter
(OFD) Measurements. Transaxial sonogram of the fetal head at the
level of the paired thalami (arrows) and cavum septi pellucidi (arrowhead),
with BPD (calipers 1) and OFD (calipers 2). Note how the calipers for
the BPD are placed from the outer aspect of the skull to the inner aspect
of the skull, “leading edge to leading edge.”
FIG. 42.6 Head Circumference (HC) Measurement. HC measurement
(calipers and tracing dots) on transaxial sonogram of the fetal head at
the same level as for the biparietal diameter measurement. Note how
the HC measurement is obtained by tracing around the bone.
he rationale for the corrected-BPD is that it represents the
BPD of the standard-shaped head (one with an OFD/BPD ratio
of 1.265) of the same cross-sectional area. 41 he same tables or
formulas used to determine gestational age from the BPD are
used to determine gestational age from the corrected-BPD
(Table 42.4).
he HC is the length of the outer perimeter of the cranium,
made on the same transaxial image of the fetal head. It can be
measured by using an electronic ellipse 50 (Fig. 42.6, Table 42.5)
or calculated from the outer-edge-to-outer-edge analogs of the
BPD and OFD:
HC = 1. 57 × [( Outer-to-outer BPD) + ( Outer-to-outer OFD)]
20 13.2 60 24.2
21 13.4 61 24.5
22 13.6 62 24.9
23 13.8 63 25.3
24 14.0 64 25.7
25 14.3 65 26.1
26 14.5 66 26.5
27 14.7 67 26.9
28 14.9 68 27.3
29 15.1 69 27.7
30 15.4 70 28.1
31 15.6 71 28.5
32 15.8 72 29.0
33 16.1 73 29.4
34 16.3 74 29.9
35 16.6 75 30.3
36 16.8 76 30.8
37 17.1 77 31.2
38 17.3 78 31.7
39 17.6 79 32.2
40 17.9 80 32.7
41 18.1 81 33.2
42 18.4 82 33.7
43 18.7 83 34.2
44 19.0 84 34.7
45 19.3 85 35.2
46 19.6 86 35.8
47 19.9 87 36.3
48 20.2 88 36.9
49 20.5 89 37.4
50 20.8 90 38.0
51 21.1 91 38.6
52 21.4 92 39.2
53 21.7 93 39.8
54 22.1 94 40.4
55 22.4 95 41.0
56 22.8 96 41.6
57 23.1 ≥97 42.0
58 23.5
59 23.8
BPDc, Corrected-BPD.
Values from Doubilet PM, Benson CB. Improved prediction of
gestational age in the late third trimester. J Ultrasound Med.
1993;12(11):647-653. 44
Although the BPD is simpler to measure than the corrected-
BPD or HC, it has the disadvantage of being the only one of the
three measurements that disregards head shape. his means that
two heads of equal widths but diferent lengths will have the
same BPD, but the longer head will have a greater corrected-BPD
and HC than the shorter head (Fig. 42.7). he fetus with the
longer head will therefore be assigned a greater gestational age
based on the corrected-BPD or HC; however, both fetuses will