Diagnostic ultrasound ( PDFDrive )

CHAPTER 42 Fetal Measurements 1449
TABLE 42.6 Gestational Age Estimation
by Femur Length (FL)
FL (mm)
Gestational
Age (Weeks)
FL (mm)
Gestational
Age (Weeks)
FIG. 42.8 Femur Length Measurement. Electronic calipers measure
the ossiied diaphysis of the femur. Note how the bone is imaged parallel
to the transducer.
Because composite age formulas use a combination of two
or more measurements in conjunction to estimate gestational
age, a potential disadvantage of using such formulas is that an
abnormal measurement or anomaly might be obscured. For
example, in a fetus with a skeletal dysplasia manifested by
shortened long bones and a normal head size, the gestational
age based on the composite formula will be an underestimation,
falling between that predicted by the HC and that predicted
by the short FL. As a result, the FL might not appear to be
abnormally small when compared to this improperly calculated
gestational age.
Gestational Age Estimation by Ultrasound:
Most Accurate Approach at Each Stage
of Pregnancy
When sonographic, as opposed to clinical, dating is used to
assign gestational age, it is important to use the most accurate
method or measurement available (Table 42.7). his is most
relevant at the time of the initial scan in a woman’s pregnancy,
since, as noted previously, it is inappropriate to reassign gestational
age at the time of the second or subsequent scan during the
pregnancy.
If the initial scan is performed in the early irst trimester,
when ultrasound demonstrates no embryo or an embryo less
than 5 mm in length, two approaches to gestational age assignment
are equal in accuracy, both with 95% conidence ranges of ±0.5
weeks: date the pregnancy based on MSD (see Table 42.1) or on
sac contents (see Table 42.2). For the rest of the irst trimester,
when the CRL measures 5 to 80 mm, dating based on CRL (see
Table 42.3) is very accurate, with a 95% conidence range of ±0.7
weeks. 53
In the second and third trimesters, there are several options
for sonographic dating, including measurements of the head,
femur, or abdomen, as well as composite formulas based on
10 13.7 45 24.5
11 13.9 46 24.9
12 14.2 47 25.3
13 14.4 48 25.7
14 14.6 49 26.2
15 14.9 50 26.6
16 15.1 51 27.0
17 15.4 52 27.5
18 15.6 53 28.0
19 15.9 54 28.4
20 16.2 55 28.9
21 16.4 56 29.4
22 16.7 57 29.9
23 17.0 58 30.4
24 17.3 59 30.9
25 17.6 60 31.4
26 17.9 61 31.9
27 18.2 62 32.5
28 18.5 63 33.0
29 18.8 64 33.6
30 19.1 65 34.1
31 19.4 66 34.7
32 19.7 67 35.3
33 20.1 68 35.9
34 20.4 69 36.5
35 20.7 70 37.1
36 21.1 71 37.7
37 21.4 72 38.3
38 21.8 73 39.0
39 22.2 74 39.6
40 22.5 75 40.3
41 22.9 76 40.9
42 23.3 77 41.6
43 23.7 ≥78 42.0
44 24.1
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
measurements of two or more of these. When both the width
and length of the head (BPD and OFD) are visible, the
optimal approach to sonographic age assignment in the second
trimester is to use either the corrected-BPD or the HC, and,
in the third trimester, use the corrected-BPD, the HC, or
the FL. When either the anterior or posterior aspects of the
cranium are not well seen on ultrasound, so that the OFD
and HC cannot be measured accurately, it is best to date
based on the BPD or FL in the second trimester and on
the FL in the third trimester. 44,54,55 hese approaches are the
most accurate single-body-part sonographic dating methods
and are preferable to composite age formulas for reasons
presented earlier.