Diagnostic ultrasound ( PDFDrive )

1452 PART IV Obstetric and Fetal Sonography
TABLE 42.9 Approach to Fetal Weight
Estimation
Body Parts Imaged
Formula Used for Weight
Estimate
HEAD, ABDOMEN, AND FEMUR
OFD measurable
Formula 1, using corrected-
BPD in place of BPD
OFD not measurable Formula 1
HEAD AND ABDOMEN
OFD measurable
OFD not measurable Formula 2
ABDOMEN AND FEMUR
— Formula 3
FORMULA 1 a
Formula 2, using corrected-
BPD in place of BPD
Log ( EFW > ) = 1. 4787 − 0.
003343 AC × FL + BPD
+ 0. 0458 AC + 0.
158 FL
FORMULA 2 a
10 2
Log ( EFW > ) = 1. 1134 + 0. 05845 AC − 0.
000604 AC
10
− 0. 007365 BPD 2 + BPD + 0. 00595 BPD × AC + 0.
1694 BPD
FORMULA 3 a
2
Log10 ( EFW) = 1. 3598 + 0. 051AC + 0. 01844 FL − 0.
0037 AC × FL
a Formulas from Hadlock FP, Harrist RB, Sharman RS, et al. Estimation
of fetal weight with the use of head, body, and femur
measurements—a prospective study. Am J Obstet Gynecol.
1985;151(3):333-337. 26
AC, Abdominal circumference (cm); BPD, biparietal diameter (cm);
EFW, estimated fetal weight, in grams (g); FL, femur length (cm);
OFD, occipitofrontal diameter (cm).
of gestational age (Table 42.10), several of which appear in the
literature. 71-76
here is some debate about a number of aspects of fetal weight
assessment in relation to gestational age. Should a weight table
or chart based on neonatal weights or on estimated fetal weights
be used? Should the table or chart be derived solely from pregnancies
of low-risk mothers? Should “population norms” or “customized
norms” be used?
Most weight norms (tables or charts) are derived from large
data sets of neonatal birth weights from babies born at a known
gestational age. 71-76 At least one chart, on the other hand, was
produced using estimated fetal weights instead of neonatal birth
weights. 77,78 A rationale for the latter is that several studies have
shown that fetuses that deliver preterm are smaller, on average,
than those of the same gestational age that remain in utero. 79-83
Babies born preterm thus represent an abnormal group with a
negatively skewed weight distribution. his supports the argument
that fetuses should be compared to fetuses, not to babies. 84
A large international study, INTERGROWTH-21st, used
another approach to produce weight norms derived from a healthy
population. 85,86 Although it is based on birth weights rather than
2
TABLE 42.10 Fetal Weight Percentiles in
the Third Trimester
Gestational
Age (Weeks)
WEIGHT PERCENTILES (GRAMS)
10th 50th 90th
25 490 660 889
26 568 760 1016
27 660 875 1160
28 765 1005 1322
29 884 1153 1504
30 1020 1319 1706
31 1171 1502 1928
32 1338 1702 2167
33 1519 1918 2421
34 1714 2146 2687
35 1919 2383 2959
36 2129 2622 3230
37 2340 2859 3493
38 2544 3083 3736
39 2735 3288 3952
40 2904 3462 4127
41 3042 3597 4254
42 3142 3685 4322
43 3195 3717 4324
With permission from Doubilet PM, Benson CB, Nadel AS, Ringer
SA. Improved birth weight table for neonates developed from
gestations dated by early ultrasonography. J Ultrasound Med.
1997;16(4):241-249. 71
fetal weights, the study population used to generate these norms
consists of babies born at or beyond 33 weeks to mothers with
no known pregnancy-related risk factors. he INTERGROWTH-
21st norms, however, are of limited use for assessing fetal weight
in relation to gestational age, because they do not cover the
period prior to 33 weeks’ gestation.
Perhaps the biggest controversy in weight assessment is whether
the estimated weight of a fetus should be compared to norms
from the overall population or to customized norms 87-91 derived
from a subgroup of fetuses similar to that fetus. For example, if
an African-American fetus has an estimated weight of 1200 g
at 30 weeks’ gestation, should the weight percentile be determined
from overall population norms or from African-American norms?
In that example, the population-based percentile would be lower
than the customized percentile, because African-American fetuses
and neonates are smaller, on average, than those in the general
population. 92 Although there are proponents of customized
norms, 93-95 a major concern with this approach is that it can do
harm by inadvertently normalizing the weight of a fetus who is
small on a pathologic basis. 96,97 his viewpoint is supported by
the fact that at least some population groups with small babies
have elevated rates of postnatal complications 98 and by the
INTERGROWTH-21st inding that neonates of healthy mothers
do not difer in size based on ethnic background. 99
Overall, our recommended approach is to use population
norms. Although it would theoretically be preferable to use norms