Diagnostic ultrasound ( PDFDrive )

CHAPTER 42 Fetal Measurements 1451
percentile, attempt to determine whether the fetus is constitutionally
small or small because of a pathologic etiology, then manage
the pregnancy accordingly.
Estimation of Fetal Weight
Numerous formulas have been published for estimating fetal
weight based on sonographic measurements, most of which use
one or more of these fetal body parts: head (BPD or HC), abdomen
(AD or AC), and femur (FL). 19-24,26-28,56 Other measurements,
such as thigh circumference, have been used as well. 28 Formulas
that estimate fetal weight using three-dimensional (3D)
sonography 57-59 and 3D magnetic resonance imaging (MRI) have
also been published. 60,61
he accuracy of a weight prediction formula is determined
by assessing how well the formula works in a group of fetuses
scanned close to delivery. An important measure of a formula’s
performance is its 95% conidence range. If the 95% conidence
range is ±18%, for example, the estimated weight will fall within
18% of the actual weight in 95% of cases, and the error will be
greater than 18% in only 5% of cases. he narrower the conidence
range is, the more reliable is the formula.
Many published studies provide information on this measure
of a formula’s accuracy 62-65 (Table 42.8). he following points are
noteworthy:
TABLE 42.8 Accuracy of Fetal Weight
Prediction Formulas
Body Part(s)
Included in
Formula
Formula
95% Conidence
Range (%) a
Abdomen Campbell and Wilkin 19 ±17.1-23.8 56,62
Head and
abdomen
Abdomen
and femur
Head,
abdomen,
and femur
Head,
abdomen,
femur, and
thigh
Higginbottom et al. 20 ±23.8 56
Hadlock et al. 56 ±22.2 56
Vintzileos et al. 28 ±22.8 28
Warsof et al. 21 ±17.4-21.2 21,56,69
Shepard et al. 22 ±18.2-18.3 22,62
Thurnau et al. 23 ±19.8 56
Jordaan 24 ±25.8 56
Hadlock et al. 56 ±18.2 56
Hadlock et al. 26 ±18.2 26
Birnholz 27 ±17.7 63,b
Vintzileos et al. 28 ±21.2 28
Hadlock et al. 56 ±16.4 56
Hadlock et al. 26 ±16.0 26
Hadlock et al. 56 ±15.0-15.4 56
Hadlock et al. 26 ±14.8-15.0 26
Vintzileos et al. 28 ±17.6 28
Vintzileos et al. 28 ±15.6-17.8 28
a Computed as two standard deviations (2 SD) of the relative error, as
reported in the study(ies) referenced, unless otherwise indicated.
b Based on the fraction of cases in which the estimated weight falls
within 10% of the actual weight.
• he accuracy of weight prediction formulas improves as
the number of measured body parts increases up to three,
achieving greatest accuracy when measurements of the
head, abdomen, and femur are used. here is no apparent
improvement by adding the thigh circumference as a fourth
measurement 66 and no proven beneit from using 3D
sonography or MRI.
• Even when based on measurements of the head, abdomen,
and femur, sonographic weight prediction has a rather
wide 95% conidence range of at least ±15%. Based on the
abdomen and either the head or femur, the range is at
least ±16% to 18%. Precision is considerably worse when
only the abdomen is used.
• A number of factors have been studied to determine their
efect on accuracy of weight prediction. Accuracy appears
to be worse in fetuses that weigh less than 1000 g than in
larger fetuses. 63 Over the rest of the birth weight range,
however, accuracy is fairly constant. 26,56,62,67 Weight prediction
is less accurate in diabetic than in nondiabetic mothers:
in diabetic mothers, formulas that use measurements of
the head, abdomen, and femur have a 95% conidence
range of ±24%, 68 wider than the range of ±15% in the
general population. 26,56 he presence of oligohydramnios
or polyhydramnios has no impact on accuracy. 23,63,69 Scan
quality may have an efect on accuracy. Studies have shown
a trend toward greater accuracy in scans that were rated
“good” compared with those rated “poor” based on ability
to visualize anatomic landmarks. 63,70
Recommended Approach
An attempt should be made to image all three key fetal anatomic
regions—head, abdomen, and femur—at the appropriate anatomic
levels (Table 42.9). If measurements of all three structures can
be obtained, Formula 1 in Table 42.9 should be used to estimate
fetal weight. his formula should be used with the corrected-BPD
when the OFD is available, and with the BPD itself if not. An
alternative approach, equally accurate but more cumbersome,
would be to use Formula 1 when the OFD is unavailable, and a
formula based on HC, AC, and FL when the OFD is available.
If the abdomen and only one of the head or the femur can be
appropriately imaged, Formula 2 or 3 should be used. If the
abdomen cannot be measured, or both the head and femur cannot
be measured, then a weight estimate should not be calculated.
Using the approach outlined in Table 42.9, an accuracy of ±15%
to 18% can be achieved for weight estimation.
Weight Assessment in Relation
to Gestational Age
When an ultrasound is performed in the third trimester, best
estimates of gestational age and fetal weight should be established.
he gestational age may be based on a prior ultrasound, clinical
dating criteria, or current measurements; fetal weight is always
calculated from current measurements. he two values should
be assessed in relation to one another to determine whether the
fetus is appropriate in size for dates. his can be accomplished
by using a table that provides norms for fetal weight as a function