Diagnostic ultrasound ( PDFDrive )

CHAPTER 42 Fetal Measurements 1453
derived from estimated fetal weights, 77,78 norms derived from
birth weights are better established and are based on larger study
populations. 71-76
he weight gain between two ultrasound examinations can
be estimated as the diference between the two estimated weights.
Adequacy of weight gain can be assessed by comparing this
diference to established normal fetal growth rate as a function
of gestational age. INTERGROWTH-21st data indicate that
median fetal weight gain per week decreases progressively from
33 weeks of gestation onward, with a maximum rate of 270 g per
week at 33 weeks, down to 100 g per week at 41 weeks. 86 Other
older growth tables suggest that weight gain may increase until
36 weeks, then decline steadily thereater. 71,72 he longer the time
is between scans, the more accurate the sonographic estimate of
interval weight gain will be. When two scans are performed
within 1 week of each other, weight gain cannot be determined
reliably, because weight prediction is too imprecise to detect
small changes in growth. Furthermore, estimating weight too
close to the time of the prior scan could raise unnecessary concern
by a spurious inding that the fetus has grown subnormally or
even lost weight. hus there is little or no value in computing
an estimated weight at the time of the second scan 1 week ater
the irst. Instead, it is recommended that fetal weight gain only
be assessed ater an interval of at least 2 weeks’ duration. 10,11
When several examinations have been performed, fetal growth
can be depicted graphically by means of a trend plot or growth
curve. One form of growth curve plots the estimated fetal weight
versus gestational age, with the curve for the fetus being
examined superimposed on lines depicting the 1st, 10th, 50th,
90th, and 99th percentiles (Fig. 42.10A). An alternative mode
of display plots the estimated fetal weight percentile versus
gestational age (Fig. 42.10B). In this latter format, the graph for
a normally growing fetus will be a horizontal line, indicating
maintenance of a particular weight percentile throughout gestation.
A down sloping line suggests subnormal growth rate.
Calculation of weight percentiles and plotting of growth curves
are most easily accomplished by computer, using an obstetric
ultrasound sotware package that performs these tasks. 100-102
Alternatively, similar results can be achieved by means of a calculator
and manual plotting of data.
FETAL GROWTH ABNORMALITIES
The Large Fetus
he large-for-gestational-age (LGA) neonate (or fetus) is deined
as one whose weight is above the 90th percentile for gestational
age. 71,103-105 Macrosomia, a related entity, is most oten deined
on the basis of a weight above 4000 g; other weight cutofs (4100 g,
4500 g) are sometimes used. 74,76-80,106 hese growth disturbances
occur with diferent frequencies and are associated with diferent
morbidities and mortalities in diabetic mothers as compared to
the general population. herefore these two patient populations
are considered separately.
General Population
About 10% of all infants have birth weights above the 90th
percentile for gestational age and are considered LGA infants.
EFW (kg)
A
EFW percentile
B
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
25 30 35 40
Gestational age (wks)
100
90
80
70
60
50
40
30
20
10
0
25 30 35 40
Gestational age (wks)
99th
percentile
90th
percentile
50th
percentile
10th
percentile
1st
percentile
FIG. 42.10 Fetal Growth Curves. (A) Estimated fetal weight plotted
against gestational age, superimposed on 1st, 10th, 50th, 90th, and
99th percentile curves. The fetus depicted here has a normal growth
pattern, with estimated fetal weights between the 50th and 90th percentile
over four sonograms. (B) Estimated fetal weight (EFW) percentile
against gestational age.
Of all newborns, 8% to 10% have birth weights over 4000 g and
thus are classiied as “macrosomic,” and 2% weigh over
4500 g. 104,106-109 hese rates, however, vary considerably among
diferent patient subgroups, depending on presence or absence
of risk factors. Risk factors for LGA and macrosomia include
maternal obesity, history of a previous LGA infant, prolonged
pregnancy (>40 weeks), excess pregnancy weight gain, multiparity,
and advanced maternal age. 103,104,107,110-112
Large fetuses have an increased incidence of perinatal morbidity
and mortality, in large part because of obstetric complications.
Shoulder dystocia, fractures, and facial and brachial plexus palsies
occur more frequently as a result of traumatic delivery. 106,110,113,114
he incidence of perinatal asphyxia, meconium aspiration,
neonatal hypoglycemia, and other metabolic complications is
signiicantly increased in these pregnancies. 103,106,107,110
he most straightforward approach to diagnosing fetal LGA
and macrosomia is to use the estimated fetal weight computed
from sonographic measurements. An estimated weight above
the 90th percentile for gestational age suggests LGA, and a weight
estimate above 4000 g suggests macrosomia. Although weight
estimation is somewhat less accurate in large than in average-sized
fetuses, 62,115 this approach has been demonstrated to be moderately
good for diagnosing LGA and macrosomia. It has a positive