Diagnostic ultrasound ( PDFDrive )

CHAPTER 42 Fetal Measurements 1455
As a group, SGA fetuses have a poor prognosis, with increased
perinatal morbidity and mortality rates. heir mortality rate is
four to eight times that of non-SGA fetuses. 134-136 Half of surviving
SGA fetuses have serious short- or long-term morbidity, including
meconium aspiration, pneumonia, and metabolic disorders. 134,137-139
he risk, however, is dependent on the cause of the small size.
Constitutionally small fetuses carry no elevated risk, unlike those
who are small because of a pathologic condition. Furthermore,
the potential to improve outcome by appropriate management
of pathologically small fetuses varies depending on causation.
For example, when the small size is caused by placental insuficiency,
early delivery may improve outcome, but this intervention
is unlikely to afect outcome in chromosomally abnormal fetuses.
he term fetal growth restriction (FGR), also called intrauterine
growth restriction, is used diferently by diferent authors. Some
use FGR and SGA synonymously, considering all fetuses below
the 10th percentile for gestational age to be growth restricted.
Others use FGR to refer to pathologically small fetuses.
Terminology aside, determination of the cause(s) of small
fetal size is oten diicult, so that all SGA fetuses should be
classiied as suspected FGR. 140 he approach to SGA or FGR
involves three steps: (1) diagnosis: identify small fetuses; (2)
classiication: attempt to determine the cause of the small size;
(3) management: institute monitoring and decide on timing of
delivery.
he most direct approach to identifying SGA fetuses is to
diagnose SGA if the estimated fetal weight falls below the 10th
percentile for the best estimate of gestational age. Several other
criteria for diagnosing SGA or FGR have been proposed, including
sonographic measurements and ratios, 141 as well as a multiparameter
scoring system. 142,143 None of the individual parameters
has a high PPV, 141 and the performance characteristics of the
scoring system 143 are not good enough to make up for its complexity
and cumbersome nature. hus the straightforward approach
of using the estimated fetal weight percentile is the preferred
method for diagnosing SGA fetuses.
Once SGA has been diagnosed, an attempt should be made
to determine its cause through evaluation of both the mother
and the fetus. Maternal assessment should include physical
examination and blood tests, directed toward diagnosis of
hypertension, renal disease, and other maternal conditions that
can cause FGR. Fetal assessment begins with a careful sonographic
examination, looking especially for indings suggestive of a
chromosomal or viral cause (e.g., holoprosencephaly, clenched
hands, rocker-bottom feet, intracranial calciications). If such a
inding is present, amniocentesis or umbilical blood sampling
can conirm the diagnosis of a chromosomal abnormality. A
viral cause of FGR may also be diagnosed by these procedures,
in some cases. 144
Growth-restricted fetuses, other than those with a lethal
condition, such as trisomy 13 or 18, should be carefully monitored
for the remainder of the pregnancy. he monitoring is typically
performed at weekly or semiweekly intervals. Sonographic features
to be followed include amniotic luid volume, biophysical proile
score, estimated fetal weight percentile, and fetal Doppler. A
worsening trend in one or more of these features should prompt
consideration of early delivery.
ASSESSMENT OF FETAL WELL-BEING
It has been shown that when a fetus is suspected of being growth
restricted or having another condition that could afect the
TABLE 42.12 Sonographic Criteria for Large-for-Gestational Age (LGA) and Macrosomia in
Diabetic Mothers: Performance Characteristics
(%) PREDICTIVE VALUES (%)
Sensitivity Speciicity Positive Negative
CRITERIA TO PREDICT LGA a
Elevated HC 129 50 80 64 70
Elevated AC/BPD 130 83 60 71 75
High EFW 129 78 78 74 81
Elevated BPD 129 13 86 75 57
Elevated AC 105,127,129 71-88 81-85 56-78 81-96
Elevated AC growth 105 84 85 79 89
Low FL/AC 105,130 58-79 75-80 68-83 75-76
Elevated AC, high EFW 129 72 71 89 89
CRITERIA TO PREDICT MACROSOMIA
Elevated AC 127 84 78 41 96
Low FL/AC 131 48-64 60-74 36-42 80-83
Elevated TD-BPD 126 87 72 61 92
High EFW 68 48 95 77 84
a Predicted values for criteria for LGA computed using Bayes theorem, 112 assuming an LGA prevalence rate of 10%.
AC, Abdominal circumference; AC/BPD, abdominal circumference to biparietal diameter ratio; BPD, biparietal diameter; EFW, estimated fetal
weight; FL, femur length; FL/AC, femur length to abdominal circumference ratio; HC, head circumference; TD, thoracic diameter.
With permission from Doubilet PM, Benson CB. Fetal growth disturbances. Semin Roentgenol. 1990;25(4):309-316. 117