Ultrasound Assessment of Fetal Macrosomia
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- Estimated Fetal Weight
(EFW):
- There is a large
standard deviation in mean differences of actual versus estimated fetal
weight (1).
- Sonographic
estimated fetal weight is a poor predictor of actual fetal weight.
Predictive value is only 64% (2).
- Most formulae for
estimated fetal weight overestimate birth weight.
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- Estimated Fetal
Weight + Abdominal Circumference (AC).
- If EFW + AC exceeds
the 90th percentile, macrosomia is correctly diagnosed in 88.8% of
fetuses (3).
- It appears that AC
growth is accelerated from 32 weeks in a group of large for gestational
age (LGA) fetuses (4).
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- Abdominal
Circumference (AC) (4):
- Grows at a rate of ± 1.2 cm / week is an optimal cut-off
point for detecting LGA infants (4). Sensitivity = 83.8%, specificity =
85.4%, positive predictive value = 78.8% and negative predictive value
= 89% (4).
- AC of >35 cm may
identify >90% of fetuses with macrosomia that are at risk for
shoulder dystocia (5).
- AC minus BPD of
2.5cm or more predicted all cases of shoulder dystocia in one series
(6) but was not predictive in another series (5).
- AC of >2 standard
deviations is also a good predictor of LGA infant
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- FL/AC Ratio:
Ratio correctly identified 89% of
LGA fetuses compared to 63% in non-diabetic fetuses.
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- HC/AC Ratio:
Gestational age dependent (accurate gestational age is essential).
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- Fetal Soft Tissues:
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Using all the above data it is possible to over-react to
false information and therefore increase the rate of cesarean section.
One worker suggests that in a diabetic mother, a sonographic EFW of over 4200
grams with morphometric changes of increased AC in comparison to HC and FL
should be used to persuade the clinician to do an elective cesarean section
(8).
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