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EVOLUTION OF TECHNIQUES FOR FETAL ASSESSMENT:-
Twenty years ago fetal health in the antepartum and intrapartum
period was being assessed by a combination of clinical tests,
largely nonspecific, which clinicians generally regarded as
bearing some relation to fetal outcome and a scattering of
newer, purportedly more specific tests relating to some
objective measurement of the fetus. The clinical evaluations
included categorization as “High” or “Low” risk pregnancy,
estimates of fetal size or fundal height, passage of meconium,
irregularity of auscultated fetal heart rate, and subjective
evaluation of fetal movement. The more objective evaluations,
just gaining degree of clinical use in the early 1970s, included
continuous fetal heart rate monitoring, static ultrasound
imaging (B-scans), hormonal tests, such as estriol and Human
Chronic Somatomammotropin (HCS) levels and test of pulmonary
maturity in amniotic fluid, such as the lecithin: sphingomyelin
(L:S) ratio, creatinine concentration and the “shake” or “ foam”
test.
There has been a remarkable refinement and enlargement (or
extension) of many of these tests in the last two decades,
primarily due to the development of reasonably priced, high
resolution, real-time ultrasound and of an understanding of the
relationship between aspects of fetal behavior or state and
fetal health. Although “ intervention” and early delivery had
been practiced for decades in certain pregnancies (e.g., Rh
alloimmunization, maternal diabetes mellitus), the combination
of a number of tests of fetal condition and of pulmonary
maturity, has allowed the important concept of optimal timing of
delivery in complicated pregnancies. This process allows the
obstetric team to make decisions regarding the continued risk of
remaining in utero versus the potential risk of sequelae of
prematurity.
DETERMINAITON OF FETAL GROWTH:-
Clinical estimates of fetal weight and progression of fundal
height measurements still have a place in management today,
particularly as screening tools. The hormonal measurements
(estriol and HCS) have all but disappeared.
The standard technique for ultimately determining fetal size
today is to obtain sonographically determined fetal
measurements: head diameter and circumference, abdominal
circumference, and femur length. These are modern ultrasound
imaging machines. The percentile based on either known menstrual
dates or sonographically determined dates is also given.
Both the estimated fetal weight (EFW) and centiles are of value
in confirming whether a fetus is small or large for gestational
age. Serial measurements can be used to detect the
appropriateness or otherwise of fetal growth. The EFW has a
standard deviation of about 10 percent, relatively similar to
that of age equivalents for each of the above four measurements
allowing the clinician to look for internal consistency and also
to note head/ abdomen discordance, which is usually seen with
later gestational intrauterine growth restriction.
DOPPLER VELOCIMETRY: -
Many ultrasound machines are equipped with a device for
measuring Doppler velocimetry in the fetal vessels, particularly
the umbilical artery and middle cerebral artery. This device
gives an index of vascular impedance in downstream beds and in
the umbilical artery. Increased systolic: diastolic indices are
related to decreased fetal placental blood flow. In the middle
cerebral artery the resistance may decrease as the fetus
compensates for nutrient or oxygen limitations, so indicating
increased cerebral blood flow. There are many strong advocates
of the use of Doppler velocimetry in clinical management, and
abnormal tests have been related to increased stillbirth and
other morbid outcomes. The role of Doppler velocimetry in
clinical management utilizing other biophysical tests is still
being established.
DETERMINATION
OF ADEQUACY OF FETAL OXYGENATION:-
Soon after the introduction of continuous fetal heart rate
monitoring for intrapartum assessment, the technique began to be
used for antepartum assessment also. The most popular test
was the contraction stress test (CST or Oxytocin Challenge
Test,) whereas a small group of Europeans concentrated on the
presence of fetal heart rate variability in the monitor
tracings, and thus foreshadowed the Non Stress Test (NST) by a
decade or more. These tests served well for the 70’s. But many
voiced concerns over the high false- positive (i.e. falsely
abnormal) rates, upto 80 percent for the NST and 50 percent for
the CST. The biophysical profile (BPP) was therefore a welcome
introduction, with a false-positive rate less than the previous
tests. The accuracy of this test is enhanced by combination of
factors viz. fetal movement, fetal flexion tone, fetal
breathing, amniotic fluid volume, and the NST.
There has now evolved a hierarchy of tests of fetal assessment,
of increasing complexity and decreasing false- positive rates.
The series consists of (a) fetal movement counting (kick
counts), (b) the NST, and (c) the BPP, or modified BPP. On
certain occasions the CST may be used as a follow-up to an
abnormal or suspicious NST, or it may be used subsequent to an
equivocal BPP.
DISEASE
SPECIFIC TESTING:-
Of great importance is the relatively recent realization that antepartum testing needs to be disease specific. For example,
the volume of amniotic fluid can decrease over a period of days
in postdate pregnancy. So evaluation needs to be done at least
twice weekly for this indication. In the case of fetal anemia
due to Rh problem the NST will be abnormally smoothed and lack
accelerations, and one can usually provoke late decelerations
with contractions. Again, in fulminant preeclampsia weekly NST
testing may not be adequate to detect fetal deterioration when
there is a rapid decrease in placental function.
APPROPRIATE
INTERVENTION AND TIMING OF DELIVERY:-
The above tests for determining adequacy of fetal growth and
fetal oxygenation have allowed many babies to be delivered
before death or damage in utero has occurred. However, such
successes would not have been possible without a number of other
paralleled developments: (a) detection of fetal developmental
defects by genetic testing and ultrasound imaging: (b) tertiary
referral centers (c) neonatal intensive care units (d) tests of
fetal pulmonary maturity such as the L: S ratio, phosphotidylglycerol, and rapid surfactant tests e)
glucocorticoid treatment for accelerating fetal pulmonary
maturity (primarily betamethasone) and more recently, (f)
neonatal surfactant administration.
We are almost certainly not at the end of the evolutionary trail
in the development and application of these technologies. There
is no doubt however, that our ability to determine the condition
of the fetus, and more accurately detect its state of health,
and our ability to rationally intervene and optimally time
delivery, has vastly improved in recent decades.
The most important challenges for the future are to refine
these techniques to prevent over intervention due to the
prevalence of false-positive results in many of the tests, and
to ensure a proper understanding of these tests by the providers
of obstetric health care.
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