Assessment Gestational Age

8/2/2019 Assessment Gestational Age

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ASSESSMENT OF

GESTATIONAL AGEby Ultrasound

DR.AKRAM ABD ELGHANY

MD,ALAZHAR UNIVERSITYCONSULTANT OBS.&GYN.

PORTSAID G.HOSPITAL

EGYPT


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Accurate determination of

gestational age is fundamental toobstetric care and is important in

a variety of situations:

antenatal test(AFP,BHCG,NT) i

when dates are inaccurate, test

results will be incorrect andmisleading.1


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Fetal growth assessment, either

clinically or by ultrasound evaluation,

relies on accurate assessment of

gestational age.

Fetal growth retardation ormacrosomia may be missed owing to

errors in gestational age assignment.

Interpretation of antenatal biophysicaltesting subjected to variation with

gestational age as well.


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Obstetric management is

dependent on gestational ageProper decisions regarding presumed

preterm labor or postdatepregnancies are only possible when

gestational age is accurately

estimated.Timing ofrepeat cesarean section

requires accurate dates.4,5


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METHODS OF GESTATIONAL AGE

ASSESSMENT

The historical information and

the physical examination. The maternal sensation of fetal

movement (quickening).

Assessment of uterine size bybimanual examination in thefirst trimester.


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Detection of fetal heart tones by

Doppler (1012 weeks). Auscultation (1921 weeks). Fundal height measurement.

Both the history and the

physical examination arefraught with error, even in

the best circumstances.


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Menstrual history was

considered reliable inonly 18% of women. Even among women with

known LMP, neonatal age

assessment differed markedlyfrom that assigned by certain

menstrual dates in 15% .


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20% to 40% of women cannot

relate the LMP with certainty.6,7 due to: oligomenorrhea,metrorrhagia.

bleeding in the first trimester ofpregnancy. pregnancy following use of oral

contraceptives or intrauterine devices. pregnancy in the postpartum period.


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1.Physical examination inaccurate,

especially with advancing

gestational age.2. Bimanual examination in the first

trimester may be accurate within 2

weeks.3. fundal height measurement is only

accurate within 4 to 6 weeks.

4. the inaccuracies of history andphysical examination may limit their

usefulness in assessment of

gestational age.


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Timed ovulation,either by basal body

temperature recording or semiquantitative

assessment of LH surge, predicts gestationalage within 4 to 6 days.Ovulation induction with agents such as

clomiphene citrate and Pergonal,accuratelypredicts gestational age.In vitro fertilization the most accurate means

of predicting gestational age (1 day).in most pregnancies, the date of ovulation or

conception cannot be accurately predicted and

gestational age must be established by other

methods.


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Clinical Predictors of Gestational Age

Estimated Range for

Parameter 95% of Cases

In vitro fertilization 1 day

Ovulation induction 46 days

LH surge indicator 46 days

Basal body temperature 46 days

Certain menstrual history 2 weeksBimanual examination (first trimester) 2 weeks

First fetal heart tones by Doppler 2 weeks

Quickening 24 weeks

First fetal heart tones by auscultation 24 weeksFundal height measurement between 4 weeks

18 and 32 weeks' gestation

Fundal height measurement after 32 46 weeks

weeks' gestation


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Ulrasound assessment of gestational

age is feasible in a majority ofpregnancies with greater accuracy

than physical examination.

In the first trimester, gestational sacmean diameter and crown-rump

length measurements have becomethe primary means of evaluating

gestational age.15,16,17,18,19


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In the second and third trimesters,

fetal head, body, and extremitymeasurements have been

commonly used to assess

gestational age.Those parameters most commonly

measured include biparietal

diameter,head circumference

,abdominal circumference and

femur length.


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First Trimester Assessment

the gestational sac mean diameter andcrown-rump length are used to establishfetal age.

Both parameters are useful because eachmeasures a different aspect of the first-

trimester pregnancy and may be used atdifferent times during the first trimester.


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GESTATIONAL SAC MEAN DIAMETER

The gestational sac is the firstidentifiable structure imaged in the firsttrimester.

transabdominal ultrasound as early as 5weeks' , as early as 4 weeks' gestation bytransvaginal ultrasound.15,16,47

The gestational sac is an echo-free spacecontaining the fluid, embryo, andextraembryonic structures.


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The sac is measured inside the hyperechoic rim,

including only the echo-free space.

The gestational sac is imaged first in the

longitudinal plane, obtaining long axis and

anteroposterior measurements perpendicular

to each other. Then, in the transverse plane at

the level of the anteroposterior measurement,

the width measurement is obtained.

The three measurements are averaged toobtain the gestational sac mean diameter.

The accuracy was found to be 1 week.16


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CROWN RUMP LENGTH(CRL)

The embryo is measured along itslongest axis to obtain the CRL

measurement accurately date pregnancybetween 7 and 13 weeks' gestation.

measurement of the fetal length from the

tip of the cephalic pole to the tip of thecaudal pole.

The fetus should be at rest and

assuming its natural curvature.


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At 5 to 6 weeks' distinct landmarks

cannot be identified but heart motion

can be detected centrally.

As the pregnancy continues, the

head can be easily identified from therest of the body.

After 12 weeks' gestation excessive

curvature of the fetus leads to

erroneous shortening of CRL

measurement.


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Gestational age assessment by CRL was

extremely accurate, approaching 3 to 4 days.

Subsequent studies have suggested that the CRLis somewhat less accurate; however, the accuracy

is still within 5 to 7 days.49,50,51,52

Variations in the measurement of CRL can beattributed to differences in fetal growth patterns.

Such differences are related to factors similar to

those that influence birth weight curves, including

maternal age and parity, prepregnancy maternal

weight, geographic location, and population

characteristics


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Technical factors can lead to

errors in CRL measurements.

incorporation of the yolk sac or

lower limbs in the CRL

measurement,

excessive curling or extension of

the fetus, tangential section of the trunk.17


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The crown-rump length is measured

along the longest axis of the fetus


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Second and Third Trimester

Assessment

fetal head (BPD and HC),

body (AC),

extremity (FL)measurements.


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BIPARIETAL DIAMETER (BPD)

The BPD is imaged in the transaxialplane of the fetal head at the level of

thalami in the midline, equidistant fromthe temporoparietal bones and usually thecavum septum pellucidum anteriorly.58,

59 the most commonly accepted method is

measurement from leading edge to leading

edge (outer-to-inner) .


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Between 12 and 26 weeks' the BPD is

accurate 10 to 11 days.

After 26 weeks' the accuracy of BPD

measurement progressively decreases and is

3 weeks near term.

Biologic variation, occur because of

differences in maternal age, parity,

prepregnancy weight, geographic location,

and specific population characteristicscontribute to inaccuracy in the BPD

measurement.


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Technical factors interobserver error, different techniques

of measurements, and single versusmultiple measurements influence theaccuracy.60,61,62

BPD measurement is most accurate inassessing gestational age when the head

shape is appropriately ovoidovoid. If the head is rounded (brachycephalic)

or elongated (dolicocephalic), BPDmeasurements would overestimate orunderestimate gestational age,

respectively.


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To determine whether head shape is

appropriate, the BPD and the

frontooccipital diameter ratio( thecephalic index) (CI) Is calculated with a

mean value of 0.78 (2 SD) of 0.70 to

0.86..

In the fetus with an abnormal cephalic

index (noted in


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HEAD CIRCUMFERENCE(HC)

Although tracing of the outer perimeter of the head is

the most reliable means of measuring HC, the following

formula using biparietal and fronto-occipital diameters

may be used to calculate HC with a maximum error of

6%:63,65

D1 + D2 / 2 x3.13 The accuracy of HC measurement is comparable with

that of BPD measurement.30

in fetuses with abnormal head shape, either

brachycephaly or dolicocephaly, HC may be a more

accurate predictor of fetal age than BPD.30,65


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Transaxial image of the fetal head for

biparietal diameter and head

circumference measurements


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ABDOMINAL CIRCUMFERENCE (AC)

AC is obtained in the transaxial view of the fetalabdomen,at the level of the fetal liver, using theumbilical portion of the left portal vein as alandmark,The fetal stomach is at the same level,

which is slightly caudad to the fetal heart andcephalad to the kidneys.

The AC measurement is taken from the outermostaspects of the fetal soft tissues. (1) tracing the

outer perimeter of the AC by the trackball on theultrasonic equipment (2) the same equation as forHC using transverse and anteroposterior diameters

of the fetal abdomen.


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The AC may be used to estimate

gestational age but is less accurate than

head measurements (BPD or HC).33

the accuracy of AC in estimating

gestational age is greatest in the secondtrimester, with decreasing accuracy

near term.

Biologic and technical factors may

contribute to the inaccuracy of AC

measurements.


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AC is the growth parameter mostcommonly affected in pregnancies

complicated by abnormal fetal growthpatterns.33

A macrosomic fetus will have increased

AC relative to gestational age, A growth-retarded fetus will have

diminished AC measurements.

Estimation of gestational age by AC willlead to inaccuracies in fetuses displayingeither of these growth patterns.


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HC/AC ratioHC/AC ratio

a predictor of head-to-

abdomen symmetry orasymmetry in order to

identify the type ofabnormal growth.


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Transaxial image of the upper fetal

abdomen


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FEMUR LENGTH(FL)FEMUR LENGTH(FL)

the largest of the long bones, leastmoveable, and easiest to image.

It is measured along the long axis of thebone; a straight measurement of theosseous portion is taken from one end tothe other, disregarding bone curvature.

The ultrasound beam shoud be

perpendicular To the shaft. the measured ends shoud be blunt in

appearance and the distal femoral

epiphysis should not be included.

FL t l di t t ti l


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FL accurately predict gestational age

between 14 weeks' and term.39

The accuracy of the FL and BPD issimilar in the third trimester. Although

there is controversy regarding the

accuracy of the FL prior to 26 weeks'

gestation.38,39

the accuracy of FL is greatest in thesecond trimester and least near term.

Bi l i d t h i l f t l d


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Biologic and technical factors may leadto inaccuracies of FL measurements.

ultrasound imaging may lead tooverestimation of FL, particularly

when the femur is in the far field

or lateral margins of the image.

Tangential section of thefemur, failing to visualize the entirelength of the shaft, leads tounderestimation of FL.


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Artifactual bowing of the femur may

also occur on ultrasound imaging and

lead to a shortened FL measurement.

The distal femoral epiphysis becomes

echogenic in the third trimester,Inclusion of the distal epiphysis will

falsely overestimate FL.67,68

FL is useful when head measurementis difficult to obtain due to fetal

position.


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FL/BPD ratioFL/BPD ratio The FL/BPD ratio (normal values 79 6%) is useful as an internal verificationof the measurements obtained.

Abnormal ratio is an indicator ofpathologic entities,

microcephaly (FL/BPD abnormallyhigh)

hydrocephalus or short-limb dysplasia(FL/BPD abnormally low).


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femur length and abdominal

circumference (FL/AC) have beencompared in order to diagnose fetal

growth abnormalities (macrosomiaand fetal growth retardation),34

there is much overlap between

normal and abnormal values of thisratio.75,76,77,78


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The femur length is measured

between the arrows


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ASSESSMENT OF

GESTATIONAL AGE

The accuracy of a single parameter is

dependent on the gestational age at

the time of ultrasound examination. To improve the accuracy of

gestational age assessment growth-

adjusted sonographic age79 and

averaging multiple parameters are

used.80,81


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Ultrasound Predictors of Gestational AgeUltrasound Predictors of Gestational Age

Estimated Range for

Parameter* 95% of Cases

Gestational sac mean diameter week

Crown-rump length 57 days

BPD, 1226 weeks 1011 days

HC, 1226 weeks 1014 days

AC, 1226 weeks 1014 days

FL, 1226 weeks 1020 days

BPD, 2742 weeks 23 weeks

HC, 2742 weeks 23 weeks

AC, 2742 weeks 23 weeks

FL, 2742 weeks 23 weeks


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Growth-Adjusted Sonographic Age

Gestational age estimation using a singleBPD is accurate 10 to 11 days in thesecond trimester. Gestational age can bemore accurately predicted by obtaining

paired BPD measurements (the first from20 to 26 weeks' gestation and the secondfrom 31 to 33 weeks' gestation) and

assigning gestational age by a method 79known as growth-adjusted sonographic age(GASA).


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In approximately 90% of fetuses, BPD growthfrom 20 to 33 weeks' gestation tends to progresswithin narrow percentile ranks.21,25

BPD growth patterns can be subdivided intothree types: large ( 90th percentile); average(10th to 90th percentile); and small ( 10th

percentile). Paired BPD measurements obtained at different

gestational ages allows categorization of the

specific cephalic growth pattern. Thefirst measurement should be obtained between20 and 26 weeks' gestation, and the secondmeasurement should be obtained between 30

and 33 weeks' gestation.


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The first BPD measurement will not

distinguish the fetus with large, average, or

small BPD growth, and, therefore, the fetus isassigned a mean gestational age based on an

assumed average BPD growth pattern.

The second BPD measurement identifies thespecific type of growth pattern. Forexample, in the fetus with average growth the

second BPD measurement will fall between

the 10th and 90th percentiles, confirming the

gestational age assignment from the first BPD

measurement.

BPD h i h ll f i l


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BPD growth in the small-for-gestational agefetus will follow a slow growth pattern and thesecond BPD measurement will be less than or

equal to the 10th percentile for the gestationalage assigned by the first BPD. Since the firstBPD measurement failed to recognize the

small growth pattern and, therefore,underestimated gestational age, the secondmeasurement allows the gestational ageassessment to be adjusted based on theBPD growth pattern. Such a fetus with aslowed growth pattern would have thegestational age advanced by 1 week at the time

of the second BPD measurement.


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Dates in the large-for-gestational age fetus

may be adjusted by GASA at the time of the

second BPD measurement, decreasinggestational age assignment by 1 week if the

BPD measurement is greater than or equal to

the 90th percentile.

Use of GASA increases the

accuracy of gestation by BPDmeasurement to within 3 to 5 days.79


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Multiple Fetal Growth Parameters

when two or more parameters predict thesame end point, the probability of correctlypredicting that end point is increased.

The BPD, HC, AC, and FL measurementswere obtained and the mean gestationalages of combinations of these parameterswere averaged to obtain a mean gestational

age. The use of multiple parameters improved

the accuracy of gestational age assessmentcompared with any single parameter.80


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If the gestational age estimatesderived from all of the parameters are

similar, assignment of gestationalage from the average of all the

parameters will improve accuracy.

If gestational age estimates of thevarious parameters are quite

different, averaging multipleparameters will decrease the accuracyof the best predictor(s).


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Averaging of fetal growth parameters

should be avoided when certain

conditions are suspected,

fetal macrosomia,

intrauterine growth retardation (bothsymmetric and asymmetric),

congenital anomalies (skeletaldysplasias, hydrocephalus, and

others).

M l i l G i


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MultipleGestations

During the last 10 weeks of pregnancy there isa decrease in the growth rate for twin fetusescompared with singleton fetuses.

The femur continues to grow normally

throughout pregnancy in twin gestations, whilethe head (BPD and HC) and abdominal (AC)growth rates decrease in the last 10 weeks ofpregnancy.

FL measurement may be a more reliableparameter to use for gestational age assessmentin twin gestations during the third trimest.

G id li d d f


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When menstrual datesfall

within the confidence limitsof the ultrasound assessment,

the role of ultrasound is toconfirm menstrual dates.

Guidelines recommended for

the assessment ofgestationalage


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When menstrual datesfall

outside the confidence limits ofultrasound assessment, assignmentof dates should be based on

ultrasoundassessment ofgestational age.


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When menstrual dates areunknown, assignment of

dates should be based onultrasoundassessment of

gestational age.


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Obstetric management must appreciatethis potential for error.

A patient presenting in spontaneous laborat 33 3 weeks' gestation should bemanaged as if the pregnancy may be as

little as 30 weeks' gestation, rather than asadvanced at 36 weeks' gestation.

The patient presenting for prenatal care at

39 3 weeks' gestation, should bemanaged for the potential of postdates

pregnancy.

Use of the multiple parameters method of


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Use of the multiple parameters method ofassessing gestational age is valid whenthe gestational age estimates of thevarious ultrasound parameters are

similar.

If the gestational age estimates of one orseveral parameters is greater than 2weeks different than the estimates of theother parameters, either the abnormalultrasound parameters should be excludedor a different method should be used toestimate gestational age.

Wh th i lt d


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When the various ultrasoundparameters predict different

gestational ages the fetus should befurther evaluated to explain thesedifferences,

an abnormally small FL measurement maysuggest short-limb defects.

A large BPD may be secondary to

hydrocephalus. an abnormally small or large AC

measurement may suggest asymmetricintrauterine growth retardation or macrosomia.

h diff l d i (CI


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the different ultrasound ratios (CI,

HC/AC, and FL/BPD) may be used

to identify abnormally small or

large parameters.

In the instance of an abnormalcephalic index, the HC should be

used to estimate gestational age,rather than the BPD measurement.

CONCLUSIONS


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CONCLUSIONS Assessment of gestational age is

fundamental to obstetric care andshould be a carefully thought-out

process. Assessment should depend on history

and physical examination, as well asultrasound evaluation.

Ultrasound is a reliable method forestablishing the length of pregnancy and in

this way can improve obstetric care.


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Assessment Gestational Age