8/2/2019 Assessment Gestational Age
1/61
ASSESSMENT OF
GESTATIONAL AGEby Ultrasound
DR.AKRAM ABD ELGHANY
MD,ALAZHAR UNIVERSITYCONSULTANT OBS.&GYN.
PORTSAID G.HOSPITAL
EGYPT
8/2/2019 Assessment Gestational Age
2/61
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
8/2/2019 Assessment Gestational Age
3/61
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.
8/2/2019 Assessment Gestational Age
4/61
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
8/2/2019 Assessment Gestational Age
5/61
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.
8/2/2019 Assessment Gestational Age
6/61
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.
8/2/2019 Assessment Gestational Age
7/61
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% .
8/2/2019 Assessment Gestational Age
8/61
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.
8/2/2019 Assessment Gestational Age
9/61
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.
8/2/2019 Assessment Gestational Age
10/61
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.
8/2/2019 Assessment Gestational Age
11/61
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
8/2/2019 Assessment Gestational Age
12/61
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
8/2/2019 Assessment Gestational Age
13/61
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.
8/2/2019 Assessment Gestational Age
14/61
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.
8/2/2019 Assessment Gestational Age
15/61
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.
8/2/2019 Assessment Gestational Age
16/61
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
8/2/2019 Assessment Gestational Age
17/61
8/2/2019 Assessment Gestational Age
18/61
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.
8/2/2019 Assessment Gestational Age
19/61
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.
8/2/2019 Assessment Gestational Age
20/61
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
8/2/2019 Assessment Gestational Age
21/61
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
8/2/2019 Assessment Gestational Age
22/61
The crown-rump length is measured
along the longest axis of the fetus
8/2/2019 Assessment Gestational Age
23/61
Second and Third Trimester
Assessment
fetal head (BPD and HC),
body (AC),
extremity (FL)measurements.
8/2/2019 Assessment Gestational Age
24/61
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) .
8/2/2019 Assessment Gestational Age
25/61
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.
8/2/2019 Assessment Gestational Age
26/61
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.
8/2/2019 Assessment Gestational Age
27/61
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
8/2/2019 Assessment Gestational Age
28/61
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
8/2/2019 Assessment Gestational Age
29/61
Transaxial image of the fetal head for
biparietal diameter and head
circumference measurements
8/2/2019 Assessment Gestational Age
30/61
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.
8/2/2019 Assessment Gestational Age
31/61
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.
8/2/2019 Assessment Gestational Age
32/61
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.
8/2/2019 Assessment Gestational Age
33/61
HC/AC ratioHC/AC ratio
a predictor of head-to-
abdomen symmetry orasymmetry in order to
identify the type ofabnormal growth.
8/2/2019 Assessment Gestational Age
34/61
Transaxial image of the upper fetal
abdomen
8/2/2019 Assessment Gestational Age
35/61
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
8/2/2019 Assessment Gestational Age
36/61
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
8/2/2019 Assessment Gestational Age
37/61
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.
8/2/2019 Assessment Gestational Age
38/61
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.
8/2/2019 Assessment Gestational Age
39/61
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).
8/2/2019 Assessment Gestational Age
40/61
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
8/2/2019 Assessment Gestational Age
41/61
The femur length is measured
between the arrows
8/2/2019 Assessment Gestational Age
42/61
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
8/2/2019 Assessment Gestational Age
43/61
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
8/2/2019 Assessment Gestational Age
44/61
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).
8/2/2019 Assessment Gestational Age
45/61
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.
8/2/2019 Assessment Gestational Age
46/61
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
8/2/2019 Assessment Gestational Age
47/61
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.
8/2/2019 Assessment Gestational Age
48/61
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
8/2/2019 Assessment Gestational Age
49/61
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
8/2/2019 Assessment Gestational Age
50/61
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).
8/2/2019 Assessment Gestational Age
51/61
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
8/2/2019 Assessment Gestational Age
52/61
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
8/2/2019 Assessment Gestational Age
53/61
When menstrual datesfall
within the confidence limitsof the ultrasound assessment,
the role of ultrasound is toconfirm menstrual dates.
Guidelines recommended for
the assessment ofgestationalage
8/2/2019 Assessment Gestational Age
54/61
When menstrual datesfall
outside the confidence limits ofultrasound assessment, assignmentof dates should be based on
ultrasoundassessment ofgestational age.
8/2/2019 Assessment Gestational Age
55/61
When menstrual dates areunknown, assignment of
dates should be based onultrasoundassessment of
gestational age.
8/2/2019 Assessment Gestational Age
56/61
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
8/2/2019 Assessment Gestational Age
57/61
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
8/2/2019 Assessment Gestational Age
58/61
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
8/2/2019 Assessment Gestational Age
59/61
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
8/2/2019 Assessment Gestational Age
60/61
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.
8/2/2019 Assessment Gestational Age
61/61
references 1.Cowchock FS:Use ofalpha-fetoprotein in prenataldiagnosis.Clin ObstetGynecol19:871,1976 2. Li ley AW:Liquoramni i in themanagementofthepregnancy complicatedby rhesussensitization.Am JObstetGynecol82:1359,1961 3.Queenan JT:Amnioticfluid analysis.Clin ObstetGynecol14: 505,1971 4.Goldenberg RL,Nelson K:Iatrogenicrespiratory distresssyndrome.Am JObstetGynecol123:617,1975 5.Hack M, Fanaroff AA, KlausMH etal:Neonatalrespiratorydistress following electivedelivery:A preventabledisease?Am JObstetGynecol126:43,1976 6. Campbell S, WarsofSL, Lit tleD, CooperDJ:Routineultrasound screening for theprediction ofgestationalage.ObstetGynecol 65:613,1985 7.DewhurstCJ,Beazley JM, Campbell S:Assessment offetal maturityand dysmaturity.Am J ObstetGynecol113:141,1972 8. Sabbagha RE: Ultrasound in managing thehig h - r isk pregnancy.In SpellacyWD(ed): ManagementoftheHigh-Risk Pregnancy,pp 137167.Baltimore,University Park Press, 1976 9.Hertz RH,SokolRJ, Knoke JD etal :Clinicalestimation ofgestationalage:Rulesfor avoiding preterm delivery.Am JObstetGynecol 131:395,1978 10.BeazleyJM,Underhi llRA:Fallacyofthe fundalheight.BrMed J 4:404,1970 11.BellET,Loraine JA:Timeofovul ation in relation to cyclelength.Lancet1:1029,1965 12.Queenan JT,O'BrienGD, BainsLMet al:Ultrasound scanning of ovariesto detectovulation in women.FertilSteril34:99,198013.RossavikIK, Gibbons WE: Variabi lity ofovarian folliculargrowth in naturalmenstrualcycles.FertilSteril44:195,1985 14.MoghissiKS:Predictionand detection ofovulation.FertilSteril34:89,1980 15. Hellman LF, KobayashiM, FillistiL et al:Growth and development of thehumanfetus priort o thetwentiethweek of gestation.Am JObstetGynecol103:789,1969 16.JouppilaPC:Lengthand depthoftheuterus and thediameterofthegestation sacinnormalgravidasduring earlypregnancy.Acta ObstetGynecolScand50 (suppl):29,197117. RobinsonHP,Fleming JEE: A criticalevaluation ofsonarcrown-rump lengthmeasurements.BrJObstetGynaecol82:702,197518.Dru mmJE,ClinchJ, MacKinzieG:Theultrasonicmeasurementoffetalcrown-rump lengthasamethodof assessinggestationalage.BrJ ObstetGynaecol 83:417,1976 19. MacGregorSN, Tamura RK, SabbaghaREetal:Underestimationofgestationalageby conventionalcrown-rump length growth curves. ObstetGynecol70: 344,1987 20.Campbel lS:Thepredi ction of fetalmaturity by ultrasonicmeasurementofthebipari etaldiameter.JObstetGynaecolBrCommonw 76:603,1969 21.SabbaghaRE,TurnerH,RockettH et al:SonarBPD and fetalage:Definition of therelationship.ObstetGynecol43:7, 1974 22.Campbell S, Newman GB:Growth ofthe fetalbiparietaldiameterduring normalpregnancy.JObstet GynaecolBrCommonw 78: 513,1971 23.SabbaghaRE,Hughey M:Standardization of sonarcephalometryand gestationalage.ObstetGynecol52:402, 1978 24.SabbaghaRE,Barton FB,Barton BA:Sonarbiparietaldiameter:I.Analysisofpercentilegrowth differences in two normal populationsusingsame meth o d o logy.Am JObstetGynecol126:479,1976 25.SabbaghaRE,Barton BA,Barton FB etal:Sonarbiparietaldiameter:II.Predicti v e o f three fetalgrowthpatternsleading to acl oser assessmentofgestationalageand neonatalweight.Am JObstetGynecol126:485,1976 26. HugheyM,SabbaghaRE:Cephalometryby real ti meimaging:A criticalevaluation.AmJ ObstctGynecol 131: 825, 1978 27.Kurt z AB,Wapner RJ,KurtzRJetal.Analysis of biparietaldiameter asan accurateindicatorof gestationalage.JClin Ultrasound 8:319, 1980 28.HadlockFP,DeterRL,HarristRB,Park SK:Fetalbiparietaldiameter:A criticalreevaluation o f t h erelation to menstrualage bymeansofrealtimeultrasound.J Ultrasound Med 1:97, 1982 29.DoubiletPM, GreenesRA:Improved predi ction ofgest ationalagefrom fetalheadmeasurement.AJR142:797,1984 30.Hadlock FP,DeterRL,HarristRB, ParkSK:Fetalhead circumference:Relation to menstrualage. AJR 138:649,1982 31.CampbellS,Wilken D:Ultrasonicmeasurement offetal abdomen circumferencein theestimationof fetalweight. Br JObstetGynaecol82:689,1975 32. Tamura RK, SabbaghaRE: Percentile ranksofsonarfetalabdominalcircumferencemeasurements.Am JObstetGynecol 138:475,1980 33.Hadlock FP, DeterRL,HarristRB,ParkSK:Fetalabdominal circumference as apredictorof menstrualage. AJR 139:367,1982 34.Hadlock FP, DeterRL,HarristRB etal:Adate-independentpredictorofintraut erinegrowth retardation:Femurlength/abdominalcircumference ratio.AJR141:979,1983 35. EriksenPS, Secher NJ,Weis-BentzonM:Normalgrowth of thefetalbiparietaldiameter and the abdominaldiameterin alongitudinalstudy.ActaObstetGynecolScand 64:65,1985 36.QueenanJF,O'BrienGD,CampbellS:Ultrasound measurementof fetal limbbones.AmJ ObstetGynecol138:297,1980 37.O'Brien GD, Queenan JT,CampbellS:Assessmentofgestationalagein the second trimesterby real-timeultrasound measurementofthefemurl ength.Am J ObstetGynecol139:540,1981 38.HadlockFP,HarristRB,DeterRL, Park SK:Fetal femur length asapredictorofmenstrualage.A JR138: 875,1982 39.Jeanty P,Rodesch F,DelbekeD,DumontJE:Estimation ofgestationalagefrom measurementoffetallong bones.JUltrasound Med 3:75,1984 40. Jeanty P. Dramaiz-Wilmer M,Dramaiz-Wilmer M,DelbekeD et al:Ultrasonic evaluation offetalvent ricular growth.Neuroradiology21:127,1981 41.Mayden KL,TortoraM,BerkowitzRLetal:Orbitaldiameters:Anewparameter forprenatal diagnosisand dating.Am JObstetGynecol144:289,1982 42.JeantyP,CantraineF,Cousaert E etal:Thebinoculardi stance:A new way to estimatefetalage.J Ultrasound Med 3:241,1984 43.Goldstein I,ReeceEA,Pilu et al: Cerebellar measurements with ultrasonography in the evaluation offetalgrowth and development.Am JObstetGynecol156:1065,1987 44.ChitkaraU,Rosenberg J,Chervenak FA etal:Prenatalsonographic assessmentofthefetalthorax:Normalvalues. Am J Obstet Gynecol156: 1069,1987 45.ReeceEA, SciosciaAL,Green Jetal:Embryoni ctrunk circumference:Anew biometricparameterfor estimation ofgestational age.AmJObstetGynecol156:713,1987 46.GoldsteinI,Reece EA,HobbinsJC: Sonographicappearanceofthe fetalheel ossi fication centersandfootlength measurements provide independentmarkersforgestationalage estimation.Am J ObstetGynecol 159: 923, 198847.SchwimerSR,Lebovic J:Transvaginalpelvic ultrasonography.J Ultrasound Med 3:381,1984 48.Robinson HP:Sonarmeasurement offetalcrown-rump lengthasmeansofassessingmaturity o f f irsttrimesterofpregnancy.BrMed J 4:28,1973 49.Drumm JE:Theprediction of deliverydat eby ultrasonicmeasurementoffetal crown-rumplength.BrJObstetGynaecol84: 1, 1977 50.YerushalmyJ:Relatio n o f birth weight,gestationalage,andtherateofintrauterinegrowt htoperinatalmortal ity.Clin Obstet Gynecol 13:107,1970 51.SmazalSF,Weisman LE,HopplerKDetal:Comparativeanalysis ofultrasonographicmethodsofgestational ageassessment.J Ultrasound Med 2: 147, 198352.Kopta MM,May RR,CraneJP:Acomparison of the reliability o f t h eestimateddateofconfi nementpredictedbycrown-rump length and biparietaldiameter.Am JObstetGynecol145:562, 1983 53.Lubchenco LO,Hansman C,DressierJ,Boyd E:Intrauterinegrowth asestimated from live-born weightdataat2442 weeksofgestation.Pediatrics32:793,1963 54.BrennerWE,EdelmanDA,Hendri cks DH:Astandard offetalgrowth fortheUnit ed StatesofAmerica.Am JObstetGynecol126:555,1976 55. WilliamsRL,CreasyRK,Cunningham GC etal:Fetalgrowth and perinat alviability in California.ObstetGynecol59: 624,1982 56.Mantoni M,Pedersen JF:Fetalgrowth delay in threatened abortion:An ultrasound study. BrJObstetGynaecol89:525,1982 57.Pedersen JF,Pedersen LM:Early growth retardation in diabeticpregnancy.BrMed J1:18, 1979 58.Shepard M,Filly RA: Astandardized plane forbiparietaldiametermeasurement.J Ultrasound Med 1:145,1982 59.Hadlock FP, DeterRL, HarristRB,ParkSK:Fetalbiparietaldiameter:Rationalchoiceofpl aneof section forsonographicmeasurement.AJR138:871,1982 60.Cooperberg PL, ChowT,KiteV,Austin S:Biparietaldiameter:A comparisonofrealti me and conventionalB-scan techniques.JClin Ultrasound 4:421,1976 61. LuntRM, Chard L:Reproducibility ofmeasurementoffetalbiparietal diameterby ultrasoniccephalometry.JObstetGynaecolBr Commonw 81:682,1974 62.Davison JM,LindT,FarrV,Whitti ngham TA:The limitations ofultrasonicfetalcephalometry.J ObstetGynaecol BrCommonw 80:769,1981 63. Hadlock FP,Kent WR,Loyd JLetal:An eval uation oftwo methods formeasuring fet al headand bodycircumferences.J Ultrasound Med 1: 359, 198264.Hadlock FP,Det er RL, CarpenterRJ,ParkSK:Estimatingfetalage:Effect ofheadshape on BPD. AJR137:83,1981 65.Shields JR,MedearisAL, BearMB: Fetalheadand abdominalcircumferences: Ellipsecalculationsversuspl animetry.J Cli n U ltrasound15:237,1987 66. CampbellS,Thorns A:Ultrasound measurement of thefetalheadtoabdomencircumferenceratioint heassessmentof growth retardation.BrJObstetGynaecol 84:165,1977 67. GoldsteinRB,FillyRA,SimpsonG:Pitfall sinfemurlengt h measurements.J Ultrasound Med 6: 203,1987 68.Chinn DH, Bolding DB,Callen OW et al:Ultrasonographicidentificationoffetal lowerextremityepiphysealossification centers.Radiology147:815,1983 69.Wolfson RN, PeisnerDB,Chik LL, SokolRJ:Comparison of biparietaldiameter and femur lengthin the third trimester:Effectsof gestationalageand variation in fetalgrowth.JUltrasound Med 5: 145, 1986 70.Winter J,Kimme-Smith C, Ki ng W:Measurementaccuracy ofsonographic sectorscanners.AJR144:645, 1985 71.GambaJL,BowieJD,Dodson WC, Hedlund LW: Accuracy o fu l trasound in fetalfemurlength determination:Ultrasoundphantomstudy.InvestRadiol 20:316,1985 72. JeantyP,Beck GJ,Chervenak FA etal:Acompari son of sectorand linear array scannersfo r the measurementofthefetalfemur.JUlt rasound Med 4: 525, 1985 73.HohlerCW,QuetelTA:Comparison ofultrasound femurlength and bipari etaldiameterin latepregnancy.Am JObstetGynecol141:759, 1981 74.AbramowiczJ,JaffeR:Comparison between lateral and axial ultrasonicmeasurementsofthefetalfemur.Am JObstetGynecol159:921, 1988 75.OttWJ:Fetalfemur length,neonatalcrown-heell ength, andscreening forintrauterinegrowth retardation.ObstetGynecol 65:460,1985 76. Vintzileos AM,NecklesS,CampbellWA etal:Three fetalponderalindexes in normalpregnancy.ObstetGynecol 65:807,1985 77. Benson CB,DoubiletPM,Saltzman DH,Jones TB:FL/ACratio:Poor predictor ofintrauterine growth retardation.InvestRadiol 20:727,1985 78. Benson CB,DoubiletPM,Saltzman DHetal: Femur length/abdominalcircumferenceratio:Poor predictorof macrosomicfetusesindiabeticmothers.J Ultrasound Med 5: 141, 1986 79.SabbaghaRE,Hughey M,Depp R: Growth adjustmentsonographicage(GASA):A simplified method.ObstetGynecol51:383,1978 80.HadlockFP,DeterRL,HarristRB, ParkSK:Estimating fetalage:Computer-assistedanalysisofmultiple fetalgrowthparameters.Radiology152:497,1984 81. HadlockFP,HarristRB, Shah YP etal:Estimating fetal ageusing multipleparameters:A prospectiveevaluationinaraciallymixed population.AmJObstetGynecol156:955,1987 82.Leveno KJ,Santos-RamosR, Duenholter JH etal: Sonalcephalometryintwins: A t able ofbiparietaldiameters for normal twi n fetuses anda comparisonwithsingletons.Am JObstetGynecol135:727,1979 83.CraneJF,Tomich PG,KoptaM:Ultrasound growth patternsin normaland discordanttwins. Obstet Gynecol55:678,198084.Leveno KJ,Santos-RamosR,Duenholter JH etal:Sonar cephalometryin twin pregnancy:Discordancyofthe biparietaldiameterafter28 weeks'gestation.Am J ObstetGynecol 138: 615, 1980 85.Socol M,TamuraR,Sabbagha REetal:Diminished biparietaldiameterand abdominal circumference growth in normaltwins.ObstetGynecol 64:235,1984 86.GrumbachK,Coleman BG,Arger PH etal:Twin and singleton growth patternscompared using ultrasound.Radiology 158:237,1986