PAPERS

Antenatal screening for intrauterine growth retardation withumbilical artery Doppler ultrasonography

R B Beattie, J C Dornan

AbstractTo assess the usefulness ofcontinuous wave Dopplerultrasonography as an antenatal screening tool forthe detection of intrauterine growth retardation andfetal compromise 2097 singleton pregnancies werestudied. Umbilical artery velocity waveforms wereobtained at 28, 34, and 38 weeks of gestation, fromwhich the pulsatility index, A/B ratio, and resistanceparameter were calculated.No abnormal features or indices of neonatal

outcome were adequately predicted. The most sen-sitive index for being delivered of a growth retardedinfant (<5th centile birth weight for gestation) wasan A/B ratio at 34 weeks (sensitivity 40%, specificity84%). Other measures that show poor neonatalnutritional state (ponderal index, skinfold thickness,and ratio of mid-arm circumference to head circum-ference) were even less well predicted. Acute andchronic hypoxia as determined by Apgar score, pHin blood from the cord artery, and packed cellvolume correlated poorly with umbilical artery wave-form indices, and there was no obvious differencebetween the indices of those who subsequentlyrequired operative or instrumental delivery for fetaldistress and those requiring no intervention. Therewere three unexplained stillbirths in the series,in each of which the fetus had shown waveformpatterns that suggested increased peripheral resist-ance, though the technique did not appear to beuseful for predicting the time of subsequent death.

Screening for small for dates babies in a threestage programme was of no value regardless of thethreshold or index chosen. Obstetricians shouldresist the temptation to introduce screening withDoppler ultrasonography until its proper role hasbeen determined.

IntroductionDoppler ultrasonography has been used in obstetrics

for the real time non-invasive direct measurement ofthe shape of the velocity waveform in the utero-placental, fetoplacental, and fetal circulations. Theassociation of a reduced diastolic component of theumbilical artery waveform with intrauterine fetalgrowth retardation has been suggested. Trudinger et aland Reuwer et al found that fetuses identified clinicallyand biophysically as being growth retarded invariablyshowed reduction, absence, or reversal of the diastolicwaveform component, which was associated with apoor perinatal outcome. `

We designed a study to assess the feasibility of usingcontinuous wave Doppler ultrasonography of theumbilical artery as an antenatal screening tool for thedetection of intrauterine fetal growth retardation andfetal compromise. Various indices have been used todescribe the shape of the waveform in the umbilicalartery,46 and the three most commonly used are shown

in figure 1. These dimensionless indices describe theshape of the waveform and reflect the distal vascularimpedance. All three are independent of the angle ofinsonation but only the pulsatility index can be used todescribe waveforms with "zero diastolic" or reverseddiastolic flow. As diastolic flow reduces, the resistanceparameter approaches unity or a negative value and theA/B ratio approaches infinity. The pulsatility indexvaries from cycle to cycle owing to minor differences inheart rate, and therefore an average from more thanone cycle should be obtained. When diastolic wave-forms are low small changes in the measured diastoliccomponent will have a pronounced effect on the finalratio and therefore large- values of all indices areinaccurate.

+e

ii

Systole

-D iastole

FIG 1-Calculation ofwaveform indices in Doppler ultrasonography.Pulsatility index=A -Bltime averaged mean; A/B ratio=A/B; resis-tance parameter=A -BIA

Patients and methodsIn all, 2097 ultrasonically dated singleton preg-

nancies were entered into the study between 1 August1986 and 31 July 1987 and were examined at 28, 34,and 38 weeks' gestation, as is the pattern for "sharedcare" patients attending our unit. The only selectioncriteria applied were that they should be singletonpregnancies and that mothers should have attendedour hospital within seven days of their 28th gestationalweek. This provided a sample of 62% of the 3400singleton pregnancies for the year, and excludedprivate patients, late bookings, those with altered dateswho attended after 29 weeks, and late referrals fromother hospitals. The attendance rate of the undeliveredpatients was 95% and 99% at 34 and 38 weeks'gestation, respectively.

There was no statistically significant differencebetween the women who were screened and thosewith singleton pregnancies who were not screened inthe known determinants of fetal outcome such as age,parity, social class, and smoking habit. The mean ageof women in the study group was 26-3 (SD 5S5) yearsand mean parity 1 3 (SD 1-4); 256 (12%) requiredcaesarean sections and 170 (8%) required instrumentalor operative intervention for fetal distress.

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Department of Obstetricsand Gynaecology, Queen'sUniversity of Belfast,Belfast BT12 6BBR B Beattie, MD, researchfellowJ C Dornan, MD, seniorlecturer

Correspondence to: DrBeattie.

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Ethical approval- for the study was granted by theQueen's University of Belfast ethics committee. Allpatients with confirmed singleton pregnancies werefirst approached by their midwives or obstetricians atthe booking visit. The outline of the study wasexplained verbally and a written note of explanationgiven. They were invited to join the study, and if theyconsented their antenatal chart was marked for ease ofidentification. Further explanation of the study andbasic principles of Doppler scanning were givenverbally by the ultrasonographers at the first scan andinformed verbal consent thus obtained. The mainreasons for refusal to enter the study were concernsabout further delay while attending the antenatal clinicand distrust of the safety of a new technique for fetalmonitoring.A Doptek 9000 spectrum analyser (Doptek Limited,

England) was used for measuring the waveforms inthe umbilical artery. The frequency of the wall vesselfilter was 200 Hz. The sonograms were obtained fromthe periumbilical region of the mother's abdomen andwaveform pattern recognition was used to identify theumbilical cord. The mothers were examined whilelying in a semirecumbent position, having rested in anon-smoking waiting room for at least 15 minutesbefore examination. To reduce distortion of theDoppler sonogram a sample of approximately 10 cycleswas obtained during fetal apnoea and in the absence ofany audible or visible artefacts due to fetal activity. Asuccessful Doppler examination was achieved withinan average of five minutes. A light pen was used tomark the length, systolic peak, and end diastoliccomponents of the three best cycles. These wereselected on the basis of absence of high intensity areasnear the baseline due to vessel wall movements; aconcentration of high intensity outline near the maxi-mum frequency, which indicates insonation of thecentre of the vessel lumen; and an absence of highfrequency "noise," which causes inaccurate computertracing of 7/8 of the maximum frequency envelope thatis used to calculate the pulsatility index.The pulsatility index, A/B ratio, resistance para-

meter, and duration of the cycle were all automaticallycalculated and averaged for each study. We defined avalue >90th centile at each gestational age as abnormalas this would produce the largest possible group thatcould effectively be followed up in a screening studyand because the frequency distribution of the wave-form indices was unknown. The results were stored ona microcomputer with an integral hard disc. A hardcopy of each trace was also produced.The results were not seen by the clinicians and all

measurements were made by one of three observers: aresearch midwife, a medical technician, and anobstetric research fellow. A concurrent reproducibilitystudy showed that the interobserver and intraobservererrors were non-significant (Kruskal-Wallis test, p=0 65 interobserver, p=034 intraobserver for pulsa-tility index).As well as the traditional measures of perinatal

outcome (birth weight for gestational age, Apgar score,crown-heel length, and occipitofrontal head circum-ference) a number of additional variables wererecorded. To identify fetuses with poor intrauterinenutrition the ponderal index,5 ratio of mid-arm circum-ference to head circumference,9 and skinfold thick-ness'O were recorded. Acidosis at birth was assessedfrom pH in blood from the umbilical artery, and thepacked cell volume of the fetal blood was measured, aschronic hypoxia is associated with polycythaemia. "

At delivery the midwife or obstetrician took a bloodsample from the umbilical artery after double clampingthe cord and measured the pH on a blood gas analyser(Instrumentation Laboratory 1312); the packed cellvolume was determined using a standard centrifuge.

On the day after delivery skinfold measurements weretaken as the average of two readings from the leftsubscapular sites and triceps with a pair of Harpendenskinfold calipers, which can be read to 0 1 mm. Thesewere left on the skin for 30 seconds or until the readinghad stabilised. The mid-arm circumference was takenas the average of two measurements from the left sideof the body, measured to the nearest 3 mm.The details of the course and outcome of the

pregnancy were entered into a data file on the micro-computer that contained no information about Dopplerstudies on the patients; this file was linked to the resultsof Doppler studies by the antenatal number of eachpatient. The files were combined and analysed with thestatistical package for the social sciences, extendedversion (SPSS X).

ResultsThe frequency distributions of the three Doppler

indices were positively skewed at 28, 34, and 38 weeks,and this was not corrected by logarithmic trans-formation. At 28 weeks the pulsatility index showed abimodal distribution with the smaller mode comprisingabout 15% of all cases." This pattern was not found at34 and 38 weeks. As expected all three indices of distalvascular impedence reduced with increasing gesta-tional age, implying falling placental impedance,'3although there were six cases with a zero diastolicwaveform component.

In view of the frequency distribution and theinfluence of gestational age, graphs for pulsatilityindex, A/B ratio, and resistance parameter at specificages were constructed. A value >90th centile forgestational age was defined as abnormal, and para-metric range definitions were derived for comparisonwith other studies (table I).

TABLE i-Reference ranges for umbilical artery Doppler ultrasono-graphv

V'alues obtained

Weeks' At 10th At 90th Mean Mteangestation ccntile ccntile -2SD +2SD

P28 0-96 1-73 0-66 184Pulsatiliryindex )34 0-82 1-28 0-60 1-52

138 0-75 1-17 0-56 1-35

128 2-43 4-29 1-60 5-05A/B ratio 34 2 09 3-44 1-40 4-08

138 1-91 3-00 1-41 3-40

128 0-58 0-76 0-54 0-81Resistance parameter 34 0-52 0-70 0-46 0-76

138 0-47 0-66 0-42 0-71

INTRAUTERINE FETAL GROWTH RETARDATION

The sensitivity, specificity, and positive and nega-tive predictive values of the pulsatility index, A/Bratio, and resistance parameter in identifying thefetuses with a subsequent birth weight <5th centile forgestational age were determined (table II). All threeindices were significantly related to the birth weight ofthe fetus in relation to the 5th centile for gestational age(p<OO5). The most sensitive index was the A/B ratio,although a sensitivity of only 31% was achieved at 28weeks and 40% at 34 weeks. When any of the threeresults were abnormal the sensitivity remained low(43%); 17% to 19% of the population was identified asbeing at risk.

Curves for receiver operating characteristics wereproduced for all waveform indices at all three gesta-tional ages. Figure 2 shows the relation between thesensitivity and specificity of the best single index, theA/B ratio at 34 weeks. Selecting threshold values lowerthan the 90th centile to define an abnormal waveformindex resulted in improved sensitivity but at consider-able loss of specificity.

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TABLE iI-Screeningfor intrauterine fetal growth retardation with Doppler ultrasonography

Predictive value

X (df= 1) p Value Sensitivity Specificity Positive Negative

28 weeks' gestation*Pulsatilityindex 18.1 p<0-0001 28 89 11 97A/B ratio 26-5 p<00001 31 90 12 97Resistance parameter 5-4 p<0 0198 14 94 9 96Anyindex 32 9 p<00001 43 84 11 97

34 weeks' gestationtPulsatility index 23-4 p<0 001 32 89 12 97A/Bratio 23 9 p<0-0001 40 84 11 97Resistance parameter 24 5 p<00001 33 88 12 97Any index 22-9 p<0-0001 43 82 10 97

38 weeks' gestationtPulsatilityindex 11 5 p<00007 31 86 9 97A/Bratio 14 6 p<0l0001 30 89 11 97Resistance parameter 8 2 p<0 0042 31 84 8 96Any index 14 3 p<00002 40 82 9 97

*Use of any index identified 17% of the population as being at risk.tUse of any index identified 19% of the population as being at risk.

1010

+40

80- '~~~~~+ 5 ~

8

+95

20- 9*7

0 1

0 20 40 60 80 100

Specificity (%)

FIG 2-Sensitivity and specificity ofA/B ratio at different percentiles(40th to 97th) used as threshold to define abnormal waveforms

FETAL NUTRITION

A ponderal index (birth weight (g) x 100/crown-heellength (cm)3) <2-32 has been defined as representing a

malnourished neonate.'4 In our series, the X2 test failedto reach significance for any of the indices of nutrition.Scatterplots for ponderal index, ratio of mid-armcircumference to head circumference, and subscapularskinfold thickness showed the poor ability of umbilicalartery waveform analysis to identify fetuses that subse-quently showed morphometric evidence of intra-uterine growth retardation. This is illustrated in figure3 for the ponderal index. If the technique of umbilicalartery Doppler velocimetry is to be of value for screen-

ing the fetuses with an abnormal outcome should havehad pulsatility indices >90th centile and those withnormal outcome should have had waveform indiceswithin the area representing normal neonatal outcome.

2-2-

.814-

10 15 20 25 30 35 40 45 50

Ponderal index

FIG 3-Pulsatility index at 38 weeks related to ponderal index of

neonate. Ruled off area (bottom right) represents fetuses with normal

neonatal outcome and Doppler index

The distribution shows the poor sensitivity and specifi-city of the technique. A similar pattern was foundfor the other indices of nutrition.

FETAL HYPOXIA

Chronic fetal hypoxia is associated with compen-satory polycythaemia (packed cell volume >0 65).Only four of the 13 babies affected had shownabnormal velocity waveforms (fig 4); the A/B ratio at 38weeks was the best index but was not significantlyrelated to outcome (X2 test with Yates's correction).

20 30 40 50 60 70Packed cell volume

FIG 4-Pulsatilitv index at 38 weeks related to packed cell voluamen inblood taken from umbilical arter- at birth. Ruled oJf area (bottom left)represents fetuses with normal packed cell volume atnd Doppler index

Thus the test is not discriminant for haemorrheologicalevidence of chronic hypoxia.

ACUTE FETAL HYPOXIA

Acute fetal hypoxia determined by the traditionalApgar scores and acidaemia at birth determined by pHof blood in the cord artery could not be usefullyanticipated with Doppler analysis. When the X2 testwas applied to fetal acidaemia (pH <7 1) and abnormalwaveform indices, it was significant for any index onlyat 38 weeks (p<005). An abnormal pulsatility index at38 weeks identified only 31% of fetuses with subse-quent acidaemia at birth.

FETAL DISTRESS

Fetal distress was diagnosed clinically in 170 (8%)cases; in these instrumental or operative delivery wasrequired. There was no significant difference in theoccurrence of abnormal waveform indices between thegroup with fetal distress and the rest of the population,which required no intervention. The ranges and distri-butions of values for the pulsatility index at 38 weekswere almost identical in fetuses clinically diagnosed asdistressed and those not.

PERINATAL MORTALITY

Thirteen babies died, giving a perinatal mortalityrate of6 3 per 1000. Seven were neonatal deaths and sixwere stillbirths, and perinatal death was not adequatelypredictable on the basis of Doppler studies (table III).

TABLE III-Perinatal death related to results of Doppler ultrasound

Gestationi Doppler result

Stillbirths*:Anencephaly 34 weeks NormalPlacental abruption 33 weeks NormalPlacental abruption 38 weeks AbnormalUnexplained 28 weeks AbnormalUnexplained 33 weeks AbnormalUnexplained 38 weeks Abnormal

Neonatal deathst:Neural tube defect NormalMultiple anomalies; small for dates NormalFallot's tetralogv AbnormalPreterm; respiratory disease svndrome NormalPreterm; hypoplastic lungs AbnormalPlacental abruption NormalRhesus hvdrops Normal

*M4ortality was 2-9/1000.tMortality was 3-4/1000.

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The association between major fetal anomalies andabnormal fetoplacental waveforms has been documen-ted, and it is not surprising that the neonate in ourstudy with a congenital cardiovascular defect (Fallot'stetralogy) had abnormal Doppler results before birth.In all three unexplained stillbirths abnormal wave-forms were the only risk factor identified, although thetime interval between the abnormal Doppler result andfetal death varied from six days to six weeks. About 200other fetuses (10% of the study population) were alsoidentified in this way as being at risk but had a normaloutcome.

ZERO DIASTOLIC COMPONENT

With the Doptek 9000 analyser and a 200 Hz filter,six results showed an absent diastolic waveform com-ponent at 28 weeks. In three cases the waveformindices reverted to normal and the outcome of preg-nancy was normal. One fetus was small for dates andtwo were unexplained stillbirths.

DiscussionIn recent years there has been increasing interest in

Doppler ultrasound as useful in obstetric practice fordiagnosing intrauterine fetal growth retardation, peri-natal asphyxia, and fetal distress. To provide useful,comparable results researchers must define cut offpoints for abnormal waveform indices based on theknowledge of the distribution of these variables in alarge unselected population and the effect of advancinggestation. A fixed value cannot arbitrarily be selectedfrom a study of a small population; indices may not benormally distributed, and this is the case for Dopplerwaveform indices in the umbilical artery. Hence, non-parametric statistics are appropriate for descriptiveand comparative statistics in studies that use Dopplerwaveform analysis.There has been some concern about the way online

spectrum analysers calculate the pulsatility indexand it cannot be assumed that normal range values canbe generalised to different systems. For the Doptek9000 spectrum analyser at least, umbilical arteryDoppler indices should be graded, based on ourcentile values at 28, 34, and 38 weeks. Using the samemodel of spectrum analyser we observed an obstetric-ally normal population of90 mothers at intervals oftwoweeks from 20 weeks until delivery in parallel withour main study. Analysis of these data showed that thisseries could be superimposed on the results from thestudy group, and thus it is reasonable to use ourreference ranges for the study group to cover the periodfrom 28 to 38 weeks (table I).

In screening for intrauterine fetal growth retardationas defined by birth weight <5th centile for gestationalage the technique described was not satisfactorilysensitive. A sensitivity of only 43% was achieved forany abnormal result (>90th centile) of any of theindices (pulsatility index, A/B ratio, resistance para-meter) at any of the gestational ages studied (28, 34,and 38 weeks) to identify a fetus with intrauterine fetalgrowth retardation. This is unacceptably low for acondition affecting only 5% of fetuses if over half arenot identified. Sensitivity improved when percentilethresholds for an abnormal waveform were loweredbut specificity decreased appreciably; thus regardlessof the cut off point or index chosen the technique isunsuitable for detection of intrauterine fetal growthretardation.

For intrauterine fetal growth retardation defined asbirth weight <10th centile and with the same threewaveform indices, Mulders et al found the screeningtechnique applied at 34 weeks' gestation to be of nomore value in detecting growth retardation thanphysical examination combined with ultrasonography

(sensitivity 53%, specificity 88% for pulsatility index< I 1 and A/B ratio <3 0).7 Fleischer et als5 reportedbetter results for the same definition of intrauterinefetal growth retardation when an abnormal A/B ratiowas used (sensitivity 78%, specificity 83%).

Intrauterine fetal growth retardation is, however, animprecise multifactorial condition and does not necess-arily imply diminished fetoplacental circulation; itlacks a universally accepted definition and maybe difficult to detect even in the neonate; thusother indices of neonatal nutritional state have beendeveloped. None of those used in this study (ponderalindex, ratio of mid-arm circumference to head circum-ference, and skinfold thickness) correlated well withthe abnormal Doppler results obtained, suggestingthat the screening technique described cannotadequately detect fetal malnutrition.

At 28 weeks' gestation a subgroup of fetuses showeda basic difference in fetoplacental haemodynamics thatwas reflected in an elevated umbilical artery pulsatilityindex. This has not been recognised previously, andmay be related to a transient phase of abnormality inplacental development.

Blood viscosity, a determinant of flow, is governedmainly by the packed cell volume, deformability oferythrocytes, total serum concentration of protein,and plasma concentration of fibrinogen.'6 In chronicfetal hypoxia there is increased fetal erythropoiesis"with an increase in the larger immature erythrocytes,and thus high viscosity has been reported in theumbilical blood of acidotic neonates'7 and in cases offetal growth retardation."8 Chronic fetal distress corre-lates well with reduced umbilical venous blood flow."If the umbilical artery Doppler technique couldidentify fetuses with chronic hypoxia, elevated wave-form indices would have been associated with a highpacked cell volume in blood from the umbilical artery;this association was not found.The Apgar score has traditionally been used as a

measure of fetal outcome, though the correlationbetween this subjective score and pH in the umbilicalartery is poor20 and a low pH correlates better with theneurological condition of the neonate than does a lowApgar score." In this series we found a poor correla-tion between the last result of Doppler ultrasono-graphy before delivery and fetal acidaemia at birth,defined as a cord pH <7- 1, and there was no correla-tion with a 5 minute Apgar score <7.A comparative study of the same three umbilical

artery waveform indices at 34 weeks' gestation sugges-ted that they may predict fetal distress better thanintrauterine fetal growth retardation.7 Fetal distress isa complex diagnosis and is often based on a clinicaldecision aided by a fetal heart tracing taken duringlabour. In our study the range and distribution of theantenatal Doppler indices did not differ in fetuses whowere subjected to operative or instrumental inter-vention for fetal distress compared with those notrequiring any intervention. Trudinger et al suggestedthat the availability of umbilical artery waveformanalysis is associated with improved obstetric decisionmaking, reduced fetal distress, and improved neonataloutcome.' When it is to be used in an unselectedpopulation, we cannot concur.The low incidence of unexplained stillbirths in

our study (3/2097) suggests that their associationwith waveform abnormalities should not be over-emphasised; a much larger trial would be required toprove the association. These results, however, suggestthat there may be a primary or secondary disturbancein fetoplacental impedance in at least some of thefetuses that are currently classified as unexplainedstillbirths.Whether the fetus or the placenta determines the

fetoplacental blood flow has long been debated. Some

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workers have suggested a fetal role in the causation ofabnormal umbilical artery waveforms in pregnanciescomplicated by major congenital abnormalities.Meizner et al studied umbilical and uterine arterywaveforms, using continuous wave Doppler ultra-sound to derive A/B ratios, and found that 17 out of 32cases of major congenital abnormalities had abnormalumbilical waveforms whereas only two had equivocaluterine waveforms, suggesting that a fetal mechanismwas operative in over half the cases.'' Trudinger andCook reported on 26 fetuses with major abnormalitiesand found that though half had abnormal umbilicalartery waveforms all had normal uterine waveforms.'<They suggested that in the group with abnormal arterywaveforms abnormal fetuses had triggered obliterationof small arteries in the placenta. Rochelson andcolleagues speculated that when uterine artery patternsare abnormal, suggesting reduced perfusion, theremay be placental damage or failure of adequate villousdevelopment."4 Normal uterine and abnormal umbili-cal waveforms mav be due to failure of placentalangiogenesis or an acute vasospasm or thrombosis.The pathological change that occurs in the umbilical

artery waveform, thought to be the result of increasedvascular impedance distal to the vessel, is a progressivereduction in the diastolic component of the waveform;in some cases it may be absent or reversed such thatfetoplacental flow seems to occur only during systole.It should, however, be remembered that the popularterminology of "absent" or "zero diastolic" flow doesnot imply absence of flow in diastole but rather aforward or reverse flow with a frequency shiftedvelocity waveform of less than the frequency of thevessel wall filter for that system. It is thereforemandatory that this frequency is specified in allpublications relating to these terms.An absent diastolic waveform component from the

umbilical artery has been associated with altered leftventricular function in cases of intrauterine growthretardation and multiple congenital abnormalities withhigh perinatal morbidity and a perinatal mortality rateof 40-42%.241 2 The loss of the normal forward diastoliccomponent in the umbilical artery in late pregnancyhas a multifactorial basis. In the trisomic fetus thehistological appearance of the placenta and the associa-ted sonogram is immature, comparable with that seenearly in the second trimester.24Comparison with other studies is difficult as the

frequency of the wall filter is often unspecified. Ourresults are broadly in keeping with other studies,which show a perinatal mortality of 33%, and we agreethat though absence of a diastolic component mayprecede obvious fetal growth retardation and mayidentify a fetus at risk, it does not aid in timingdelivery and may in fact be associated with a com-pletelv normal outcome of pregnancy. Obstetric inter-vention solely on the basis of this finding is unjustified.

This study suggests that it is unhelpful to useumbilical artery Doppler ultrasonography to screen forpoor fetal growth as measured in neonates by thetraditional criteria as well as by ponderal index, ratio ofmid-arm circumference to head circumference, andskinfold thickness. These may no't be the most relevantfeatures of outcome. Doppler ultrasonography ofumbilical artery waveforms may be of clinical use aspart of a wider biophvsical profile of the fetus. It maybe the answer-but to what question?

Until we more fully understand the physiologicalbasis and relevance of these abnormal waveforms, theuse of Doppler analysis of umbilical artery waveformsin clinical practice should be a tentative one. In ourseries only eight of 5730 Doppler studies had an absent

diastolic component, a waveform pattern with verypoor prognostic significance. Two of these were asso-ciated with subsequent unexplained stillbirths and onewith a growth retarded fetus; the three other fetuseswere appropriately grown and delivered in goodcondition.

Biophysical assessment of the fetus has allowed us tostudy fetal growth, fetal behaviour, and the fetalenvironment, and Doppler ultrasound is helping usstudy the normal and abnormal dynamic physiologicalstate of the fetus. We concur with Neilson's cautionthat the proper role of Doppler ultrasonography hasnot yet been determined.26 Obstetricians should resistthe temptation to introduce it prematurely as apanacea.

We thank the Northern Ireland Council for OrthopaedicDevelopment for microcomputer equipment and software.We also thank C Clements, medical technician, and BMcGrath, research midwife.

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2 Rcuwer PJHM, Sijmons hA, Rietman GW, Van Tiel MWAM, Bruinsc HW.Intrauterine growth retardation: prediction of perinatal distress by Doppleruiltrasound. Larcet 1987;ii:415-8.

3 rrudinger BJ, (;iles W'B, Cook CM, Cionnelly A, Thompson RS. Umbilicalartery flow velocity waveforms in high-risk pregnancy. Lancet 1987;i:188-90.

4 Gosling RG, King DH. Cotitinuous wave ultrasound as an alternative andcomplemcnt to x-rays in vascular examinations. In: Reneman RS, ed.Cardiovascular applications of ultrasound. New York: Elsevier NorthHolland, 1974:266.

5 Gosling RG. Extraction of physiological information from spectrum analysedDoppler-shiftcd contintiotis vavc uiltrasouind signal obtained non-invasivclvfrom the arterial tree. In: Hill DM, WVatson BW, eds. IFE medical electronicmionogTraph 21. Stcvenage: Peter lPeregrinus, 1976:73-125.

6 Pourcelot L. Applications cliniques de lexamen Doppler transcutane. In:Peroneau P, ed. I'elocimetric ultrasonore Doppler. Paris: INSERM, 1975:213-40.

7 Mulders LGM, W'ijn PFF, Jongsma HW, Hein I'R. A comparative study cofthree indiccs of umbilical blood flow in relation to prediction of growthretardation. P1'-rinat Med 1987;15:3-12.

8 Lubchenco l O, Bard H. Incidence of hypoglycemia in newborn infantsclassified by birthweight and gestational age. Pediatrics 197 1;47:831.

9 Sasanow SR, Georgieff MK, Pereira GR. Mid-arm circumference and mid-arm circumference/head circtimference ratios: standard curves for anthro-pometric assessment of neonatal nutritional status. 7 Pediatr 1986;109:311-5.

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24 Rochelson B, Schulman H, Farmakides G, et al. TIhe significance of absentend-diastolic selocity in umbilical artery selocity waveforms. Am ] ObstetGtvnecol 1987;156:1213-8.

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