International Journal of Gynecology & Obstetrics 73 Ž2001. 15᎐20
Article
Assessment of fetal weight based on ultrasonic femur
length after the second trimester 夽
Mehrdad Honarvar U , Mahboobeh Allahyari, Sedigheh Dehbashi
Department of Obstetrics and Gynecology, Shiraz Uni¨ ersity of Medical Sciences, Shiraz, Iran
Received 8 August 2000; received in revised form 23 October 2000; accepted 25 October 2000
Abstract
Objecti¨ e: The ultrasonic measurement of the fetal femur length is a sensitive and precise variable for estimation
of fetal growth and development. The objective of this study is to predict fetal weight in fetuses of more than 24
weeks gestation by ultrasound measurement of the femur length. Method: In this study, pregnant mothers were
identified by the criteria of normalities, such as: well-known LMP, regular menstrual cycles, no use of OCP for the
last 3 months, no use of alcohol or cigarettes, no drug abuse, no history of diabetes or chronic HTN. Multiple
gestations, congenital anomalies and still-births were excluded. Birth-weight measurements Žadjusted for maternal
age, baby’s sex, parity and week of gestation. were taken immediately after birth. Result: The relation between fetal
weight and fetal femur length has been determined by cross-sectional analysis of 900 normal fetuses ŽG 25 weeks
gestation. using real time ultrasonography. Mathematical modeling of the data has demonstrated that the femur
growth curve is non-linear beyond 24 weeks gestation. With the aid of a scientific calculator the data were analyzed
and a simple second-grade equation has been derived: EFW Žkg. s 0.042FL2 Žcm. q 0.32FLy 1.36, S.D.; "235 g
ŽHonarvar’s Formula 2.. With the use of this data, the error in estimation of EFW given FL is "235 g. Conclusion:
This simple, new and accurate equation appears to be clinically reliable and easy to use and suggests that previous
夽
This paper was presented in part at the First Asian Pacific International Congress of Anatomists Ž1st APICA., Seoul, South
Korea, 22᎐24 August 1996 and the 15th World Conference of Family Doctors ŽWONCA98., Dublin, Ireland, 14᎐18 June 1998 and
the First Emirates International Congress of Perinatology Ž1st EICOP., Abudhabi, United Arab Emirates, 9᎐12 November 1998.
U
Corresponding author. E-mail address: alziba@yahoo.com ŽM. Honarvar..
0020-7292r01r$20.00 䊚 2001 International Federation of Gynecology and Obstetrics. All rights reserved.
PII: S 0 0 2 0 - 7 2 9 2 Ž 0 0 . 0 0 3 6 8 - 4
16
M. Honar¨ ar et al. r International Journal of Gynecology & Obstetrics 73 (2001) 15᎐20
normal ultrasonic fetal femur length curves for another population may underestimate or overestimate normal fetal
weight for the Iranian population. Thus, our formula is an excellent means to estimate true fetal weight. 䊚 2001
International Federation of Gynecology and Obstetrics. All rights reserved.
Keywords: EFW, estimated fetal weight; GA, gestational age; FL, femur length
1. Introduction
Determination of fetal weight is important in
all pregnancies. When the delivery of an infant is
anticipated, accurate antenatal assessment of the
fetal weight can be a useful adjuvant for establishing a plan of management that will
minimize the perinatal morbidity and mortality
rate w1x.
A major problem in the decision-making
process seems to be the inability to estimate fetal
weight accurately prior to delivery w2x. Recently, a
number of equations for estimating fetal weights,
based on ultrasound measurements obtained
within 48᎐72 h prior to delivery, have been devised w3᎐5x and tested.
Besides it is possible to identify abnormal fetal
growth with the measurement of normal growth
curves for femur length during pregnancy. It is
noticed that the growth rate of femur length
illustrates a decreasing growth rate as gestational
age increases. This growth rate can be used in
following the progressive growth of the fetus w6x
and infants who deviate from physiologic norms
of FL for GA have increased perinatal mortality
and morbidity. Thus, symmetric growth retardation can be identified in all instances by shortened ultrasonic femur lengths.
On the other hand, the fairly common practice
of using mean values determined for one population as the basis for judging the individual fetus
drawn from another population compounds the
problem of lack of sensitivity in two ways. It
ignores the fact that all normal fetuses within a
population do not conform to mean values and it
does not take into account interpopulation variation in fetal anthropometric characteristics. For
example, Raman et al. w7x have shown a significant difference in limb length growth of Indians
compared with non-Indians. For this reason, the
present report is a detailed account of our experi-
ence with estimating EFW in fetuses of more
than 24 weeks gestation in an Iranian population.
In the Iranian population this is one major ethnic
group. Our aim was to make a prospective study
of growth of the femur in the Iranian ethnic
group and observe any differences in growth vs.
other populations. Thus, we intend to device a
growth chart of femur lengths applicable for clinical use in our local setting.
2. Methods
Gestational age assessment in our study was
performed by ultrasonography by using
crown᎐rump length. CRL was measured in the
first trimester Žbefore the 12th week of gestation.
to date of pregnancy. Therefore, in each case
gestational age was documented by the last menstrual period and correlated with ultrasonic CRL
measurements to within "5 days.
Pregnant mothers participated in this study demonstrated criteria of normality such as:
1. regular menstrual cycles, at least for three
menses;
2. no use of OCP, for at least 3 months previous;
3. live-born neonates;
4. no anomaly known perinatally or postnatally;
5. no use of alcohol or cigarettes;
6. no drug abuse;
7. no family H.X of dwarfism; and
8. no diabetes or chronic HTN in mothers.
The patients who have the following criteria were
excluded:
1. multiple gestations Ži.e. twins.;
2. gestational duration ) 40 week;
M. Honar¨ ar et al. r International Journal of Gynecology & Obstetrics 73 (2001) 15᎐20
Table 1
Obstetrics data for normal patients Ž N s 900.
Demographic
data
Parity
Para 0
Para 1
Para 2 or more
Number
%
Male
neonates
374
256
270
41.5 196
28.5 121
30 126
Female
neonates
178
135
144
Economical class Žmonthly income.
Low Ž- $100.
72
8
Middle Ž$100᎐$300. 702
78
High Ž) $300.
126
14
Mean patient age
23.78 years
3. maternal chronic disease; and
4. non-Iranian ethnic groups.
Following delivery, neonatal actual birth
weights ŽABWs. were immediately obtained and
recorded. The patient population is described in
Table 1.
This study consists of 900 pregnant women and
1062 femur length measurements were obtained,
162 of these were studied serially and had at least
two measurements made throughout their preg-
17
nancies. In these patients the femur length measured just before birth was the one used to correlate with birth weight.
The FL is defined from the proximal to the
distal metaphysis, obtained within 48᎐72 h prior to
deli¨ ery. For measuring FL, the present study utilized the technique originally described by
O’Brien et al. w6x. In this connection, the long axis
of the fetus is identified first and the transducer is
then turned 90⬚ to produce a cross-sectional image
of the fetal trunk. After that several femur length
measurements are made, within a 2-mm range,
and the average measurement is considered optimal. All examinations were performed by one of
us using a linear-array real-time system with a
3.5-MHz focused transducer ŽHitachi EUB 40..
The distribution of FL for each week of gestational age is presented in Table 2.
Our research was conducted as a longitudinal
prospective study of a ‘normal’ population referred from the prenatal centers at the urban and
rural areas to the large university hospital in
Shiraz. Therefore, our measurements taken in
this referral center are indicative of a diverse
population and represent the entire Iranian population, not just the population around Shiraz.
Table 2
Distribution of femur length for each week of gestational age
Week
y2 S.D.
Žmm.
y1 S.D.
Žmm.
Mean FL
Žmm.
q1 S.D.
Žmm.
q2 S.D.
Žmm.
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
39.48
40.3
41.7
43
51
48.75
50.52
53.77
53.84
57.46
56.46
60.4
58.93
65.22
65.64
68.42
41.54
42.9
44.75
46.2
51.75
51.72
53.66
55.38
56.72
60.13
60.08
63.1
62.86
66.56
67.17
70.26
43.6
45.5
47.8
49.4
52.5
54.7
56.8
58.3
59.6
62.8
63.7
65.8
66.8
67.9
68.7
72.1
45.66
48.1
50.85
52.6
53.25
57.67
59.94
61.21
62.48
65.47
67.32
68.5
70.73
69.24
70.23
73.94
47.72
50.7
53.9
55.8
54
60.65
63.08
64.13
65.36
68.14
70.94
71.2
74.67
70.58
71.76
75.78
M. Honar¨ ar et al. r International Journal of Gynecology & Obstetrics 73 (2001) 15᎐20
18
Fig. 1. EFW against FL.
3. Results
In this research the equation for EFW as a
function of FL was generated from the data by
means of the regression analysis method ŽFig. 1..
From 25 to 40 weeks gestation the correlation of
fetal femur length growth with fetal weight seems
best to be approximated by a non-linear relationship w7x. So, at this period the linear quadratic
function is the optimal model for predicting fetal
weight from femur length Ž r s 0.98, S.D.s "235
g..
With the aid of a scientific calculator the data
were analyzed and the regression equation for
Table 3
Distribution of estimated fetal weight for mean femur lengths at each week of gestational age EFW Žkg. s 0.042FL2
Žcm. q 0.32FLy1.36, S.D.; "235 g ŽHonarvar’s Formula 2. a
Week
y2 S.D.
y1 S.D.
Mean
q1 S.D.
q2 S.D.
FL Žcm.
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
355
496
659
770
1000
1170
1337
1455
1567
1829
1908
2085
2177
2272
2348
2657
590
731
894
1005
1235
1405
1572
1690
1802
2064
2143
2320
2412
2507
2583
2892
825
996
1129
1240
1470
1640
1807
1925
2037
2299
2378
2555
2647
2742
2818
3127
1060
1201
1364
1475
1705
1875
2042
2160
2272
2534
2613
2790
2882
2977
3053
3362
1295
1436
1599
1710
1940
2110
2277
2395
2507
2769
2848
3025
3117
3212
3288
3597
4.36
4.55
4.78
4.94
5.25
5.47
5.68
5.83
5.96
6.28
6.37
6.58
6.68
6.79
6.87
7.21
a
" 1 S.D.s "235; "2 S.D.s "470.
M. Honar¨ ar et al. r International Journal of Gynecology & Obstetrics 73 (2001) 15᎐20
Table 4
A comparison of the mean ABWs to our predicted weights
Week
Mean ABWs
EFW
da
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
Mean
762.5
1031.28
1084.54
1326.92
1344.66
1562.22
1810.00
1985.29
2149.66
2279.68
2410.25
2630.10
2733.13
2870.78
2940.64
3250.52
825
966
1129
1240
1470
1640
1807
1925
2037
2299
2378
2555
2647
2742
2818
3127
y62.5
65.28
44.46
86.92
y125.34
y77.78
y3
60.29
112.66
19.32
32.25
75.1
86.13
128.78
122.64
123.52
22.63
a
ds difference ABWs and EFW ŽABWsy EFW.. Kolmogoro¨ ᎐Smirno¨ Z two-sample test: d was not statistically
significant ŽK᎐S Z s 0.436, P; 0.991..
our data is:
EFW Ž kg. s 0.042FL2 Ž cm. q 0.32FLy 1.36,
S.D.; "235 g Ž Honarvar’s Formula 2 .
where, EFW is in weeks ŽG 25 weeks gestation.
and FL is in centimeters. The S.D. tends not to
increase as femur length increases.
Predicted EFW values of our study for specific
FL measurements are presented in Table 3.
After that Kolmogorov᎐Smirnov Z two-sample
test for goodness of fit between EFW and mean
ABWs was performed and differences were not
statistically significant ŽK᎐S Z s 0.436, P; 0.991..
The comparison of mean ABWs and that of EFW
derived from Honarvar’s Formula is shown in
Table 4.
There was no difference in either the distribution of parity or in age between patients who
delivered male infants and those delivering female infants. It was found that gender seems not
to affect limb lengths w4x. There were 374 patients
who were nulliparous, 256 patients primiparous
and 270 patients para 2 or greater. The mean age
for all patients was 23.78 years ŽTable 1..
19
4. Discussion
When there is a need for deciding on the
viability of the fetus or its chances of survival,
evaluation of fetal weight seems to be of great
importance. The problem of the large fetus may
arise in conjunction with abnormalities of labor
and with fetuses in breech presentations. Many
centers now believe that the large fetus or the
term fetus in breech presentation may benefit
from operative delivery w4x.
The mean delivery weights in our study were
compared with that of two normal ultrasonic fetal
weight tables. So that, an analysis of the mean
delivery weights in our study confirms the findings
of Hadlock w8x and Ott w9x in part.
The estimates of fetal weight by means of predicted normal ultrasonic weight performed by
Hadlock are more likely to be lower than actual
neonatal weight. Conversely, the estimates of fetal weight performed by Ott were more likely to
be an overestimate of the actual birth weight w10x.
After 29 weeks of gestation, the increase in the
confidence interval with increasing fetal weight is
seen, but before 29 weeks of gestation the confidence interval is variable.
The absolute mean error involved in the calculation of fetal weight by ultrasonic methods ranges
from 5 to 8.4% w11x.
There are major controversies in the reported
gestational age variability Ž"2 S.D.. during the
period 23᎐40 weeks. For example, Hadlock w12x
reported that variability during this period is
Ž"3᎐3.5 weeks., but Yeh et al. w13x reported a
95% confidence interval of "5 days. Our study,
based on analysis of over 900 fetuses, indicates
that the variability during this period is "5 days
w14x. Besides, recently some others have shown
that the correlation coefficient of gestational age
ŽGA. vs. FL is statistically greater than that of the
GA vs. BPD w7,15x.
Finally, this article suggests that previous normal ultrasonic fetal femur length curve of one
population is unsuitable and inappropriate for
another population. The estimated quadratic
growth curves of femoral length amongst the three
ethnic groups clearly showed that the rate of
growth of the Indian fetuses was faster than that
20
M. Honar¨ ar et al. r International Journal of Gynecology & Obstetrics 73 (2001) 15᎐20
of the Malays and Chinese Žnon-Indian fetuses ..
Their femur lengths grew faster by 0.15 mmrweek,
which is highly significant Ž P- 0.001.. The growth
curve was, however, similar amongst the three
ethnic groups w7x. Therefore, it is recommended
that one should not use mean values determined
for one population as the basis for judging the
individual fetus drawn from another population,
because of the significant interpopulation variation observed in fetal femur lengths and this
should be taken into account when growth charts
are being used and fetal weight formulae are
being calculated using limb lengths. In conclusion, in each population, fetal femur length must
be studied independently for making a better
operational and functional decision in the field of
obstetrics and gynecology.
w5x
w6x
w7x
w8x
w9x
w10x
w11x
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