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Original Scientific Research: Can the anterior-posterior thigh diameter be used as an indicator for fetal age using two-dimensional sonography?
by Saad Ramzi Ismail, MDS, BSc Diagnostic Imaging, MSC Medical Ultrasound, ARDMS, CARDUP, RT(R), CAMRT, ASRT
The author wishes to express his deepest appreciation to his project supervisors Dr. Karen Hofmann and Karen Pollard for their guidance and advice. In addition thank you to Dr. Philip Hughes, S Desilva, A Zammit, Gary Kachure, Dr. K Game, Dr. V Botha, and Dr. S Benade for their assistance. Grateful thank you to Ms. Jean Spitz, Valletta Lawrence, and Siti Arabiah.This research is dedicated to the memories of my parents and brothers.
No financial support was granted to this research and no commercial affiliation was involved.
Abstract
This study evaluated the usefulness and direct correlation of a simple new method of predicting fetal age by measurement of the anterior-posterior thigh diameter (APTD) in a normal 18-to28 week pregnancies using two-dimensional sonography. Little published research exists in the area of fetal thigh biometry, specifically in the use of the anterior-posterior fetal thigh diameter (APTD). The only study I found was that of fetal thigh circumference. Continuing review of existing practices needs to be coupled with evaluation of alternate or additional methodology.
Material and Methods
This was a quantitative prospective study of 55 patients in High Level General Hospital, Alberta, Canada. Anterior-posterior thigh diameters (APTD) were sonographically measured. The normal range for each week of pregnancy was determined for reliability.
Results
Significant correlation was found between (APTD) and fetal age from simple line regression analysis, with 99.9% confidence intervals at each week from 18 to 28 weeks gestation. There was a correlation of 1 mm APTD per 1 week of fetal age. In addition R >0.93, P< than 0.001. The residual scatter plots confirmed the APTD validity.
Conclusion
APTD is a reliable and valid method for assessing fetal age in a normal pregnancy and may be particularly useful when other parameters are unable to accurately predict fetal age. An accurate linear measurement of multiple fetal parameters allows a more complete profile of fetal growth and estimated date of delivery (EDD). APTD may also be useful in identifying fetal growth problems. All of the values of fetal age lie directly on the “best-fit” regression line. Since the coefficient of determination (Rsq) is very high, this model is very effective.
Keywords: Fetal age, APTD, and parameters. Figures, tables and graphs will open in a new window.
Introduction
There is no existing literature regarding fetal thigh diameter versus fetal age and estimated date of delivery (EDD). There are many parameters that can be used by sonography, including biparietal diameter (BPD), abdominal circumference (AC), head circumference (HC), and femur length (FL). It is important to find new parameter to measure fetal growth that correlate with fetal age so that fetuses that are not growing well can be identified and treated.[1] Multiple factors may influence the fetal biometry including, for example, pathological factors that affect the fetal head meauserments[1] Fetal organ sizes remains small during early pregnancy, followed by a period of rapid growth with rate and time vary for individual organs.[2] Studies have shown that this period of growth can be affected by external and internal factors.[3, 4] The reason for choosing the second trimester for this studies because it is more convenient to the patient and provide excellent window to the fetal thigh.
The reliability of multiple parameters
Studies have supported the use of multiple parameters to improve the accuracy of fetal age and weight estimation.[5] Studies have provided a logical explanation of why it is necessary to measure the fetal leg. It suggested that some times measuring the fetal head is impossible, as when it is too low in the pelvic cavity and therefore alternate methods must be used.[6] Formula of femur length multiplied by the square root of the cross sectional area of thigh has shown a significant correlation with fetal weight.[6 ]The validity of estimated fetal weight is reported to be either below or above the normal limits by using fetal biometry formulas.[7] Multiple measurements formulas of fetal weight showed more accuracy than single measuerment.[8] Fetal thigh calf circumference ratios showed excellent results in evaluating fetal growth in high-risk patients with unknown due dates [9,10 &11] Using more than one fetal biometry can increase the reliability and accuracy in determining fetal age and estimated date of delivery especially in using long bone biometry from 12 to 40 weeks gestation.[12,13] Study has found that limb volume to be a reliable predictor of intrauterine growth restriction.[14] Femur length (FL) is a reliable measurement, but it can be effected by skeletal dysplasias.[14]
Fetal Pathology and Biometry
With the use of fetal measurements, wide ranges of pathological conditions can be discovered[15]; among these are chromosomal abnormalities (Trisomy 21, fetal nasal pathology).[15] The ratio of femur to foot length has proven a useful parameter in assessing dysplastic limb reduction and fetal growth.[16, 17, 18, & 21]. Studies of Goldstein et al have shown that there is significant correlation between femur length (FL) and orbital diameter (OD) and this may aid in future research regarding fetal orbital abnormalities[19]. The fetal kidney length between 24 to 38 weeks gestational period is more accurate fetal biometry than biparietal diameter (BPD) and head circumference [20]. A combination of more than one parameter should be used to increase the reliability, sensitivity, and accuracy of fetal biometry. Fetal macrosomia can be predicted by using single ultrasonic biometry[21, 22]. Gestational diabetes mellitus can affect the fetal weight[23]. Accurate measurement of fetal age is the most useful contribution sonogram has made to obstetric practice[24]. So far crown rump length (CRL), biparietal diameter (BPD) and femur length (FL) are considered the measurements of choice [25, 26] All these measurements were acquired before 1985 and in some cases before the electronic calipers were available, resulting in a need to update these procedures by using new sonographic equipment in determining age.
Limitations
Before a new parameter can be used, it must be shown to correlate with fetal age in normal pregnancies. Some fetal positions can reduce the ability to measures specific areas of the fetal body; for example in the occipital anterior or occipital posterior position, it will not be possible to obtain a biparietal diameter (BPD). Serial measurements of biparietal diameter and or head circumference alone are of no value because of the “brain sparing” effect.[27, 28 & 29] The reliability of the ratio of head circumference to abdominal circumference to predict intrauterine growth restriction is limited[26, 27]. There are situations, (for example; pre term labour, diabetes, breech presentation, or previous caesarean section) when it is important for the attending physician to have a single estimate of the fetal size or weight at one point in time. Gestational diabetes mellitus (GDM) can be associated with high birth weight and therefore can effect overall fetal measurements[23, 28 &. 29] Head measurements can be used as genetic markers for frontal lobe hypoplasia[30].
Material and Method
Fifty-five uncomplicated pregnancies were studied prospectively and quantitatively. In High Level General Hospital (North-Western Health Centre), High Level, Alberta between March 21, 2005 and May 10, 2005.This study was approved by the ethic committee of Charles Sturt University, Australia .The patients chosen by randomized selection of normal pregnancies. All patients have had an early (first trimester) sonogram at 12 week gestation to confirmed the accuracy of the gestational age. The author did the study. The author made comparable data and tables with the Hadlock et al tables for femur length.[31, 32] The growth of the fetal anterior-posterior thigh diameter (APTD), outer to outer skin surface was sonographically measured at the middle point of the fetal femur in sagital section and compared with the fetal age from 18 to 28 weeks gestation. The selection of the second trimester period was chosen because soft tissue accretion of the fetal thigh begins to accelerate towards the end of this period. The inclusion criteria for this study were; singleton uncomplicated pregnancies with a normal fetus and an informed consent form, read and signed by the patients and approved by the hospital and Charles Sturt University Ethical Committee. The patients’ ages ranged between 18 and 35 years, with a mean age of 26.5, the study population were mixed of different ethnic groups in Alberta, Canada (eg, Germans, natives Indians, Mennonites, Irish, Hispanics, Ukrainians and east Indians).
The radiologists reported major congenital malformations, chromosomal abnormalities, and maternal complications. The author did not release any pathological information to the patients, and patients were asked to obtain their reports from their physicians. Routine transabdominal sonography was done, including femur length (FL), biparietal diameter (BPD), abdominal circumference (AC), and head circumference (HC). In addition, the author measured the fetal anterior- posterior thigh diameter (APTD), from the middle point of the fetal femur in sagittal section of the fetal thigh using the femur length as a landmark. The anterior-posterior thigh diameter (APTD) measurements were analyzed and compared with fetal age using the Hadlock’s tables for femur length.[31, 32] Equipment use in this study was ATL 5000 and Philips Alegra 4500 (Bothell, WA). With 5 to 3 MHz transducers the fetal age of each patient was determined by using Dr Hadlock measurements of the Femur length (FL).[31, 32] The comparison was made between the anterior posterior thigh diameter and the fetal age .The correct diameter of the fetal thigh was measured in the same portion of fetal thigh every time by measuring the mid point of the femur.11 groups were studied, each group divided to 5 patients and each 5 patients were in the same gestational period from 18 to 28 weeks.
Protocol
Starting with transducer at the fetal abdominal circumference:
1. Move transducer inferiorly to transect the fetal bladder.
2. Rotate transducer 30 degrees to view the fetal femur.
3. Rotate transducer until a sagittal view of the fetal thigh be obtained (figure 1).
4. Exclude distal femoral epiphyses (usually present after 32 weeks gestation).
5. Make sure to identify the fetal knee. If a double line is seen in the fetal thigh, measure the inner line or repeat the scan until a smooth (sagittal) line of the fetal thigh is obtained (figure 2). This double line can be corrected by obtaining a perfect sagittal view of the fetal thigh. Otherwise the curve of the thigh adds extra false line to the real outer skin surface of the fetal thigh in lateral or medial section. The thigh is convex in the anterior part and concave in the posterior part so geometrically we are dealing with a cylinder and not a flat surface.
6. Use real-time sonographic equipment with 3.0, 3.5, and 5.0 MHz transducers frequencies to obtain the images. Posterior shadowing from the femur bone is limited and want effect the over all measurements.
7. Freeze-frame and electronic calipers are more sensitive tools to provide accurate measurements of the fetal thigh.
8. Using zoom capability to outline the fetal thigh (outer skin surface) will allows you to see the skin surface better therefore reduce the chances of errors.
9. Using Hadlock et al table for femur length[31, 32]. To compare with anterior-posterior thigh diameter (APTD)
10. Measurementsa. Scan the femur length (FL) at the sagittal view (figure 1 & figure 2).
b. Measure the femur length, then bring the first caliper to the exact middle point of the fetal femur; for example, if the femur length was 2.4 cm (24mm), then bring the first caliper until the measurement reads 1.2 cm (12 mm), (figure 3 & figure 4).
c. Carefully move the first caliper to the outer surface of the fetal anterior thigh (figure 5). Measure the real skin surface and not the extra double line created by the sound waves travel through the convex part of the thigh in parasiagital planes. Scanning the fetal thigh in sagittal plane can make correction and smooth the skin surface of the fetal thigh. Move the second caliper to the outer surface of the posterior thigh, and enter (figure 6).
Calculations
Each one millimetre (1mm) of the anterior-posterior thigh diameter (APTD), or the posterior-anterior thigh diameter (PATD) measurements will be equal to one-week (1 w); For example, 1.90 cm (19 mm) will be equal to19 weeks gestation, and 2.80 cm (28 mm) will be equal to 28 weeks gestation. Multiply 1.428 With any fraction of a millimetre, This number (1.428) Obtained from (10 mm divided by 7 days), for example, APTD of 2.68 cm equal to (26.8 mm) calculate to 26 weeks plus (0.8 x 1.428) = 0.1424 day, this will be added to the 26 weeks equalling 27.0 weeks and 1.4 day. The anterior-posterior thigh measurement (APTD) was found to be relatively constant, one mm equal to one week. Serial measurements should be obtained. The measurements should be repeated with zooming capability and electronic calipers; the serial measurements range should be less than 1 mm. If these measurements don’t match the fetal age obtained by using Hadlcok et al tables for femur length[31, 32] a follow -up scan should be recommended.
Results
Measurements of femur lengths from 55 patients who met the criteria were correlated with the anterior-posterior thigh diameter (APTD) and used to construct tables and graphs. There was significant correlation between the anterior-posterior thigh diameter (APTD) and fetal age Using a simple linear regression for this study, More than 99.9 % confidence intervals were found at each week of the eleven groups from 18 to 28 weeks gestation (R > 0.93), and (p less than 0.001). The anterior-posterior thigh diameter was positively correlated with fetal age (table 1) and (graph 1, graph 2 , graph 3 & graph 4 ). Eleven gestational periods from 18 to 28 weeks were analyzed, each period including 5 different measurements of the femur lengths compared to the fetal age and to the anterior-posterior thigh diameter with mean +/-2SD. Femur length measured from 2.70 centimetre (cm) to 5.50 cm over all gestational periods, the mean being 4.3. Fetal weight ranged between 310 grams and 1400 grams, the mean being 629 grams .The anterior-posterior thigh diameter (APTD) ranged between 1.80 to 2.87 cm, with the mean at 2.36 cm. Linear growth was obtained in each gestational period from 18 to 28 weeks, and compared with the Dr Hadlock’s tables.[31, 32] In addition a linear growth of fetal weight was observed in the graphs (graph 1). The anterior-posterior thigh diameter, converted to millimetres and compared with the fetal age, was found to be a consistent and valid measurement by using the scatter plots (graph 2 & graph 3). The standard errors of estimates using anterior-posterior thigh diameter (APTD) was significantly lower at (.08664) than that using Femur length at (.2436). The variability estimates from Hadlock et al[31] table for femur length versus fetal age from 18 to 30 weeks have indicated +/- 1.8 weeks to 2.4 weeks. APTD table in this study has shown +/- 3 days variability (see table 1). Adjusted R square (variance) was >.99 for both models.
Statistical Analysis
Regression – APTD (cm) and fetal age (weeks)
The standard error of estimation (SEE) is very low at (.08664.) This indicates a strong goodness of fit” of this model .The “spread” of values for the dependent variable (Fetal age) around the mean value of the independent variable is very narrow. About 70% of the values of fetal age will lie +/- .08664 from the mean of APTD (using anova method).
Discussion
The simplicity of the application found by this study is really its greatest advantage. The idea is new, the only study was done previously was the fetal thigh circumference and not the APTD. Accuracy of fetal age, weight, and estimated delivery date (EDD) will be improved if multiple predictors are used; especially when it is difficult to obtain fetal head biometry (for example, when the head is too low in the pelvis, hydrocephalus, anencephaly, and fetal renal disease). New methods for estimating fetal body weight and fetal age without head measurement are therefore required. Reliable new methods of fetal biometry can be very beneficial in reducing overall fetal biometry errors and increasing the reliability of the fetal biometry. Results of this study show that anterior-posterior thigh diameter (APTD) predicts second trimester growth with high validity, and reliability. The very simple correlation in this study of 1 mm APTD per week of fetal age is new and useful information.
Measuring thigh parameter can be a convenient method for determining fetal growth in the second trimester. The APTD may have a role in quality control of second trimester sonogram examination, and may help in the diagnosis of fetal growth abnormalities. The APTD may be used as an indicator of fetal biometric disturbance, thus enabling the physician to better manage the pregnancy. Diabetes Mellitus is one cause of intrauterine growth restriction (IUGR), and may affect the femur length (FL).[10, 23 & 27] Diabetes Mellitus may also affect the fetal body mass and consequently the abdominal circumference and fetal thigh[28] Hence, the anterior-posterior thigh diameter may be used not only as indictor for fetal age but also to detect IUGR. Renal pathology, such as hydronephrosis or congenital renal malformation, can affect the fetal abdominal circumference, making this measurement unreliable as an indicator of fetal age. Using combined parameters may be superior to the use of each measurement alone as a marker of trisomy 21. [30] In addition, it can be difficult in practice to obtain a good fetal thigh circumference, or fetal hands, feet and ears to obtain fetal biometry.
This study shows that the fetal APTD provides a more accurate linear measurement of the fetus, thus generating a more complete profile of the fetus. Significant correlations of APTD with fetal age indicate that this is a reliable method and is particularly useful when other fetal parameters may not accurately predict fetal age or if they are difficult to obtain. If the age predicted from the APTD does not match the age using the femur length, other factors such as intrauterine growth restriction or maternal and fetal nutrition deficits should be considered. The soft tissue accretion of the fetal thigh also depend on the generalized nutritional status of the infant but such increase in the soft tissue usually more marked after 30th weeks gestation. The APTD measurements were obtained from the 11 groups correlated perfectly with the fetal age and repeated five times for each gestational group between 18 and 28 weeks. Tables for femur length versus gestational age from 18 to 30 weeks were +/- 1.8 to +/- 2.4 weeks while the variability estimates in the APTD table was +/- 3 days. Both models predict the fetal age very well, but compared to FL, using APTD produces a model with better “goodness of fit” based on differences in the SEE between the two of them, and on interpreting the Best Fit Regression Lines for both Models. The “spread” of values for the dependent variable is narrower around the mean of the independent variable in the APTD model and wider in the FL model. The standard error of estimates (SEE) of .2436 obtained for FL versus Gestational age is higher than that obtained in the analysis with APTD. This indicates a weaker “goodness of fit” of this model. The “spread” of values for the dependent variable around the mean value of the independent variable is wider. 68% of the values of fetal age will lie +/- .2436 from the mean of APTD. Model Statistics (F, t, and standardized Beta) are significant for both models. Beta (APTD) =10.0 (SEE =. 037), Beta (FL) = 3.79 (SE=. 039). T =273.07 for GA x APTD Model = 96.87 for GA x FL Model
Conclusion
APTD was found to be valid and reliable index for estimating fetal age. Further research to study the relationship between APTD versus fetal weight and IUGR is needed. More research also need it to study the APTD measurements from 12 to 40 weeks gestational age and in wide population to give more statistical power. (See graph 1, graph 2 , graph 3 & graph 4), (table 1) & (figure 1, figure 2, figure 3, figure 4, figure 5, figure 6).
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