What is the average femur length at 36 weeks

Crown-rump length may be used to accurately date pregnancy between 7 and 13 weeks' gestation. The technique involves measurement of the fetal length from the tip of the cephalic pole to the tip of the caudal pole.

The fetus should be at rest and assuming its natural curvature. At 5 to 6 weeks' gestation, distinct landmarks cannot always be identified but heart motion usually can be detected centrally. As the pregnancy continues, the head can be easily identified from the rest of the body. After 12 weeks' gestation excessive curvature of the fetus may lead to erroneous shortening of CRL measurement; therefore, other measurements, such as the biparietal diameter, should be used to estimate gestational age.

Ultrasound image of a fetus at 10 weeks' gestation. The crown-rump length is measured along the longest axis of the fetus between the arrows. The correlation between sonographic CRL values and dates was first reported by Robinson and Fleming, who obtained CRLs in pregnancies of women with certain menstrual histories.

Table 3 summarizes the results of these three studies evaluating gestational age relative to CRL. TABLE 3. Obstet Gynecol , Reprinted with permission of American College of Obstetricians and Gynecologists Variations in the measurement of CRL can be attributed 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.

These include incorporation of the yolk sac or lower limbs in the CRL measurement, excessive curling or extension of the fetus, and tangential section of the trunk. The biparietal diameter BPD is one of the most commonly measured parameters in the fetus. Campbell was the first investigator to link fetal BPD to gestational age 20 ; however, since this original report, numerous publications on this subject have appeared in the literature.

The BPD is imaged in the transaxial plane of the fetal head at a level depicting thalami in the midline, equidistant from the temporoparietal bones and usually the cavum septum pellucidum anteriorly Fig. Transaxial image of the fetal head for biparietal diameter and head circumference measurements. Ultrasound image with biparietal diameter measurement between the solid arrows, outer edge to inner edge and fronto-occipital diameter measurement between the open arrows. The head circumference may be calculated using these diameters or measured directly.

Diagram of the transaxial ultrasound image of the fetal head at the level of the thalami large arrows , midline falx curved arrow , and cavum septi pellucidi open arrow.

Gestational age assignment is based on the mean BPD; however, a single BPD encompasses a range of ages in which most fetuses of that size are most likely to fall Table 4. A number of factors may contribute to variation or inaccuracy in the BPD measurement. Biologic variation, for example, may occur because of differences in maternal age, parity, prepregnancy weight, geographic location, and specific population characteristics.

Technical factors including interobserver error, different techniques of measurements, and single versus multiple measurements may likewise influence the accuracy of BPD in assessing gestational age. Furthermore, BPD measurement is most accurate in assessing gestational age when the head shape is appropriately ovoid. If the head is unusually rounded brachycephalic or unusually elongated dolicocephalic , BPD measurements would overestimate or underestimate gestational age, respectively.

To determine whether head shape is appropriate, Hadlock and co-workers 64 compared the BPD and the frontooccipital diameter. The ratio of these diameters is called the cephalic index CI , with a mean value of 0. TABLE 4. The head circumference HC measurement may be used to estimate gestational age in a similar manner to BPD measurement Table 5. TABLE 5. AJR , The accuracy of gestational age estimation by HC measurement is comparable with that of BPD measurement.

Measurement of the fetal abdominal circumference AC is obtained in the transaxial view of the fetal abdomen. The AC is measured at the level of the fetal liver, using the umbilical portion of the left portal vein as a landmark Fig.

The fetal stomach is at the same level, which is slightly caudad to the fetal heart and cephalad to the kidneys. The AC measurement is taken from the outermost aspects of the fetal soft tissues.

Measurement of the AC is performed in the same manner as that of the HC, that is, by 1 tracing the outer perimeter of the AC by the trackball on the ultrasonic equipment or by digitizer or 2 the same equation as for HC using transverse and anteroposterior diameters of the fetal abdomen. Biologic variation and technical factors may contribute to the inaccuracy of AC measurements in a manner similar to that previously described for bi-parietal diameters.

Of particular note, the abdominal circumference is the growth parameter most commonly affected in pregnancies complicated by abnormal fetal growth patterns. Variation in AC measurements in macrosomic and growth-retarded fetuses is due to differences in liver size and width of subcutaneous tissue in these two types of abnormal growth patterns. Thus, estimation of gestational age by AC will lead to inaccuracies in fetuses displaying either of these growth patterns.

TABLE 6. TABLE 7. Gestational Age. Campbell S, Thoms A: Ultrasound measurement of the fetal head to abdomen circumference ratio in the assessment of growth retardation. Br J Obstet Gynaecol , Transaxial image of the upper fetal abdomen. Ultrasound image with anteroposterior solid arrows and transverse open arrows diameter measurements.

The abdominal circumference may be calculated using these diameters or measured directly. Diagram of the transaxial ultrasound image of the fetal abdomen at the level of the abdominal circumference measurement with fetal spine curved arrow , the umbilical portion of the left portal vein solid arrow , and fetal stomach open arrow. All the fetal long bones can be adequately examined and measured by ultrasound; however, the femur is the largest of the long bones, least moveable, and easiest to image.

The femur may be adequately visualized from 14 weeks' gestation until delivery. The femoral neck and both proximal and distal epiphyseal cartilages are excluded from the measurement. Femur length FL measurements may be used to accurately predict gestational age between 14 weeks' gestation and term Table 8. Although there is controversy regarding the accuracy of the FL prior to 26 weeks' gestation, 38 , 39 the accuracy of gestational age prediction based on FL is greatest in the second trimester and least near term.

TABLE 8. Linear array image of the fetal femur. The femur length is measured between the arrows. Biologic variation may lead to inaccuracies of FL measurements in a manner similar to that of the other fetal growth parameters. In addition, several technical factors are potential sources of error in the measurement of the femur.

Linear-array ultrasound imaging provides more accurate measurements of FL. In addition, FL measurements obtained in the axial plane parallel to the ultrasonic beam have less mean absolute error than those obtained in the lateral plane, perpendicular to the ultrasonic beam 1. 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 and is separated from the distal end of the diaphysis, the osseous portion of the shaft. Inclusion of the distal epiphysis will falsely overestimate FL.

Gestational age assessment by FL is particularly useful when head measurement is difficult to obtain due to fetal position. The accuracy of a single parameter is dependent on the gestational age at the time of ultrasound examination Table 9. Several methods have been employed to improve the accuracy of gestational age assessment compared with the use of a single parameter.

Two of these methods, growth-adjusted sonographic age 79 and averaging multiple parameters 80 , 81 are discussed. Several principles are important to remember when assessing gestational age by ultrasound:. When menstrual dates fall within the confidence limits of the ultrasound assessment, the role of ultrasound is to confirm menstrual dates. When menstrual dates fall outside the confidence limits of ultrasound assessment, assignment of dates should be based on ultrasound assessment of gestational age.

When menstrual dates are unknown, assignment of dates should be based on ultrasound assessment of gestational age. Growth-Adjusted Sonographic Age. Gestational age can be more accurately predicted by obtaining paired BPD measurements the first from 20 to 26 weeks' gestation and the second from 31 to 33 weeks' gestation and assigning gestational age by a method developed by Sabbagha and co-workers 79 known as growth-adjusted sonographic age GASA.

Paired BPD measurements obtained at different gestational ages allows categorization of the specific cephalic growth pattern. The first measurement should be obtained between 20 and 26 weeks' gestation, and the second measurement should be obtained between 30 and 33 weeks' gestation. The first BPD measurement will not distinguish the fetus with large, average, or small BPD growth, and, therefore, the fetus is assigned a mean gestational age based on an assumed average BPD growth pattern.

The second BPD measurement identifies the specific type of growth pattern. For example, 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. In contrast, BPD growth in the small-for-gestational age fetus will follow a slow growth pattern and the second BPD measurement will be less than or equal to the 10th percentile for the gestational age assigned by the first BPD.

Since the first BPD measurement failed to recognize the small growth pattern and, therefore, underestimated gestational age, the second measurement allows the gestational age assessment to be adjusted based on the BPD growth pattern. Such a fetus with a slowed growth pattern would have the gestational age advanced by 1 week at the time of the second BPD measurement. Similarly, dates in the large-for-gestational age fetus may be adjusted by GASA at the time of the second BPD measurement, decreasing gestational age assignment by 1 week if the BPD measurement is greater than or equal to the 90th percentile Fig.

TABLE Obstet Gynecol in press. Reprinted with permission of American College of Obstetricians and Gynecologists. Fetal growth patterns from second trimester BPD of 5. The first BPD of 5. A second BPD measurement 10 weeks later will identify the fetus as large 90 th percentile , average between 10 th and 90 th percentiles , or small 10 th percentile and lead to a closer assessment of fetal age. Predictive of three fetal growth patterns leading to a closer assessment of gestational age and neonatal weight.

Reprinted with permission of C. Mosby, St. The method of GASA has not been used when the first BPD measurement is obtained prior to 20 weeks' gestation; therefore, it is best to confine the use of GASA to pregnancies in which serial ultrasound studies are contemplated and the first measurement is obtained between 20 and 26 weeks' gestation.

Hadlock and co-workers 80 , 81 combined several measurements in an effort to increase the accuracy of gestational age assessment. The rationale for employing multiple parameters for fetal dating is that when two or more parameters predict the same end point, the probability of correctly predicting that end point is increased. The use of multiple parameters improved the accuracy of gestational age assessment compared with any single parameter Table However, if gestational age estimates of the various parameters are quite different, averaging multiple parameters will decrease the accuracy of the best predictor s.

Averaging of fetal growth parameters should be avoided when certain conditions are suspected, such as fetal macrosomia, intrauterine growth retardation both symmetric and asymmetric , and congenital anomalies skeletal dysplasias, hydrocephalus, and others. Radiology , Multiple Gestations. The detection of multiple gestations is important since multiple gestations are at greater risk for many complications, particularly fetal growth retardation.

Fetal biometric data are available for twin gestations 81 , 82 , 83 , 84 , 85 ; however, triplet and quadruplet pregnancies have not been adequately studied owing to their infrequent occurrence.

In general, ultrasound-derived fetal dating tables obtained for singleton pregnancies can be used accurately for twin pregnancies until approximately 30 weeks' gestation.

Grumbach and co-workers 86 have suggested that the femur continues to grow normally throughout pregnancy in twin gestations, while the head BPD and HC and abdominal AC growth rates decrease in the last 10 weeks of pregnancy.

Although further studies are required to confirm these findings, this study suggests that FL measurement may be a more reliable parameter to use for gestational age assessment in twin gestations during the third trimester. Gestational age estimations in twin pregnancies prior to 30 weeks' gestation should be performed in a similar manner to that for singleton pregnancies.

A simple, but uniform approach to the evaluation of gestational age should be performed in all fetuses. The ultrasound assessment of fetal age is based on the earliest ultrasound study, provided the measurement is technically adequate. Early in gestation fetal measurements have the least variability and, therefore, are most likely to predict fetal age. In the first trimester, the CRL measurement is used to estimate gestational age, whereas in the second and third trimesters fetal head BPD and HC , body AC , and extremity FL measurements are used to assess gestational age.

The following guidelines are recommended for the assessment of gestational age:. If menstrual dates are unknown, or the difference between menstrual dates and the mean gestational age predicted by single or multiple parameters is greater than the range of error of these measurements, fetal age should be established using the best ultrasound predictors either single or multiple parameters or GASA method.

In the third trimester, gestational age assessment is particularly problematic. In the pregnancy with unknown menstrual dates or a discrepancy between menstrual dates and mean gestational age predicted by multiple parameters of more than 3 weeks, fetal age should be estimated by the multiple parameters method. The first sign of pregnancy, the early gestational sac, may be seen as early as days post conception on US, where one sees a well organized sac within the endometrium.

By the 6th menstrual week, the early embryo can be identified. At this time the crown rump length is the best estimation of GA. In the first trimester the crown-rump length is accurate to within 3 to 5 days for gestational age GA determination.

The measurement is plotted on a nomogram that gives a correlation for the measurement to a GA in weeks. The biparietal diameter and femur length are most accurate; a combination of measurements is used to estimate fetal weight.