When is crown rump length measured

After 13 weeks, head circumference , biparietal diameter , and femur length measurements become more useful measurements for assessing fetal growth. The earlier in pregnancy a scan is performed, the more accurate the age assignment from crown rump length 4. If the original CRL measurement was adequate, the measurement is considered the baseline for all subsequent age measurements. If it not detected at this size on transvaginal scanning performed by an experienced operator, it is an indicator of failed early pregnancy missed miscarriage.

It has been reported that patients in whom MSD mean sac diameter is less than 5 mm greater than crown rump length i. Chromosomal anomalies, particularly trisomy 18 and triploidy are markedly associated with growth restriction, i. Please Note: You can also scroll through stacks with your mouse wheel or the keyboard arrow keys. Updating… Please wait. Unable to process the form. Check for errors and try again. Thank you for updating your details.

Log In. Sign Up. Become a Gold Supporter and see no ads. We modelled the simulated data using fractional polynomial regression of log transformed GA on CRL and compared the fractional polynomial FP terms and the predicted median GA from the equation obtained to the original dating equation reported by Verburg et al. Both equations for the median were FP models of degree 2 with powers 0 and 1 i. The predicted GA from the two equations agreed within 0. Simulated data for crown-rump length measurements in relation to gestational age with fitted centiles.

Full title: Simulated data for crown-rump length CRL measurements in relation to gestational age grey circles with 3 rd and 97 th fitted centiles.

Blue continuous lines represent the original equation fit reported by Verburg et al. All three suggested approaches make use of this fact, but in different ways. Simulated data generated from dating equation by Verburg and truncated at 9 and 14 weeks. Full title: Simulated data generated from the dating equation by Verburg et al. Panel A shows crown-rump length CRL versus gestational age for creating a size chart and panel B shows gestational age versus crown-rump length for creating a dating chart.

We applied the three proposed approaches to the truncated simulated data shown in Figure 2. Figure 3 shows a flow diagram summarising all the three methods. A Flow diagram summarising the process and methodology of the simulation study. Full title: A Flow diagram summarising the process and methodology of the simulation study to evaluate three methods to overcome the truncation problem inherent in the data set.

From the obtained equation of the median GA, we simulate CRL observations about the same number of observations for each day of GA in the un-truncated data set for each day of gestation between 7 and 9 weeks, to overcome the truncation at the bottom end of the distribution of CRL measurements.

The choice of 7 weeks as a lower limit for extrapolation was based on the desire to be able to obtain a good fit to the data at 9 weeks where the actual data is truncated and it was also the lowest limit where the fitted equations and range of gestational age remained plausible when extrapolated. We then extrapolated the mean and SD equations obtained to the rest of the data Figure 4 , panel C.

The predicted GA from this approach was compared to that originally reported by Verburg Table 2. A sensitivity analysis to establish which lower cut-off, i. We note that the choice of a cut-off affects the fit for large CRL and so has clinical implications, because it is desirable to have predictions of GA from CRL between 15 mm and 95 mm Table 2. Crown-rump length measurements in relation to gestational age with fitted centiles Approach 1. Full title: Crown-rump length CRL measurements in relation to gestational age grey circles with 3 rd , 50 th and 97 th fitted centiles Panel A.

Yellow small crosses in panels B and C represent data simulated from the fitted equation of the mean and SD from panel A. We use the model for CRL to simulate observations of CRL about the same number of observations for each day of GA in the un-truncated data set for each day of gestation at both ends of the distribution, i.

The choice of 7 weeks as a lower limit and 17 weeks as an upper limit for extrapolation was based on the desire to be able to obtain a good fit to the data between 9 and 14 weeks where the actual data is truncated. The two cut-offs at 7 and 17 weeks were also the lowest and upper limits where the fitted equations and range of gestational age remained plausible when extrapolated. Crown-rump length measurements in relation to gestational age with fitted centiles Approach 2. Full title: Crown-rump length CRL measurements in relation to gestational age grey circles with 3 rd , 50 th , and 97 th fitted centiles Panel A.

The simulated CRL measurements below 9 weeks and above 14 weeks overcomes the truncation problem presented by the data thereby allowing us to model GA as a function of CRL more efficiently and obtain the respective median and SD equation Figure 5 , panel C. The predicted GA from this approach was compared to that originally reported by Verburg Table 3.

A sensitivity analysis assessment was performed in relation to the value of the lower end cut-off of CRL. The third approach does not require simulating data. Similarly, we can obtain equations for the 3 rd and 97 th centiles Figure 6 , panel C. The predicted GA from this approach was compared to that originally reported by Verburg Table 4.

Since we do not have an equation for the SD, the full model cannot be written down simply. We describe how we obtained an equation for the SD as function of CRL that also allows prediction of any desired centiles.

Crown-rump length measurements in relation to gestational age with fitted centiles Approach 3. Panel B shows the relation between GA and CRL after interchanging the axes and fitting new models to the three sets of coordinates. Panel C shows the model obtained by simply taking the average of 2 SDs. We have described above how to obtain equations for say the 3 rd , 50 th and 97 th centiles by regressing GA on the predicted p th centile of CRL measurements. Using these equations 3 rd , 50 th and 97 th centile relating log GA and CRL we can get two estimates of the SD at a given CRL from the difference between 97 th and 50 th centiles and between the 50 th and 3 rd centiles.

Note that the two are not exactly the same but are very similar because GA was modelled on the log scale. Estimates of any desired centiles can then be obtained using the relation:. The largest difference was at the lower range of CRL i. This is notably because the model was first fit for CRL between 20 mm and 65 mm and extrapolated to the rest of the data.

Model fits beginning with lower CRL values i. Crown-rump length measurements in relation to gestational age comparing the 3 approaches with Verburg. The predicted values of median GA from approach 2 agreed within 1 day for CRL between 15 mm and 85 mm with the largest difference at the 2 extremes of CRL, i. Approach 3 agreed within 1 day for CRL between 15 mm and mm with the largest difference of 1.

The estimates obtained from the computation of SD for approach 3 were remarkably similar to those obtained from the three sets of X, Y coordinates of GA and the predicted 3 rd , 50 th and 97 th centiles for CRL Figure 6 panels B and C. Results of the full sample and the new international dating equation will be published in a separate paper. Full title: Crown-rump length CRL measurements in relation to gestational age grey small hollow circles with 3 rd , 50 th and 97 th fitted centiles Panel A.

Full title: Crown-rump length CRL measurements in relation to gestational age GA grey small hollow circles with 3 rd , 50 th and 97 th fitted centiles Panel A.

The close similarity between the two data sets is apparent. The main aim of this study was to explore the best methodology for modelling data when the outcome variable GA is truncated at both ends, i.

We evaluated 3 approaches to overcome this difficulty by generating data from an existing equation Verburg. The three approaches provided a good fit to the data Figure 6 when compared to the original equation reported by Verburg. We appreciate that the choice of which approach is the best is hard to justify through formal statistical testing.

Approach 2 was considered the best since it gives excellent results i. This potential for bias, methodological heterogeneity and limitations would affect clinical decision-making depending on the equation used; hence the need for an international dating equation and chart. 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.

However, the potential error of this method in the third trimester of pregnancy may not be acceptable. Obstetric management must appreciate this potential for error. Use of the multiple parameters method of assessing gestational age is valid when the gestational age estimates of the various ultrasound parameters are similar. If the gestational age estimates of one or several parameters is greater than 2 weeks different than the estimates of the other parameters, either the abnormal ultrasound parameters should be excluded or a different method should be used to estimate gestational age.

When the various ultrasound parameters predict different gestational ages the fetus should be further evaluated to explain these differences. For example, an abnormally small FL measurement may suggest short-limb defects, a large BPD may be secondary to hydrocephalus, and an abnormally small or large AC measurement may suggest asymmetric intrauterine growth retardation or macrosomia, respectively.

In the instance of an abnormal cephalic index, the HC should be used to estimate gestational age, rather than the BPD measurement. In conclusion, assessment of gestational age is fundamental to obstetric care and should be a carefully thought-out process.

Assessment should depend on history and physical examination, as well as ultrasound evaluation. Liley AW: Liquor amnii in the management of the pregnancy complicated by rhesus sensitization.

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Acta Obstet Gynecol Scand 50 suppl : 29, Campbell S: The prediction of fetal maturity by ultrasonic measurement of the biparietal diameter. J Obstet Gynaecol Br Commonw , Obstet Gynecol 7, Analysis of percentile growth differences in two normal populations using same methodology. Am J Obstct Gynecol , Analysis of biparietal diameter as an accurate indicator of gestational age. J Clin Ultrasound ,