Predictive of three fetal growth patterns leading to a closer assessment of gestational age and neonatal weight. Am J Obstet Gynecol Reprinted with permission of C. 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. Multiple Fetal Growth Parameters 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. 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:. 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.
Liquor amnii in the management of the pregnancy complicated by rhesus sensitization. Goldenberg RL, Nelson K: Iatrogenic respiratory distress syndrome. Neonatal respiratory distress following elective delivery: Routine ultrasound screening for the prediction of gestational age. Assessment of fetal maturity and dysmaturity. Ultrasound in managing the high-risk pregnancy. In Spellacy WD ed: Management of the High-Risk Pregnancy, pp — Baltimore, University Park Press, Clinical estimation of gestational age: Rules for avoiding preterm delivery.
Ultrasound scanning of ovaries to detect ovulation in women. Variability of ovarian follicular growth in natural menstrual cycles.
Growth and development of the human fetus prior to the twentieth week of gestation. Length and depth of the uterus and the diameter of the gestation sac in normal gravidas during early pregnancy. Acta Obstet Gynecol Scand 50 suppl: Br J Obstet Gynaecol The ultrasonic measurement of fetal crown-rump length as a method of assessing gestational age.
Underestimation of gestational age by conventional crown-rump length growth curves. The prediction of fetal maturity by ultrasonic measurement of the biparietal diameter. J Obstet Gynaecol Br Commonw Sonar BPD and fetal age: Definition of the relationship. Campbell S, Newman GB: Growth of the fetal biparietal diameter during normal pregnancy.
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Sabbagha RE, Hughey M: Standardization of sonar cephalometry and gestational age. Analysis of percentile growth differences in two normal populations using same methodology. Hughey M, Sabbagha RE: Cephalometry by real time imaging: Am J Obstct Gynecol Analysis of biparietal diameter as an accurate indicator of gestational age. J Clin Ultrasound 8: A critical reevaluation of the relation to menstrual age by means of realtime ultrasound.
J Ultrasound Med 1: Improved prediction of gestational age from fetal head measurement. Relation to menstrual age. Campbell S, Wilken D: Ultrasonic measurement of fetal abdomen circumference in the estimation of fetal weight. Percentile ranks of sonar fetal abdominal circumference measurements. Fetal abdominal circumference as a predictor of menstrual age. A date-independent predictor of intrauterine growth retardation: Normal growth of the fetal biparietal diameter and the abdominal diameter in a longitudinal study.
Acta Obstet Gynecol Scand Ultrasound measurement of fetal limb bones.
Assessment of gestational age in the second trimester by real-time ultrasound measurement of the femur length. Fetal femur length as a predictor of menstrual age. Estimation of gestational age from measurement of fetal long bones. J Ultrasound Med 3: Ultrasonic evaluation of fetal ventricular growth. A new parameter for prenatal diagnosis and dating. A new way to estimate fetal age. Cerebellar measurements with ultrasonography in the evaluation of fetal growth and development.
Prenatal sonographic assessment of the fetal thorax: A new biometric parameter for estimation of gestational age. Sonographic appearance of the fetal heel ossification centers and foot length measurements provide independent markers for gestational age estimation. Sonar measurement of fetal crown-rump length as means of assessing maturity of first trimester of pregnancy.
Br Med J 4: The prediction of delivery date by ultrasonic measurement of fetal crown-rump length. Relation of birth weight, gestational age, and the rate of intrauterine growth to perinatal mortality. Clin Obstet Gynecol Comparative analysis of ultrasonographic methods of gestational age assessment. J Ultrasound Med 2: A comparison of the reliability of the estimated date of confinement predicted by crown-rump length and biparietal diameter.
Intrauterine growth as estimated from live-born weight data at 24—42 weeks of gestation. A standard of fetal growth for the United States of America.
Calculating Conception - American Pregnancy Association
Fetal growth and perinatal viability in California. Mantoni M, Pedersen JF: Fetal growth delay in threatened abortion: Shepard M, Filly RA: A standardized plane for biparietal diameter measurement. Rational choice of plane of section for sonographic measurement.
A comparison of real time and conventional B-scan techniques. J Clin Ultrasound 4: Lunt RM, Chard L: Reproducibility of measurement of fetal biparietal diameter by ultrasonic cephalometry. The limitations of ultrasonic fetal cephalometry. An evaluation of two methods for measuring fetal head and body circumferences.
Calculating Conception Date
Effect of head shape on BPD. Fetal head and abdominal circumferences: Ellipse calculations versus planimetry. J Clin Ultrasound Campbell S, Thorns A: Ultrasound measurement of the fetal head to abdomen circumference ratio in the assessment of growth retardation. Pitfalls in femur length measurements.
J Ultrasound Med 6: Ultrasonographic identification of fetal lower extremity epiphyseal ossification centers. Comparison of biparietal diameter and femur length in the third trimester: Effects of gestational age and variation in fetal growth. J Ultrasound Med 5: Measurement accuracy of sonographic sector scanners.
Accuracy of ultrasound in fetal femur length determination: A comparison of sector and linear array scanners for the measurement of the fetal femur. J Ultrasound Med 4: Comparison of ultrasound femur length and biparietal diameter in late pregnancy. Abramowicz J, Jaffe R: Comparison between lateral and axial ultrasonic measurements of the fetal femur. Fetal femur length, neonatal crown-heel length, and screening for intrauterine growth retardation.
Three fetal ponderal indexes in normal pregnancy. Poor predictor of intrauterine growth retardation.
Poor predictor of macrosomic fetuses in diabetic mothers. Growth adjustment sonographic age GASA: Computer-assisted analysis of multiple fetal growth parameters. Estimating fetal age using multiple parameters: A prospective evaluation in a racially mixed population. Sonal cephalometry in twins: For the purposes of research and surveillance, the best obstetric estimate, rather than estimates based on the last menstrual period alone, should be used as the measure for gestational age.
The American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal—Fetal Medicine make the following recommendations regarding the method for estimating gestational age and due date:. An accurately assigned EDD early in prenatal care is among the most important results of evaluation and history taking. This information is vital for timing of appropriate obstetric care; scheduling and interpretation of certain antepartum tests; determining the appropriateness of fetal growth; and designing interventions to prevent preterm births, postterm births, and related morbidities.
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- Methods for Estimating the Due Date.
Appropriately performed obstetric ultrasonography has been shown to accurately determine fetal gestational age 1. A consistent and exacting approach to accurate dating is also a research and public health imperative because of the influence of dating on investigational protocols and vital statistics.
This Committee Opinion outlines a standardized approach to estimate gestational age and the anticipated due date. However, there is great usefulness in having a single, uniform standard within and between institutions that have access to high-quality ultrasonography as most, if not all, U. Accordingly, in creating recommendations and the associated summary table, single-point cutoffs were chosen based on expert review. Because this practice assumes a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day after the beginning of the menstrual cycle, this practice does not account for inaccurate recall of the LMP, irregularities in cycle length, or variability in the timing of ovulation.
It has been reported that approximately one half of women accurately recall their LMP 2—4. Accurate determination of gestational age can positively affect pregnancy outcomes. For instance, one study found a reduction in the need for postterm inductions in a group of women randomized to receive routine first-trimester ultrasonography compared with women who received only second-trimester ultrasonography 5.
A Cochrane review concluded that ultrasonography can reduce the need for postterm induction and lead to earlier detection of multiple gestations 6. Because decisions to change the EDD significantly affect pregnancy management, their implications should be discussed with patients and recorded in the medical record. Measurements of the CRL are more accurate the earlier in the first trimester that ultrasonography is performed 11, 15— The measurement used for dating should be the mean of three discrete CRL measurements when possible and should be obtained in a true midsagittal plane, with the genital tubercle and fetal spine longitudinally in view and the maximum length from cranium to caudal rump measured as a straight line 8, Mean sac diameter measurements are not recommended for estimating the due date.
Dating changes for smaller discrepancies are appropriate based on how early in the first trimester the ultrasound examination was performed and clinical assessment of the reliability of the LMP date Table 1. For instance, the EDD for a pregnancy that resulted from in vitro fertilization should be assigned using the age of the embryo and the date of transfer.
For example, for a day-5 embryo, the EDD would be days from the embryo replacement date. Likewise, the EDD for a day-3 embryo would be days from the embryo replacement date. Using a single ultrasound examination in the second trimester to assist in determining the gestational age enables simultaneous fetal anatomic evaluation. With rare exception, if a first-trimester ultrasound examination was performed, especially one consistent with LMP dating, gestational age should not be adjusted based on a second-trimester ultrasound examination.
Ultrasonography dating in the second trimester typically is based on regression formulas that incorporate variables such as. Other biometric variables, such as additional long bones and the transverse cerebellar diameter, also can play a role. Date changes for smaller discrepancies 10—14 days are appropriate based on how early in this second-trimester range the ultrasound examination was performed and on clinician assessment of LMP reliability.
Because of the risk of redating a small fetus that may be growth restricted, management decisions based on third-trimester ultrasonography alone are especially problematic; therefore, decisions need to be guided by careful consideration of the entire clinical picture and may require close surveillance, including repeat ultrasonography, to ensure appropriate interval growth.
The best available data support adjusting the EDD of a pregnancy if the first ultrasonography in the pregnancy is performed in the third trimester and suggests a discrepancy in gestational dating of more than 21 days. As soon as data from the LMP, the first accurate ultrasound examination, or both are obtained, the gestational age and the EDD should be determined, discussed with the patient, and documented clearly in the medical record.
Determination of gestational age by ultrasound.
For the purposes of research and surveillance, the best obstetric estimate, rather than estimates based on the LMP alone, should be used as the measure for gestational age. The American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal—Fetal Medicine recognize the advantages of a single dating paradigm being used within and between institutions that provide obstetric care. Table 1 provides guidelines for estimating the due date based on ultrasonography and the LMP in pregnancy, and provides single-point cutoffs and ranges based on available evidence and expert opinion.
No part of this publication may be reproduced, stored in a retrieval system, posted on the Internet, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission from the publisher. Methods for estimating the due date. American College of Obstetricians and Gynecologists. Women's Health Care Physicians. Recommendations The American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal—Fetal Medicine make the following recommendations regarding the method for estimating gestational age and due date: As soon as data from the last menstrual period LMP , the first accurate ultrasound examination, or both are obtained, the gestational age and the EDD should be determined, discussed with the patient, and documented clearly in the medical record.
Introduction An accurately assigned EDD early in prenatal care is among the most important results of evaluation and history taking.