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.
- Methods for Estimating the Due Date - ACOG.
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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.
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. Crown-rump length CRL is the measurement of the length of human embryos and fetuses from the top of the head crown to the bottom of the buttocks rump. It is typically determined from ultrasound imagery and can be used to estimate gestational age. The embryo and fetus float in the amniotic fluid inside the uterus of the mother usually in a curved posture resembling the letter C.source link
The measurement can actually vary slightly if the fetus is temporarily stretching straightening its body. The measurement needs to be in the natural state with an unstretched body which is actually C shaped. The measurement of CRL is useful in determining the gestational age menstrual age starting from the first day of the last menstrual period and thus the expected date of delivery EDD.
Different babies do grow at different rates and thus the gestational age is an approximation. Recent evidence has indicated that CRL growth and thus the approximation of gestational age may be influenced by maternal factors such as age, smoking , and folic acid intake. In that situation, other parameters can be used in addition to CRL. The length of the umbilical cord is approximately equal to the CRL throughout pregnancy. 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.
Assessment of Gestational Age by Ultrasound | GLOWM
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.
- Methods for Estimating the Due Date;
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- Crown-rump length - Wikipedia.
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Assessment of Gestational Age by Ultrasound
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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:
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