# Dating scan first trimester

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A scan usually takes around 20-30 minutes. 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. Conclusion Accurate dating of pregnancy is important to improve outcomes and is a research and public health imperative.\
&#x20;![dating scan first trimester](https://image.slidesharecdn.com/1sttrimesterscan-140118084205-phpapp01/95/1st-trimester-scan-14-638.jpg?cb=1390035742)\\

The CRL is very accurate in the first trimester, but after 13 weeks your solo can curl up and stretch out, so measuring the length becomes more difficult. Ultrasound Obstet Gynecol 2005;26:504—11. This scan checks for structural abnormalities anomalies in the baby. J Womens Health Larchmt 2005;14:248—52. It has been reported that approximately one half of women accurately recall their LMP. Sol 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.

The sonographer estimates when your baby is due the estimated date of delivery, or EDD based on the baby's measurements. First- and second-trimester ultrasound assessment of gestational age.\
&#x20;![dating scan first trimester](http://resources.ama.uk.com/glowm_www/uploads/1487849033_fig10B.jpg)\\

**Ultrasound scans in pregnancy** - Because decisions to change the EDD significantly affect pregnancy management, their implications should be discussed with patients and recorded in the medical record.\
&#x20;![dating scan first trimester](http://posterng.netkey.at/esr/viewing/index.php?module=viewimage\&task=\&mediafile_id=494850&201301291913.gif)\\

Pettker, MD; James D. Goldberg, MD; and Yasser Y. Copel, MD; and the Society for Maternal—Fetal Medicine. This document reflects emerging clinical and scientific advances as of the date issued and is subject to change. The information should not be construed as dictating an exclusive course of treatment or procedure to be followed. Methods for Estimating the Due Date ABSTRACT: Accurate dating of pregnancy is important to improve outcomes and is a research and public health imperative. As soon as data from the last menstrual period, the first accurate ultrasound examination, or both are obtained, the gestational age and the estimated due date EDD should be determined, discussed with the patient, and documented clearly in the medical record. Subsequent changes to the EDD should be reserved for rare circumstances, discussed with the patient, and documented clearly in the medical record. When determined from the methods outlined in this document for estimating the due date, gestational age at delivery represents the best obstetric estimate for the purpose of clinical care and should be recorded on the birth certificate. 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. 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. Subsequent changes to the EDD should be reserved for rare circumstances, 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. Introduction 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. Appropriately performed obstetric ultrasonography has been shown to accurately determine fetal gestational age. 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. Background Traditionally, determining the first day of the LMP is the first step in establishing the EDD. By convention, the EDD is 280 days after the first day of the LMP. 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. In one study, 40% of the women randomized to receive first-trimester ultrasonography had their EDD adjusted because of a discrepancy of more than 5 days between ultrasound dating and LMP dating. Estimated due dates were adjusted in only 10% of the women in the control group who had ultrasonography in the second trimester, which suggests that first-trimester ultrasound examination can improve the accuracy of the EDD, even when the first day of the LMP is known. 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. 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,. 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, 11. 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. If pregnancy resulted from ART, the ART-derived gestational age should be used to assign the EDD. 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 261 days from the embryo replacement date. Likewise, the EDD for a day-3 embryo would be 263 days from the embryo replacement date. Clinical Considerations in the Second Trimester 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. 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. Conclusion Accurate dating of pregnancy is important to improve outcomes and is a research and public health imperative. 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. Subsequent changes to the EDD should be reserved for rare circumstances, discussed with the patient, and documented clearly in the medical record. When determined from the methods outlined in this document for estimating the due date, gestational age at delivery represents the best obstetric estimate for the purpose of clinical care and should be recorded on the birth certificate. 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. Fetal imaging: executive summary of a joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal—Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging workshop. Fetal Imaging Workshop Invited Participants. A comparison of recalled date of last menstrual period with prospectively recorded dates. J Womens Health Larchmt 2005;14:248—52. Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. Am J Obstet Gynecol 2002;187:1660—6. Last menstrual period versus ultrasound for pregnancy dating. Int J Gynaecol Obstet 2004;87:38—9. First trimester ultrasound screening is effective in reducing postterm labor induction rates: a randomized controlled trial. Am J Obstet Gynecol 2004;190:1077—81. Ultrasound for fetal assessment in early pregnancy. Cochrane Database of Systematic Reviews 2015, Issue 7. Predicting delivery date by ultrasound and last menstrual period in early gestation. New charts for ultrasound dating of pregnancy and assessment of fetal growth: longitudinal data from a population-based cohort study. Ultrasound Obstet Gynecol 2008;31:388—96. First- and second-trimester ultrasound assessment of gestational age. Am J Obstet Gynecol 2004;191:975—8. First- vs second-trimester ultrasound: the effect on pregnancy dating and perinatal outcomes. Am J Obstet Gynecol 2008;198:703. Br J Obstet Gynaecol 1975;82:702—10. Fetal crown—rump length: reevaluation of relation to menstrual age 5—18 weeks with high-resolution real-time US. Gestational age in pregnancies conceived after in vitro fertilization: a comparison between age assessed from oocyte retrieval, crown—rump length and biparietal diameter. Ultrasound Obstet Gynecol 2000;15:41—6. Ultrasound dating at 12—14 weeks of gestation. A prospective cross-validation of established dating formulae in in-vitro fertilized pregnancies. Ultrasound Obstet Gynecol 2005;26:504—11. Accuracy of gestational age estimation by means of fetal crown—rump length measurement. Am J Obstet Gynecol 1993;168:903—8. Estimation of gestational age by transvaginal sonographic measurement of greatest embryonic length in dated human embryos. Ultrasound Obstet Gynecol 1994;4:457—62. Underestimation of gestational age by conventional crown—rump length dating curves. New crown—rump length curve based on over 3500 pregnancies. Ultrasound Obstet Gynecol 2010;35:650—5. Standardization of sonar cephalometry and gestational age. Sonographic prediction of gestational age: accuracy of second- and third-trimester fetal measurements. AJR Am J Roentgenol 1991;157:1275—7. Conceptional age, menstrual age, and ultrasound age: a second-trimester comparison of pregnancies of known conception date with pregnancies dated from the last menstrual period. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. American College of Obstetricians and Gynecologists. Computer assisted analysis of fetal age in the third trimester using multiple fetal growth parameters. J Clin Ultrasound 1983;11:313—6. 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. Requests for authorization to make photocopies should be directed to Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, 978 750-8400. ISSN 1074-861X The American College of Obstetricians and Gynecologists 409 12th Street, SW, PO Box 96920, Washington, DC 20090-6920 Methods for estimating the due date. American College of Obstetricians and Gynecologists.
