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.
Can your pregnancy scan give the wrong due date?
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.
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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.
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. Ultrasonography dating in the second trimester typically is based on regression formulas that incorporate variables such as the biparietal diameter and head circumference measured in transverse section of the head at the level of the thalami and cavum septi pellucidi; the cerebellar hemispheres should not be visible in this scanning plane the femur length measured with full length of the bone perpendicular to the ultrasound beam, excluding the distal femoral epiphysis the abdominal circumference measured in symmetrical, transverse round section at the skin line, with visualization of the vertebrae and in a plane with visualization of the stomach, umbilical vein, and portal sinus 8 Other biometric variables, such as additional long bones and the transverse cerebellar diameter, also can play a role.
Conclusion Accurate dating of pregnancy is important to improve outcomes and is a research and public health imperative. Fetal Imaging Workshop Invited Participants. A comparison of recalled date of last menstrual period with prospectively recorded dates. J Womens Health Larchmt ; Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination.
Am J Obstet Gynecol ; Last menstrual period versus ultrasound for pregnancy dating. Int J Gynaecol Obstet ; The healthcare professionals will stick to this date as far as possible because it is then used as the baseline for growth throughout pregnancy. Your baby's growth can slow down and then have spurts.
Predicting delivery date by ultrasound and last menstrual period in early gestation.
I've not even had my 20 week scan yet. I have scans every few weeks so it will probably keep on changing.
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My baby will come when it's ready anyway, maybe weeks before or maybe weeks after my due date. These can be carried out from week 5 or 6 of pregnancy, which is a notoriously tricky time to be accurate on dates. Measurements at such an early stage aren't accurate which is why they wait for 12 weeks to date you.
Expert Jan Steward, who is director of Ultrasound Direct and Babybond, agrees that early scans carry a much greater risk of inaccuracy. We always recommend mums get a definitive dating scan closer to 12 weeks as this will be much more accurate.
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Just half a millimetre at such an early stage can change your date by a week. Your week scan will be far more accurate. The sonographer then said that I was only about 4. Then at the week scan I got an EDD of 15 march, so that early scan was completely out.
What Is An Ultrasound?
Well, you'd think so, wouldn't you. At around 12 weeks you can get more accurate measurements. By talking to the health care professionals you should get a clearer picture as to what is happening. Thanks so much for your post — we always welcome feedback, positive or negative, as we always aim to make our articles as accurate and as helpful as possible.
How an Ultrasound Can Estimate Your Conception Date
We spent a lot of time talking to sonographers before writing this article, so we're surprised that you think what they're saying is not in line with what you have studied. When deliveries before 37 weeks were excluded, crown-rump length measurement of mm corresponding to After that time, BPD at least 21 mm showed a similar error 7. Femur length was slightly less accurate than crown-rump length or BPD. Regression models using a combination of any two or three ultrasonic variables did not improve accuracy of prediction.