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Most accurate dating ultrasound

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.

Wrong due date from ultrasound scan - why it happens | MadeForMums

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.

Can your pregnancy scan give the wrong due date?

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.

First trimester ultrasound dating

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.


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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.

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.

This is where you discover that your due date isn't based on the date you conceived, it's actually calculated from when your last menstrual period started your LMP. It's because it gives a definite date for health professionals to work with.

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There would be so many variables if it was calculated from when you think you conceived, especially as everyone's cycles are different and the fact that you don't necessarily conceive on the day you have sex! But this will only give you a rough idea. This is especially true if you have an irregular cycle or have been on the Pill for example.

Experts say that the scan date can sometimes come out as less pregnant than suggested by your LMP. But then again with my first baby I was spot on with dates, he always measured big and he was a week overdue Babies are ready when they're ready - try not to worry easier said than done. But Nigel also says that the estimated and that is all it is date is normally accurate plus or minus about a week.


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  5. 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. 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.

    How accurate is a due date that is determined by ultrasound?

    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.