Are second and third trimester ultrasound dating and measurements accurate?

Ultrasonic studies have proven useful in determination of gestational age (GA) in first and second trimester, but their accuracy in third trimester is not reliable because of biologic variations like racial differences in fetal biometric measurements and inter-population variations [2] . radiologykey.comИзображение:radiologykey.comUnfortunately, ultrasound technology, like most technologies, isn’t perfect, especially when it comes second and third trimester dating and measurements. In terms of dating, the ultrasound’s accuracy depends on the trimester, explains Kara Manglani, CNM, a nurse-midwife practicing in New York City and blogging at The Fertile Times. Although ultrasound dating in pregnancy is accurate in the second trimester, it is less precise than in the trimesterfirst of pregnancy, when pregnancy is datedby crown -rump length measurement.Ultrasonic studies have proven useful in determination of gestational age (GA) in first and second trimester, but their accuracy in third trimester is not reliable because of biologic variations like racial differences in fetal biometric measurements and inter-population variations [ 2] .Sonographic dating is based on measurement of fetal parameters. Standardized charts correlate GA with measurements of fetal parameters. Serial measurements are used to document fetal growth. In the second and third trimesters, four fetal measurements are routinely used.The main objective of a trimester ultrasound second examination is to accurately date the pregnancy, evaluate fetal anatomy, and assess placental location and the adnexae.



How accurate are ultrasound measurements in third trimester?

Clinical Considerations in the Third Trimester

Gestational age assessment by ultrasonography in the third trimester (28 0/7 weeks of gestation and beyond) is the least reliable method, with an accuracy of ± 21–30 days 19 20 24.

Why is third trimester ultrasound inaccurate?

The estimated weight that is calculated is just an estimate based upon the size (or mass) of the baby. But since those extra factors (bone density, etc.) cannot be evaluated with ultrasound, the TRUE weight of the baby cannot be measured with 100% accuracy.

How accurate are ultrasound measurements in pregnancy?

In accordance with numerous other studies, our results confirmed that the ultrasound-derived EFW during labor at term is an appropriate diagnostic tool, with an average accuracy of 70% within a relative difference of ±10% to the real birth weight [9,10,11,12,13].

How accurate are ultrasound measurements at 36 weeks?

It’s important to know there is a recognised 15% error in ultrasound fetal weight estimation. This means your baby can be either 15% smaller or 15% larger than the estimated weight given at your ultrasound. Ultrasound scans are generally more accurate for assessing your baby’s size during the first half of pregnancy.

How accurate are ultrasound measurements for due date?

Determining a Due Date

Evidence suggests that ultrasounds more accurately predict your due date than using your last menstrual period—but only in the first trimester and early second trimester (until roughly 20 weeks). 3 Early ultrasound due dates have a margin of error of roughly 1.2 weeks.

How accurate are ultrasound measurements at 38 weeks?

Scans done at 40 weeks and beyond were more accurate than those done preterm. Only 81% of the scans done between 34 and 37 weeks gestation fell within the accepted 15% margin of error between EFW and actual weight compared to 100% when the scan was done after 40 weeks.

How accurate is femur length in third trimester?

Third Trimester:

After 26 weeks, all measurement are less accurate. In the third trimester, the variation in both biparietal diameter and femur measurements is +/- 14 to 24 days.

Do you subtract 2 weeks from ultrasound?

The ultrasound technician measures the “crown to rump” (head to buttocks) length of the embryo/fetus and will give you the gestational age. To translate into fetal age and estimate when you conceived, take the gestational age and subtract two weeks.

Is ultrasound dating in pregnancy accurate in the second trimester?

Although ultrasound dating in pregnancy is accurate in the second trimester, it is less precise than in the trimesterfirst of pregnancy, when pregnancy is datedby crown -rump length measurement.

How accurate are ultrasonic tests in determining gestational age in third trimester?

Ultrasonic studies have proven useful in determination of gestational age (GA) in first and second trimester, but their accuracy in third trimester is not reliable because of biologic variations like racial differences in fetal biometric measurements and inter-population variations [ 2] .

How many fetal measurements are used in the second and third trimester?

In the second and third trimesters, four fetal measurements are routinely used. The BPD measurement is greatly affected by shape of the fetal head. The BPD is determined on an axial image of the fetal head at the level of the thalamus ( Fig. 7.1 ).

What is a trimester ultrasound second examination?

The main objective of a trimester ultrasound second examination is to accurately date the pregnancy, evaluate fetal anatomy, and assess placental location and the adnexae.

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