First trimester dating ultrasound accuracy
The strong acoustic shadow behind the femoral or humeral shaft and the visualisation of both cartilaginous ends indicates that the image plane is on the longest axis and is the optimal measurement plane.
The calipers are placed along the diaphyseal shaft excluding the epiphysis.
BPD - OFD - HC | Femur and Humerus (FL HL) | Abdominal Circumference (AC) Fetal Weight Percentile Charts | Fetal Growth Percentile Charts Measurement Standards | Ultrasound Measurement Chart | Crown-Rump Length After the first trimester of pregnancy a multiparameter assessment of gestational age is advocated.
This should include at least two fetal measurements (usually biparietal diameter (BPD) and femur length (FL)) plus a consideration of additional parameters such as head circumference (HC), occipitofrontal diameter (OFD), abdominal circumference (AC) and humerus length (HL).
The abdominal circumference is measured at the level of the liver and stomach, including the left portal vein at the umbilical region.
If we assume a fetus is growing normally, biometric measurements are determined by gestational age, and we can estimate the gestational age and thus the due date.
When the date of the last menses preceding conception is known, it is usually a more accurate estimate ( /- 5 days) of EDCWhen using ultrasound to estimate gestational age, remember that we are assuming the fetus is growing normally.
Conditions which alter fetal growth will make the estimates less reliable.
Thus, a value that is above 50% indicates that the measurement is above average.
The reduced accuracy of prediction of gestational age after 20 weeks must be appreciated.
At any gestation, if the ultrasound fetal measurements of each parameter are not in agreement, the reason for this difference should be evaluated.
It is common to make at least 2 and often 4 or 5 measurements to estimate gestational age.
In most cases, BPD [Biparietal diameter], HC [Head circumference], AC [Abdominal circumference] and FL [Femur length] are used.