First trimester dating ultrasound accuracy
Although many embryonic and fetal structures can be measured, only a few measurements are easy and repeatable enough for widespread use.
Gestational Sac: The first element to be measurable is the gestation sac of the early pregnancy.
These measurements are more appropriately used in the assessment of fetal growth, particularly in the second half of the pregnancy, than in the assessment of gestational age.
It is, however, an appropriate measurement in the second trimester to demonstrate normal fetal proportions.
The abdominal circumference is measured at the level of the liver and stomach, including the left portal vein at the umbilical region.
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).Percentile Example: Out of 100 babies, a reading of forty percent (this is the percentile value) indicates that the baby is smaller than sixty other babies and larger than forty other babies. A value that reads below 50% indicates that the measurement is lower than the average.Thus, a value that is above 50% indicates that the measurement is above average.The BPD and OFD are measured on a transverse axial section of the fetal head which includes the falx cerebri anteriorly and posteriorly, the cavum septum pellucidum anteriorly in the midline and the thalami.
The BPD is measured from the outer edge of the nearer parietal bone to the inner edge of the more distant parietal bone. The long bones are measured with the bone across the beam axis.
It is common to make at least 2 and often 4 or 5 measurements to estimate gestational age.