If the gestational age and fetal development are to be estimated, measurements must be obtained and then compared with local standard values. Although there are many alternative measurements that can be made, only a few accurate and reliable.
Biparietal diameter (BPD) is one of the basic biometric parameters used to assess fetal size. BPD together with head circumference (HC), abdominal circumference (AC), and femur length (FL) are computed to produce an estimate of fetal weight.
Let's see them one by one:
Crown-rump length measurement (CRL)
The crown-rump length is the most reliable parameter for estimating gestational age up to the eleventh week. After that, the curvature of the fetus affects the reliability of the measurement. From the twelfth week onwards, the biparietal diameter is more accurate.
There is an excellent correlation between the crown-rump length and gestational age from the seventh to the eleventh week of pregnancy: biological variability is minimal and growth is not affected by pathological disorders.
Using scans in different directions, the longest length of the embryo should be found and a measurement made from the head (the cephalic pole) to the outer edge of the rump. The yolk sac should not be included.
Using scans in different directions, measure the fetus from head to buttock. Measure the longest length, ignoring any curvature.
Do not include the fetal limbs or the yolk sac in this measurement.
The gestational age can be determined from crown rump length to within approximately one week using biometric tables. Make sure that you use tables that are appropriate for your patients and not derived from some quite different population.
Biparietal Diameter
This is the most reliable method of estimating gestational age between the 12th and the 26th weeks. After that, its accuracy may be lessened by pathological disorders and biological variations that affect fetal growth. It must be considered together with other measurements, such as femoral length and abdominal circumference.
The biparietal diameter (BPD) is the distance between the parietal eminences on either side of the skull and is, therefore, the widest diameter of the skull from side to side. Using scans at different angles, the transverse section will be recognized when the shape of the fetal skull is ovoid and the midline echo from the falx cerebri is interrupted by the cavum septi pellucid and the thalami. When this plane is found, the gain on the ultrasound unit should be reduced and measurements made from the outer table of the proximal skull (the part nearest to the transducer) to the inner table of the distal skull (the part farthest away from the transducer). The soft tissues over the skull are not included. This is the "leading-edge-to-leading-edge" technique.
Be careful.
If your ultrasound unit has a programmed comparative scale relating normal growth to the biparietal diameter, check the manual. Some older units base the scale on measurements made from the outside of the skull: others use measurements from the inside of the skull.
Whichever method you use, make sure the data are appropriate for your patients and not derived from a quite different population.
Frontal-occipital Diameter
The fronto-occipital diameter along the longest axis of the skull at the level of the biparietal diameter (BPD), from outer edge to outer edge.
Cephalic index
The BPD is a reliable estimate of gestational age except when the shape of the head is abnormal or there is an abnormality of the intracranial contents (Fig. 207, pp. 262-263). The adequacy of the head shape is determined by comparing its short axis to its long axis-the cephalic index.
Cephalic index = Biparietal diameter "by" Fronto-occipital diameter × 100
Normal range (± 2 standard deviations) 70-86
Head Circumference
If the cephalic index is within the normal range, the BPD is acceptable as an estimate of gestational age. If the cephalic index is outside this range (less than 70 or greater than 86), the measured BPD should not be used to determine the gestational age. Instead, the head circumference can be used. On some ultrasound machines, this may be measured directly. It can also be calculated.
Head circumference = (Biparietal diameter + Fronto-occipital diameter) x 1.57
Abdominal Circumference
Abdominal circumference is used to detect intrauterine growth disturbances. The measurement must be taken at the level of the fetal liver, which is very sensitive to deficient nutrition. If the measurement is less than normal there has probably been intrauterine growth retardation.
It is most important that the scan shows a cross-section of the fetus that is as round as possible. Make sure that the correct level is being measured; look for the umbilical part of the left portal vein. The measurement must be made on a true transaxial plane, where the umbilical portion of the left portal vein enters and is entirely within the liver. The vein should be short, not elongated. If it is too long, the axis is too oblique.
When you have a scan at the correct level, measure the anteroposterior (AP) and transverse diameters. A medium gain setting should be used and the measurement must be from the outer edge of the fetal abdomen on one side to the outer edge on the other side. Calculate the abdominal circumference by multiplying the sum of the two measurements by 1.57.
Abdominal circumference = (Antero-posterior diameter + Transverse diameter) x 1.57
If the abdominal circumference is less than the fifth percentile, it is small. If it is greater than the 95th percentile, it is large. (With some ultrasound units it is possible to make this measurement automatically by tracing the perimeter of the abdomen.)
Fetal long Bone Measurements
When measuring bone length, it is necessary to reduce the gain. It is usually easy to see fetal long bones from 13 weeks onwards. Find a projection that shows a transverse section of one of the long bones: then scan at 90° to this to obtain a longitudinal section. Measurements are made from one end of the bone to the other end. The femur is the easiest bone to recognize and measure. If there is any doubt, also measure the limb on the other side.
The length of a bone, particularly the femoral length, can be used as a measure of gestational age when the head measurement is unreliable because of intracranial pathology. This occurs most frequently in the third trimester.
Bone length may also be compared with gestational age or biparietal diameter. A femoral or humeral measurement can be considered normal if it falls within two standard deviations of the mean for the known gestational age. It is proportional to the biparietal diameter if that measurement falls within two standard deviations of the mean biparietal diameter. A femur is short if it is more than two standard deviations below the mean. Skeletal dysplasia is likely only if the femur length is even smaller-5 mm smaller than two standard deviations below the mean.
There are limits to the accuracy of ultrasound:
• Clinical and laboratory findings must be included in the assessment.
• When there is doubt, serial measurements should be made to assess the rate of embryonic and fetal growth at intervals of at least 2 weeks, or even 3 weeks.
• Do not scan at weekly intervals. The changes will be too slight for accurate assessment.
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