Determining fetal size and age

Determination of the size and age of the fetus (fetal biometry)

When determining the gestational age and age of the fetus, it is necessary to take a number of measurements and then compare the results with standard values. Although there are many different parameters that determine the age of the fetus, only a few of them are more accurate and most acceptable.

Parietococcygeal size (CTS)

The crown-rump length is the most accurate measurement for determining the gestational age up to 11 weeks. After 11 weeks, fetal curves reduce the accuracy of measurements. From the 12th week of pregnancy, the biparietal length of the fetal head is measured.

There are good correlations between the values of the crown-rump length and the gestational age from 7 to 11 weeks of pregnancy: the spread of normal values is minimal, pathological changes in the fetus do not affect the growth dynamics.

Using scanning in different planes, the greatest length of the embryo is determined, with measurements taken from the head (cranial pole) to the outer edge of the buttocks. The yolk sac is not included in the measurements.

Using sections in different planes, measure the fetus from the head to the buttocks. Measure the greatest length, ignoring the curves of the fetus.

Fetal limbs or yolk sac should not be included in measurements.

The gestational age can be determined by measuring the crown-rump length using biometric tables with an accuracy of one week. Make sure that you use biometric tables of the population to which the patient belongs, and not tables of a completely different population.

Biparietal size

Measuring the biparietal diameter is the most accurate method for determining the gestational age between 12 and 26 weeks. After 26 weeks, the accuracy of determining the gestational age may decrease due to biological variability and possible pathological changes affecting fetal growth. In this case, measuring the biparietal diameter should be combined with measuring the femur length and abdominal circumference.

The biparietal diameter (BPD) is the distance between the most prominent points of the parietal bones on both sides, which is thus the largest diameter of the fetal head from one lateral surface of the skull to the other. Using sections at different angles, it is necessary to obtain a cross-section of the head that has a clearly ovoid shape, with the median echo from the falx cerebri interrupted by the cavity of the septum pellucidum and the thalamus. When the required section is obtained, the sensitivity level of the device is reduced and a measurement is made between the outer contour of the cranium, the closest surface, and the inner contour, the surface of the fetal head that is farthest from the sensor. The soft tissues of the fetal head are not included in the measurements. This technique is described as a "protruding-to-protruding-edge" measurement.

Be careful. If your ultrasound machine has a program in its software that calculates gestational age using the biparietal diameter, check your manual. Some older models calculate the biparietal diameter using either the outer contour of the skull or only the inner contour.

Regardless of which method you use, make sure that the measurements are specific to your patient and not specific to a completely different population of pregnant women.

Fronto-occipital diameter

The fronto-occipital diameter is measured along the largest long axis of the head at the level of the biparietal diameter (BPD) measurement, from the outer contour to the outer contour of the skull.

Head index

Basically, the BPD measurement is used to determine the gestational age, except in cases where there is a skull deformation or pathology of the internal structure of the head. The adequacy of the head shape is determined by the cephalic index - the ratio of the size along the short axis to the size along the long axis.

Cephalic index = Biparietal diameter / Fronto-occipital diameter x 100

Normal index values (± 2 standard deviations) = 70-86.

Head circumference

With normal values of the cephalic index, BPD can be used to determine gestational age. If the cephalic index has values less than 70 or more than 86, BPD measurement cannot be used to determine the gestational age. Instead, head circumference measurement is used. On some devices, head circumference is calculated automatically. You can also calculate the circumference using a formula.

Head circumference = (biparietal diameter + fronto-occipital diameter) x 1.57.

Abdominal circumference

Measuring the fetal abdominal circumference is used to detect intrauterine growth retardation. The measurement must be taken at the level of the fetal liver, which is very sensitive to trophic disorders. If the measurement values are less than the required values, intrauterine growth retardation is highly likely.

It is very important that the cut is as rounded as possible. Make sure that the cut is made at the proper level: find the umbilical section of the left branch of the portal vein. Measurements should be made in a plane located strictly transverse to the long axis of the body, at the level of entry of the left branch of the portal vein, which should be located entirely in the liver parenchyma. The vein in the section should be short, should not have an elongated, oblong shape. If the vein is too long, the cutting axes are oblique.

Once the desired cut is obtained at the proper level, measure the anteroposterior (AP) and transverse diameters. The device sensitivity level is set to medium, and the measurement should be taken from the outer contour of the fetal abdomen on one side to the outer contour of the abdomen on the other side. Calculate the fetal abdominal circumference by multiplying the sum of the two measurements by 1.57.

Abdominal circumference = (anteroposterior diameter + transverse diameter) x 1.57.

If the abdominal circumference is less than the 5th percentile, the abdomen is considered small. If the abdominal circumference is more than the 95th percentile, the abdomen is considered enlarged. (Some ultrasound machines can automatically calculate the abdominal circumference by tracing the perimeter of the abdomen.)

Measuring fetal long bones

When measuring bone length, it is necessary to reduce the level of general sensitivity. Usually, the long bones of the fetus are clearly visualized starting from 13 weeks of pregnancy. Find a projection in which you can obtain a cross-section of one of the long bones; then rotate the transducer 90° to obtain a section of the bone along its length. Measurements are taken from one end of the bone to the other. The femur is the bone most accessible for visualization and measurement. If in doubt, measure the length of the other femur.

Bone length, particularly femur length, can be used to estimate gestational age, particularly in cases where head measurements are unavailable due to intracranial pathology. This is most common in the third trimester of pregnancy.

Bone length can be compared with gestational age or biparietal diameter. Femur or humerus length values are considered normal if they fall within two standard deviations of the mean for a given gestational age. These values are proportional to the biparietal diameter if the biparietal diameter value falls within two standard deviations of the mean for a given gestational age. The femur is considered short if its length value is less than the mean by more than two standard deviations. Skeletal dysplasia is highly likely if the femur length is less than two standard deviations from the mean by only 5 mm.

There is a limit to the accuracy of the ultrasonic method:

• Clinical and laboratory tests must be taken into account.
• If there are any doubts, it is necessary to carry out dynamic measurements at intervals of 2-3 weeks.
• There is no need to repeat the tests weekly.
• The changes may be too small to register.

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