Determination of the size and age of the fetus

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

When determining the timing of pregnancy and the age of the fetus, a number of measurements must be taken and then the results compared with the 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.

Coccygeal-parietal size (CT)

Coccygeal-parietal size is the most accurate for determining the duration of pregnancy up to 11 weeks. After 11 weeks, fetal bends reduce the accuracy of measurements. From the 12th week of pregnancy, the biparietal size of the fetal head is measured.

There are good correlations between the values of the coccygeal and parietal size and the duration of pregnancy from 7 to 11 weeks of gestation: the spread of normal values is minimal, the pathological changes of the fetus do not affect the dynamics of growth.

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

Using slices in different planes, measure the fruit from the head to the buttocks. Measure the longest length, not paying attention to the bends of the fruit.

Do not include in the measurement of the fetal limb or yolk sac.

The gestation period can be determined by measuring the coccygeal-parietal size by biometric tables with an accuracy of one week. Make sure that you use the biometric tables of the particular population to which the particular patient belongs, and not the tables of a completely different population.

Biparietal size

Measurement of biparietal size is the most accurate method of determining the gestation period between 12 and 26 weeks. After 26 weeks, the accuracy of determining the gestation period may decrease due to biological variability and possible pathological changes affecting fetal growth. Measurement of biparietal size in this case should be combined with measurements of femur length and abdominal circumference.

Biparietal size (BDP) 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 side of the skull to the other. Using sections at different angles, it is necessary to obtain a transverse section of the head that is clearly ovoid in shape, while the middle echo from the cerebral crescent is interrupted by the cavity of the transparent septum and the visual cusps. When the desired cut is obtained, the sensitivity level of the instrument is reduced and a measurement is made between the outer contour of the skull, the closest surface, and the inner contour that is furthest from the head surface sensor. Soft fetal head tissues are not included in the measurements. This technique is described as a measurement "from the protruding edge to the protruding edge".

Be careful. If your ultrasound software has a biparietal diameter pregnancy test program, check your manual. On some older models of apparatus biparietal diameter is calculated either by the outer contour of the skull, or only by the internal one.

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

Frontal-occipital diameter

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

Head index

Basically, the BDP measurement is used to determine the timing of pregnancy, except when there is deformation of the skull or pathology of the internal structure of the head. The adequacy of the shape of the head is determined by the head index - by the ratio of the size along the short axis to the size along the long axis.

Head index = Biparietal size / Frontal-occipital diameter x 100

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

Head circumference

With normal values of the head index, BDP can be used to determine the gestational age. If the head index is less than 70 or more than 86, the BDP measurement can not be used to determine the gestational age. Instead, the head circumference measurement is used. On some devices, the head circumference is calculated automatically. You can also calculate the circle by the formula.

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

Circumference of the abdomen

Measurement of the circumference of the fetal abdomen is used to detect intrauterine growth retardation of the fetus. Measurement should be performed at the level of the fetal liver, which is very sensitive to trophic disturbances. If the measurement values are less than the prescribed values, the intrauterine growth retardation is very likely.

It is very important that the cut is as round 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 transversely 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 parenchyma of the liver. The vein in the section should be short, should not have an elongated, oblong form. If the vein is too long, the shear axes are oblique.

When obtaining the desired cut at the proper level, measure the anteroposterior (PP) and transverse diameter. The average sensitivity level of the device is set, and the measurement should be made from the outer contour of the fetal abdomen from one side to the outer contour of the abdomen on the other side. Count the circumference of the fetal abdomen, 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. Then the stomach is considered small. If the abdominal circumference is over the 95th percentile, it is considered enlarged. (On some ultrasound devices, it is possible to automatically calculate the length of the abdominal circumference when circling the abdominal contour around the perimeter.)

Measuring long bones of the fetus

When measuring bone length, it is necessary to reduce the level of overall sensitivity. Usually the long bones of the fetus are clearly visualized, starting from the 13th week of pregnancy. Find a projection in which you can obtain a cross-section of one of the long bones; then rotate the sensor 90 ° to obtain a cross section of the bone along the length. Measurements are made from one end of the bone to the other. The thigh is the most accessible for visualization and measurement by a bone. If there is any doubt, measure the length of the second femur.

The length of the bone, especially the length of the thigh, can be used to determine the duration of pregnancy, especially in cases where head measurements are not available due to the presence of intracranial pathology. This is most often the case in the third trimester of pregnancy.

The length of the bone can be compared with the gestational age or biparietal diameter. The lengths of the hip or shoulder length are considered normal if they fall within the range of two standard deviations of the mean value established for this period of pregnancy. These values are proportional to the biparietal diameter, if the value of the biparietal diameter falls within the range of two standard deviations of the mean value established for this period of pregnancy. The thigh is considered shortened if the value of its length is less than the average value by more than two standard deviations. Very likely the presence of skeletal dysplasia, if the length of the femur is less than the value of two standard deviations from the average value of only 5 mm.

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

• Clinical and laboratory tests should be taken into account.
• If there are doubts, it is necessary to conduct dynamic measurements at intervals of 2-3 weeks.
• Do not repeat the research weekly.
• Changes may be too small to register.

[1], [2], [3], [4]