fetal ultrasound

Ultrasound scanning (USS) is a highly informative, harmless method of examination and allows for dynamic monitoring of the fetus. USS is performed if there is a suspicion of multiple pregnancy, polyhydramnios, ectopic and non-developing pregnancy, hydatidiform mole, fetal growth retardation syndrome and congenital malformations, as well as placental pathology (abnormal attachment, premature detachment and placental insufficiency). The most optimal periods for examination are the first trimester, 16-20 and 28-34 weeks of pregnancy. In case of complicated pregnancy, USS is performed at any time.

Monitoring of pregnancy development is possible from the earliest stages. In the 3rd week of pregnancy, a fertilized egg with a diameter of 5-6 mm is visualized in the uterine cavity. In the 4th-5th week, an embryo is detected as a linear echo-positive structure 6-7 mm long. The head of the embryo is identified from the 8th-9th week as a separate anatomical formation of a round shape and an average diameter of 10-11 mm. Embryo growth is uneven. The highest growth rates are noted at the end of the first trimester of pregnancy. The most accurate indicator of the gestational age in the first trimester is the crown-rump length.

The assessment of embryo vital activity in the early stages is based on the registration of its cardiac activity and motor activity. The use of the M-method allows registering the cardiac activity of the embryo from 4-5 weeks. The heart rate gradually increases from 150-160/min at 5-6 weeks to 175-185/min at 7-8 weeks, followed by a decrease to 150/min by the 12th week. Motor activity is detected from the 7-8th week. There are 3 types of movements: movements of the limbs, trunk and combined movements. The absence of cardiac activity and motor activity indicates the death of the embryo. Ultrasound examination in the first and second trimesters of pregnancy allows diagnosing non-viable pregnancy, anembryony, various stages of spontaneous miscarriage, hydatidiform mole, ectopic pregnancy, uterine developmental anomalies, multiple pregnancies. Ultrasound scanning has an undeniable advantage in pregnant women with uterine fibroids and pathological ovarian formations.

During the assessment of fetal development in the second and third trimesters of pregnancy, the main focus is on the following fetometric parameters: biparietal diameter of the head, average diameter of the chest and abdomen, and length of the femur. The biparietal diameter of the fetal head is determined with the best visualization of the M-structure from the outer surface of the upper contour of the parietal bone to the inner surface of the lower contour. The average diameter of the chest and abdomen is measured at the level of the fetal heart valves and at the point where the umbilical vein enters the abdominal cavity, respectively. To determine the length of the femur, the sensor must be shifted to the pelvic end of the fetus and, by changing the angle and plane of scanning, achieve the best image of the longitudinal section of the thigh. When measuring the thigh, the cursors are placed between its proximal and distal ends.

Ultrasound examination is one of the most accurate methods of diagnosing fetal growth retardation syndrome. Echographic diagnosis of the syndrome is based on a comparison of fetometric parameters obtained during the examination with standard parameters for a given gestational age. The optimal and at the same time reliable method of determining the expected fetal weight using ultrasound is a formula based on measuring the biparietal size of the head and the abdominal circumference of the fetus.

The capabilities of modern ultrasound equipment make it possible to assess the activity of various organs and systems of the fetus with a high degree of accuracy, as well as to prenatally diagnose most congenital malformations.

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Ultrasound placentography

Ultrasound placentography helps to establish the location of the placenta, its thickness and structure. The placenta is located mainly on the anterior or posterior surfaces of the uterine cavity with a transition to one of its lateral walls. In a smaller percentage of observations, the placenta is localized in the fundus of the uterus. The localization of the placenta at different stages of pregnancy is variable. It has been established that the frequency of low placentation before 20 weeks of pregnancy is 11%. Subsequently, as a rule, there is a "migration" of the placenta from the lower segment to the fundus of the uterus. Therefore, it is advisable to finally judge the location of the placenta only at the end of pregnancy.

In uncomplicated pregnancy, stage I of placental structure is detected mainly from 26 weeks of pregnancy, stage II - from 32 weeks, and stage III - from 36 weeks. The appearance of echographic signs of various stages of placental structure before the established time is regarded as premature "aging" of the placenta.

Determination of the biophysical profile of the fetus

Based on ultrasound data and fetal cardiac activity recording, many authors use the concept of “fetal biophysical profile”, which includes the analysis of 6 parameters: the results of the non-stress test (NST) during cardiotocography and 5 indicators determined during real-time scanning [fetal respiratory movements (FRM), motor activity (MA), fetal tone (T), amniotic fluid volume (AFV), degree of placental maturity (DPM).

The maximum score is 12 points. The high sensitivity and specificity of the fetal biophysical function test are explained by the combination of markers of acute (NST, respiratory movements, motor activity and fetal tone) and chronic (volume of amniotic fluid, degree of placental maturity) disorders of the intrauterine fetus. Reactive NST even without additional data indicates a favorable prognosis, while with non-reactive NST, the assessment of other biophysical parameters of the fetus is of primary importance.

Indications for determining the fetal BFP include the risk of developing placental insufficiency, intrauterine growth retardation, fetal hypoxia, and neonatal asphyxia. Pregnant women with OPG gestosis, those suffering from a long-term threat of termination of pregnancy, with diabetes mellitus, and hemolytic disease of the fetus are subject to examination. Evaluation of the fetal BFP can be used to predict infectious complications in premature rupture of membranes. Determination of the fetal BFP to obtain objective information is possible from the very beginning of the third trimester of pregnancy.

Doppler study of blood flow in the mother-placenta-fetus system. In obstetric practice, the most widespread is the qualitative analysis of blood flow velocity curves, the indicators of which do not depend on the vessel diameter and the value of the insonation angle. In this case, the main importance is given to the indicators that determine the ratio of blood flow velocities in different phases of the cardiac cycle - the systolic-diastolic ratio (SDR), the pulsation index (PI), the resistance index (RI):

SDO= MSK/KDSK, PI= (MSK-KDSK)/SSK, IR= (MSK-KDSK)/MSK,

Where MSV is the maximum systolic blood flow velocity, KDSV is the terminal diastolic blood flow velocity, and MV is the average blood flow velocity. An increase in vascular resistance, which is primarily manifested by a decrease in the diastolic blood flow component, leads to an increase in the numerical values of the above indices.

The use of modern high-resolution ultrasound equipment allows us to evaluate the blood flow in most fetal vessels (aorta, pulmonary trunk, inferior and superior vena cava, arterial duct, common, internal and external carotid arteries, anterior, middle and posterior cerebral arteries, renal arteries, hepatic and umbilical veins, as well as arteries of the upper limbs). Of greatest practical importance is the study of blood circulation in the uterine arteries and their branches (arcuate, radial), as well as in the umbilical artery. Analysis of blood flow in the fetal aorta with pathological blood flow velocity curves (BFC) in the umbilical artery makes it possible to assess the severity of disorders of fetal geodynamics.

The basis of the mechanism that ensures the constancy of uterine blood flow during pregnancy progression is a decrease in preplacental resistance to blood flow. This is achieved by the process of trophoblast invasion, which consists of degeneration of the muscular layer, hypertrophy of endothelial cells and fibrinoid necrosis of the terminal sections of the spiral arteries, which is usually completely completed by the 16th-18th week of pregnancy. The preservation of high resistance of the uterine arteries, caused by the disruption or absence of trophoblast invasion, is the leading morphological substrate for disorders of the uteroplacental circulation.

Normally, the CSC in the uterine arteries after 18-20 weeks of pregnancy are characterized by the presence of two-phase curves with a high diastolic blood flow velocity. During the second half of uncomplicated pregnancy, the numerical values of the indices reflecting the resistance of the vascular wall remain fairly stable with some decrease by the end of pregnancy. In uncomplicated pregnancy, the values of the SDO in the uterine arteries after 18-20 weeks do not exceed 2.4. Characteristic signs of pathological CSC in the uterine arteries are a decrease in the diastolic component of blood flow and the appearance of a dicrotic notch in the early diastole phase. At the same time, a reliable increase in the values of SDO, IR, PI is noted.

Normally, in the second half of uncomplicated pregnancy, a reliable decrease in vascular resistance indicators in the umbilical artery (UA) is observed, expressed in a decrease in the numerical values of SDO, IR, PI. Up to 14-15 weeks of pregnancy, diastolic blood flow is usually not visualized (with a frequency filter of 50 Hz), and after 15-16 weeks it is constantly recorded.

A decrease in the vascular resistance indices in the AP during the second and third trimesters of pregnancy indicates a decrease in the vascular resistance of the placenta, which is caused by the intensive growth of its terminal bed, due to the development and vascularization of the terminal villi of the placenta. In uncomplicated pregnancy, the SDO values in the AP do not exceed 3.0.

The newest method based on the Doppler effect is color Doppler mapping (CDM). The high resolution of the method facilitates visualization and identification of the smallest vessels of the microcirculatory bed. The use of CDM makes it possible to study blood flow in the branches of the uterine artery (up to the spiral arteries), terminal branches of the umbilical artery, and the intervillous space, which allows us to study the features of the formation and development of intraplacental hemodynamics and, thus, to promptly diagnose complications associated with the formation of placental insufficiency.

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Normal fetal parameters on ultrasound

The fetal spine is visualized as separate echo-positive formations corresponding to the vertebral bodies. It is possible to identify all sections of the spine, including the sacrum and coccyx.

When examining the fetal heart, a four-chamber section is used, obtained by strictly transverse scanning of the chest at the level of the cusp valves. In this case, the right and left ventricles, right and left atria, interventricular and interatrial septa, mitral and tricuspid valve cusps and the valve of the oval opening are visualized quite clearly. It should be noted that from the end of the second trimester and throughout the third trimester of pregnancy, a functional predominance of the right ventricle over the left is observed, which is associated with the peculiarities of intrauterine blood circulation.

Registration of fetal respiratory movements helps to determine their maturity (maturity of the respiratory muscles and the nervous system that regulates them). From 32-33 weeks, fetal respiratory movements become regular and occur at a frequency of 30-70 movements/min. Respiratory movements are simultaneous movements of the chest and abdominal walls. In complicated pregnancies, the number of respiratory movements increases to 100-150/min, or decreases to 10-15/min; in this case, individual convulsive movements are noted, which is a sign of chronic intrauterine hypoxia.

The use of echography allows for the precise identification of the stomach, kidneys, adrenal glands and urinary bladder of the fetus. In a normal pregnancy, urine production in the fetus is 20-25 ml/hour.

From 18-20 weeks of pregnancy, it is possible to determine the sex of the fetus. The reliability of determining the male sex approaches 100%, and the female sex is up to 96-98%. Identification of the female fetus is based on visualization of the labia in the form of two ridges in cross-section, and of the male fetus - by determining the scrotum with testicles and/or penis.