**2. Maternal-fetal circulation**

The primary heart and vascular system appear in the middle of the third week of development. On about 22nd-23rd day, the heart begins its systolic action.

Oxygenated blood, rich in nutrients, flows through the umbilical vein from the placenta to the portal sinus. The portal sinus is a wide L-shaped vessel at the terminal end of the umbilical vein, connecting two main vessels termed the right and left intrahepatic portal veins perfusing the right and left hepatic lobes. It then goes into the ductus venosus. The ductus venosus originated from the portal sinus as the latter turned at an almost right angle into the right lobe of the liver. The ductus venosus is a branchless, hourglass-shaped vessel that ascends steeply in the direction of the diaphragm. The blood flow in the ductus venosus is regulated by the sphincter mechanism. The blood then flows in the inferior vena cava and enters the right atrium of the heart. Most of the blood from the inferior vena cava is directed to the secondary septum through the oval foramen to the left atrium. There it mixes with a relatively small amount of poorly oxygenated blood returning through the pulmonary veins from the lungs. The blood from the left atrium flows into the left ventricle and leaves it through the ascending aorta. The arteries that supply the heart, head, neck and upper limbs receive well-oxygenated blood. The small amount of well-oxygenated blood from the inferior vena cava that remains in the right atrium of the heart mixes with the poorly oxygenated blood from the superior vena cava and coronary sinus and flows into the right ventricle. This blood leaves the right heart through the pulmonary trunk. Due to the high pulmonary vascular resistance during fetal life, blood flow through the lungs is low. About 10% of the blood flows to the lungs and most of it flows through the arterial duct to the fetal aorta. The blood returns to the placenta through the umbilical arteries [3, 4].

#### **3. Vascular flow testing using Doppler ultrasonography**

Doppler examination assessing the vascular flow of the maternal-fetal circulation is an important diagnostic tool in the assessment of the well-being of the fetus. The analysis of vascular flows is also used to make decisions about the further duration of pregnancy. It is often a pregnancy complicated by diseases that threaten the life of the mother and the fetus. Due to the high risk of iatrogenic prematurity, the experience of the person performing the ultrasound examination is extremely important, taking into account the factors that may affect the parameters of the vascular flow wave. Overinterpretation of the Doppler results may expose both parents and the perinatological team to unnecessary stress, medical activities and costs [5, 6].

The safety of ultrasound examinations is based on the degree of fetal exposure, which depends on the amount and duration of ultrasound examinations and the energy used for the examination. It takes into account the control of the thermal and mechanical index and the superior principle of using the lowest dose of energy that allows correct imaging - ALARA (as low as reasonably achievable). The term thermal index describes the quotient of the power lost to the reference power that increases the tissue temperature by 1 °C. The mechanical index describes the amplitude of the ultrasound wave. Ensuring the safety of ultrasound examination is only possible through excellent knowledge of anatomy and embryology, as well as regular index control when changing the settings of the ultrasound machine.

The table contains the most frequently assessed vascular flows along with their correct imaging.


*Haemodynamic Changes during Preterm Birth Treatment DOI: http://dx.doi.org/10.5772/intechopen.96923*

The special structure of the fetal uteroplacental, umbilical and cerebral circulation ensures a constant vascular flow in the fetus, independent of the mother's heart cycle. This system gradually develops in the utero-fetal circulation. The significant effect of this phenomenon consists not only in the gradual increase in the end-diastolic velocity of the flow wave, but also in the accompanying decrease in pulsation, which is the difference between the components of the maximum systolic and end-diastolic velocity.

The correct shape of the flow wave of both the fetal middle cerebral artery and the umbilical cord artery is not characterized by the disappearance of the flow wave or its inversion. It is one of the disparities in the fetal circulatory system that does not give a compensatory break in the heart's work. If it occurs, it is called absent flow or reverse flow. Both of these phenomena are among the alarm signals of poor fetal condition [7].

#### **3.1 Mistakes in Doppler examination**

Due to incomplete bone calcification, the head of a premature fetus is susceptible to pressure. Excessive pressure with the transducer may indicate the disappearance of the end-diastolic wave in the assessed fetal middle cerebral artery [7, 8].

The assessment of the vascular flow spectrum should cover several fetal heartbeat cycles. One of the reasons for doing this are the breathing movements of the fetus, which can disrupt the normal flow spectrum. Similar phenomena can be observed when a pregnant woman breathes too deeply. In order to verify the correctness of the vascular flow, a patient should be asked to shorten her breath or even temporarily stop breathing.

Incorrect parameters of the ultrasound device settings may falsify the Doppler measurements.

Corticosteroids administered to the mother to stimulate the maturation of the fetal lungs in the event of impending preterm labor may temporarily "improve" the flow waves. In that case, it is reasonable to repeat the Doppler examination approximately 48 hours after administering the medications [7, 9].
