Challenges of the ultrasound examinations during pregnancy

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2021-1-HU01-KA220-HED-000027613 - COHRICE
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SYLLABUS

Challenges of the ultrasound examinations during pregnancy 

(Basic level)

Despite there is widespread of knowledge, books, and publications are available on the use of ultrasound in obstetrics, the current e-material is designed to summarize, outline the most important aspects of this diagnostic tool in daily clinical use, and lists the challenges ahead. There are several techniques aimed to visualize the developing fetus in utero. Among the currently available techniques the use of methods utilizing ionizing radiation are limited, due to their known mutagenic and teratogenic effects CT scan or X-rays should only be used after individual assessment considering the risk and the benefit for both the mother and the fetus. MRI scans provide the best quality images, and they overcome the limitations caused by dense bony structures, such as the skull and the rib cage, even not influenced by the fetal lie, or position, although motion artifacts originate from fetal movement can cause problems during image acquiring, the method itself is expensive, and there is few radiologists familiar with embryology and developmental anomalies.

Since the 1980s-1990s the gold standard of fetal imaging is the ultrasound (US). But how does it work?

It is based on the Piezzo-electric principle. The probe contains crystals, which under electrical current start to shake on top of each other, generating and emitting sound waves. The soundwaves are over the range what the human ear can detect, that’s why it is called ultrasound. Since air is a poor conductor of the ultrasound waves a coupling gel must be applied between the skin of the patient and the probe. Once the soundwaves penetrated the patient, they are either absorbed, or emitted. This is depending on the water content of the organs scanned, because water allows the soundwaves to pass through, while other organs reflect the US (echo). If reflection happens, the soundwaves undergo change in frequency, which returns to the probe and the machine pick it up and matches it with a color on a 2-dimensional greyscale image. High US reflecting organs (like the bones) are hyperechoic, and will appear white on the screen, in the contrary hypoechoic organs (amniotic fluid, urine, etc) will be seen as black. All other organs depending on their reflection abilities will appear on the shades of grey on the monitor.

How do you diagnose pregnancy?

Pregnancy is diagnosed with US scan, not earlier than 6. weeks (calculated from the last menstrual period-LMP), during which viable (heartbeat) in utero embryo need to be seen through a vaginal scan. If the LMP is not clear enough, the gestational age can be determined through the measurement of the crown-rump-length (CRL) of the fetus, which is the distance between the top of the head and the lowest point of the buttocks in millimeters.

What is the difference between screening and diagnostic US examinations?

There is a cardinal difference between these two examination types. Screening US examination of a population contributes the unselected examination those, who does not have any symptoms or complains. It is an examination without sorting, with a goal to differentiate pathological conditions from normal by screening the whole population. On the other hand, diagnostic examination is a process of detailed evaluation of a special population, who has abnormal blood test results, positive medical history, abnormal ultrasound findings, with a goal to confirm the presence of major and minor congenital anomalies. Although there are three planed US screenings (first trimester scan between week 11-13; second trimester scan between week 18-22, and third trimester scan between week 28-32) nowadays the most important scan is the first trimester scan. The following link contains the guideline of the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) according to who, how, and when should perform a first trimester US scan:

https://www.isuog.org/uploads/assets/uploaded/4daa1ea7-bc64-4c24-b81b17df5a684a38.pdf

What can be seen during the first trimester scan and should be seen is also discussed in detail on the website of the fetal medicine foundation:

www.fetalmedicine.org

How to assess the growth and development of the fetus in utero?

The assessment of the fetus can be achieved through the establishment of the biophysical profile, containing four measurements (biparietal diameter – BPD; head circumference – HC; abdominal circumference – AC; femoral length – FL). Comparing the measured values into a preset database (Headlock, etc.) the machine calculates a gestation age, which can be compared with the gestational age determined by the LMP. The estimated birth weight is distributed according to a Gauss curve. Fetuses under the 10-percentile range are considered small for gestational age (intra uterine growth restricted), while fetuses over the 90-percentile range are called large for gestational age (macrosomia). 

SGA can be, due to genetic predisposition (short parents), or developmental disturbance called intrauterine growth restricted (IUGR). There are two forms of IUGR, the early onset type (symmetrical) is caused by genetic anomalies, the other type is late onset form (asymmetric), often caused by placental insufficiency.

LGA is also multifactorial, but most of all maternal diabetes is the main reason for it. Fetal hyperinsulinemia, as a response to high maternal glucose levels which bypasses the placenta, triggers the attachment of insulin molecules into the insulin-like growth factor receptors. Macrosomia can result dystocia during labor therefore need to be diagnosed with US before delivery.

How can we determine the amniotic fluid?

Amniotic fluid is vital to the well-being of the fetus. It cushions the fetus from injury, helps prevent compression of the umbilical cord, and allows room for it to move and grow. In addition, its bacteriostatic action helps prevent infection of the intra-amniotic environment. The quantity of amniotic fluid at any time in gestation is the product of water exchange between the mother, fetus, and placenta, and is maintained within a relatively narrow range. Disorders of this regulatory process can lead to either polyhydramnios or oligohydramnios, in which too much or too little fluid exists, respectively. These disorders may result from abnormal fetal or maternal conditions and, conversely, may be responsible for alterations of fetal well-being as well. With the advent of real-time ultrasonography, assessment of amniotic fluid has been possible, resulting in earlier recognition of abnormal conditions and possible intervention. Because precise quantification of amniotic fluid volume is not possible with ultrasonography, various techniques for both qualitative and semiquantitative assessment have been proposed. The amount of the amniotic fluid can be objectively described by the determination of the amniotic fluid index (AFI). There are two techniques described in the lecture, how to measure the amount of the amniotic fluid with US.

How to assess the fetal circulation and the fetal wellbeing?

By definition, the Doppler effect refers to the change in wave frequency during the relative motion between a wave source and its observer, and it allows us to assess the blood flow inside the fetal vessels.

The fetal circulatory system uses 3 shunts normally. These are small passages that direct blood that needs to be oxygenated. The purpose of these shunts is to bypass the lungs and liver. That's because these organs will not work fully until after birth. The shunt that bypasses the lungs is called the foramen ovale. This shunt moves blood from the right atrium of the heart to the left atrium. The ductus arteriosus moves blood from the pulmonary artery to the aorta.

Oxygen and nutrients from the mother's blood are sent across the placenta to the fetus. The enriched blood flows through the umbilical cord to the liver and splits into 3 branches. The blood then reaches the inferior vena cava. This is a major vein connected to the heart. Most of this blood is sent through the ductus venosus. This is also a shunt that lets highly oxygenated blood bypass the liver to the inferior vena cava and then to the right atrium of the heart. A small amount of this blood goes straight to the liver to give it the oxygen and nutrients it needs. Waste products from the fetal blood are transferred back across the placenta to the mother's blood. (https://www.stanfordchildrens.org/en/topic/default?id=fetal-circulation-90-P01790)

During hypoxia, or fetal distress there is redistribution in the fetal circulation resulting centralized circulation, which is also called the brain-sparring effect. During this condition blood flow in favor of the brain, heart, and adrenals and at the expense of almost all peripheral organs, particularly of the lungs, carcass, skin and scalp. This leads to hypoxic vasodilatation in the fetal brain, which can be picked up by the assessment of the middle-cerebral artery peas systolic velocimetry, where we will find decreased resistance, and increased flow. The hypoxia also can be seen in the umbilical vessel flowmetry, where at the beginning end-diastoic stop, and later reverse flow can develop. The presence of absent end-diastolic flow (AEDF) usually occurs as a result of placental insufficiency. Flow in the umbilical artery should be in the forward direction in normal circumstances. If placental resistance increases, the diastolic flow may reduce, later becoming absent and finally reverses (https://radiopaedia.org/articles/absent-umbilical-arterial-end-diastolic-flow-2). Absent end diastolic flow velocity in the fetal umbilical artery or aorta has been associated with high mortality, increased risk of necrotising enterocolitis, and haemorrhage.

Conclusion

As technology develops the use of ultrasound becomes more and more essential for the assessment of fetal development and wellbeing. Such advances allow us to detect and diagnose conditions earlier, most preferably during the first trimester, unlike before.

Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the Foundation for the Development of the Education System. Neither the European Union nor entity providing the grant can be held responsible for them.