Despite the fact that hypoxic damage to fetal brain cells, which occurs when fetal adaptive reserves to hypoxia are exhausted, is a direct cause of fetal biological death during pregnancy and labor, keeping the fetus alive in hypoxic conditions is still an unsolved problem of obstetrics and gynecology [1, 2, 3, 4, 5, 6, 7, 8]. Unfortunately, diagnostic, preventive, and treatment standards still do not rule out the development of fetal asphyxia, postpartum encephalopathy, or even fetal death from hypoxic brain damage during pregnancy and/or childbirth [9, 10, 11, 12, 13]. Therefore, there is an urgent need to develop an innovative solution that will allow timely diagnosis of intrauterine hypoxia, assess the resistance of the fetus to hypoxia and, if necessary, decide on the use of oxygen, or on an immediate caesarean section in order to save the life of the fetus and its mother [14, 15].

In our opinion, this solution can be found by analyzing not only scientific articles, but also relevant inventions. Moreover, the key to solving this problem can most likely be found in inventions, since absolute world novelty is more inherent in the content of inventions than in scientific articles. That is why it makes sense to pay special attention of researchers to the essence of inventions developed in the beginning of the 21st century. There is every reason to believe that inventions designed to assess fetal resistance to hypoxia and increase the fetal adaptation reserves to hypoxia may be particularly promising among them [7, 16, 17].

The study was conducted on all databases for the period from the beginning of the 21st century to 26.08.2023. In addition, we studied the references and conducted a citation search. A systematic review was conducted in accordance with the quality standards described in the AMSTAR measurement tool and the PRISMA 2009 checklist [18, 19], and also the Cochrane Handbook for Systematic Reviews of Interventions [20]. The protocol of this investigation was not retrospectively recorded in the International Prospective Register of Systematic Reviews (PROSPERO). Additionally, ethical approval was not warranted as this investigation used only published literature (articles and inventions).

The methodology of the study was built on the search and analysis of existing inventions and articles. Inventions were searched in electronic databases, such as: EAPATIS, BYPATENTS, DWPI, DEPATISnet, PATENTSCOPE, Espacenet, RUPTO, USPTO, CIPO, CNIPA, KIPRIS, PatSearch, J-PlatPat, Google Patents and TPO. Articles were searched in electronic databases, such as: Google, SCOPUS, PudMed,Crossref, Web of Science and E-Library. The data obtained were carefully analyzed in accordance with all quality requirements. The search itself was conducted by such keywords as: "pregnancy", "woman", "normal", "pathological", "uterus", "placenta", "fetus", "amniotic fluid", "brain", "blood", "circulation", "heart", "umbilical cord", "function", "physiological","oxygen", "oxygenation", "saturation","antihypoxant", "hydrogen peroxide","brth", "delivery", "life", "health", "death", "clinical", "outpatient", "home", "damage", "hypoxia", "ischemia", "asphyxia", "diagnostics", "measurement", "prediction", "prognosis", "prevention","resistance", "adaptation", "treatment", "drugs", "obstetric care", "emergency care","cardiovascular system", "toxicosis", "preeclampsia", "eclampsia", "acrocyanosis", "dyspnea", "apnea", "syndrome", "abortion", "stillbirth", "uterine contractility", "Caesarian section", "placental abruption", "respiration", "breathes", "temperature","hyperthermia", "pyrexia", "stressors", "methods", "device", "screening", "monitoring", "prenatal", "labor", and "newborn".

That were searched separately for each keyword in electronic databases, but the main one is that the search was focused on inventions that address fetal hypoxia and can improve fetal resilience during childbirth. The search results are shown in the Table 1. The search for inventions and the selection of inventions was carried out independently by two researchers, and inconsistencies were resolved by consensus. Information about the essence of inventions written in Russian and English was included. The search strategy was based on the PICO model [21, 22]. Two coauthors independently selected, evaluated and extracted data. The inconsistencies in the reviews were resolved by consensus. The flowchart for selecting articles was a spiral in which each spiral turn was an iteration [23, 24].

The study inclusion criteria were limited to devices, drugs, and methods (medical technologies) that provide diagnosis, modeling, prevention, and treatment of intrauterine hypoxia, reliably increase fetal resistance to sudden acute hypoxia, urgently assess fetal adaptation to hypoxia, and increase fetal survival and viability in oxygen deficient conditions during pregnancy and childbirth by reducing the fetus’s need for oxygen and increasing the resistance of its brain cells to hypoxia by optimizing the mechanical and physical-chemical factors of interaction between the organisms of the fetus and its mother.

The exclusion criteria of the study were the absence of inventions dedicated to fetal intrauterine hypoxia, assessment of fetal resistance to hypoxia and improvement of fetal adaptive reserves to hypoxia. The risk of individual bias in judgments was reduced by relying on the essence of the invention as a generally accepted criterion of novelty. A total of 197 inventions were found, of which 27 inventions were evaluated for review.

Almost 100 years ago, the great physiologist Sir Joseph Barcroft showed that hypoxia reduces the mental and physical abilities of humans and is one of the major dangers to the life and health of fetuses during pregnancy and physiological childbirth [25, 26, 27]. The main hopes of all women and doctors for the birth of a living and healthy baby are still associated with the inhalation of fresh oxygen-enriched air by a pregnant woman, both during pregnancy and during vaginal delivery [28, 29, 30, 31]. Unfortunately, there is still no officially recognized scale for assessing the fetal adaptive capacity to hypoxia [7, 17, 29, 32, 33, 34]. Moreover, the diagnosis of intrauterine hypoxia, asphyxia, and hypoxic fetal brain cell damage is still made only after delivery, because the only assessment tool in obstetrics remains the Apgar score for the newborn [35, 36, 37]. Under these circumstances, all fetuses in all pregnant women and all newborns are left without timely diagnosis of hypoxic brain damage and without timely increase in oxygen delivery through the pregnant woman’s body, or through the fetal lungs after an emergency Cesarean section [29, 38]. Only prolonged labor, meconium-stained amniotic fluid, and preeclampsia are considered incontrovertible evidence of possible perinatal asphyxia [31]. Therefore, to reduce neonatal mortality, it has been proposed to improve prenatal care, monitor mothers during delivery with a partograph, improve emergency obstetric care, and improve the qualifications of physicians in neonatal resuscitation and management of sick newborns [34].

Nevertheless, following these recommendations cannot predict and rule out neonatal encephalopathy. Only a Caesarean section protects the baby’s brain from the tragic ordeal to which it may be subjected during a physiological birth [17, 29]. This method of urgently saving a fetus from death was proposed in 715 B.C., and to this day the widespread use of Cesarean section improves obstetric outcomes around the world [39]. Indications for a C-section are considered to be difficult labor, a double pregnancy, high maternal blood pressure, breech delivery, or problems with the placenta or umbilical cord [39, 40]. At the same time, there is still no standard assessment test to evaluate the fetal resistance to impending intrauterine hypoxia in labor, which would be the basis for indicating a C-section to rule out death and asphyxia of the newborn [17, 29, 41].

Thus, it was high time to develop innovative ways to diagnose, prevent and treat intrauterine hypoxia in order to preserve the life and health of newborns. The first recommendations of this kind for obstetricians and gynecologists were developed in the form of inventions at the beginning of the 21st century. The essence of the inventions was originally published not in the scientific literature, but in the patent literature. Therefore, these innovations were not noticed by researchers who were not researching the inventions. That is why it is very important to research not only traditional databases of scientific articles, but also relevant databases containing information about patented inventions.

In 1914, Vladimir Stange of Petrograd University in the capital of the Russian Empire found that the average healthy person can hold his breath for 45-50 seconds, while patients with heart failure can hold their breath for no more than 10-20 seconds [42]. Today it is clear to us that more than 100 years ago, the Russian researcher Vladimir Stange was the first and independent of the innovations of great physiologist Sir Joseph Barcroft proposed to evaluate the adaptation reserves of an adult to acute hypoxia by the maximum possible duration of voluntary apnea. That is why this method is currently used all over the world and is known as the "Stange test".

Oxygen therapy is widely used to combat hypoxia [43, 44]. Particularly in neonatology for neonatal asphyxia and in pediatrics, a nasal cannula with heated humidified therapy with high flow and forced ventilation is used for this purpose [45, 46]. At the same time, forced airway ventilation with respiratory gases with oxygen is not included in the standard of obstetric care for pregnant women with intrauterine hypoxia of their fetuses [47, 48, 49, 50, 51, 52]. These facts seem to suggest, as if on purpose, that forced ventilation with oxygen gas is meaningless for every pregnant woman.

Therefore, our current review aims to improve obstetric care to prevent hypoxic damage to fetal brain cells during pregnancy and delivery. In doing so, we focus on innovations underlying inventions aimed at developing medical technologies to assess fetal resistance to upcoming periods of hypoxia during labor, diagnose intrauterine hypoxia, and prevent it. The fact is that only such technology can save the lives of fetuses and newborns, because only a timely increase in fetal blood oxygenation can urgently provide the fetal brain with oxygen, since the true cause of their death is hypoxic brain cell damage of an irreversible nature [7, 15, 17, 25, 29]. It follows that oxygen is the number 1 treatment for fetal hypoxia. And to propose new technologies of obstetric care aimed at efficient oxygen supply to the fetal brain is a very worthy and long overdue task. This gap in the relevant reviews and standards of obstetric care needs to be addressed as soon as possible. There is an urgent need to supplement the existing standards of obstetric care with effective and safe medical technologies for the diagnosis, prevention, and treatment of fetal intrauterine hypoxia. Research in this area is needed because the results may provide a new way to rebalance the fetal brain’s need for oxygen and the delivery of oxygen to the fetal brain through the mother’s body in a timely manner, ultimately reducing fetal mortality during pregnancy and delivery and increasing the birth rate of healthy newborns with potentially high mental capacity [47].

Today, it can be argued that the creation of the obstetric Stange test was made possible by the introduction into clinical practice of advanced ultrasound devices for medical purposes. It was the introduction of such sensitive devices into clinical practice that unexpectedly made it possible to visualize the movement of solutions of antiseptic drugs in the abdominal cavity during its lavage in peritonitis. The point is that at the beginning of the 21st century, there was an invention based on the ultrasound visualization of the movement of a dialysis solution in the abdominal cavity, based on the change in ultrasound echogenicity of this solution of antiseptic drugs with the change in its dissolved gas content (RU Patent No. 2336833). It appeared that increase of gas content in the solution by means of overpressure provides under conditions of normal atmospheric pressure in abdominal cavity gas release and formation of gas bubbles in the solution. This process is known in physics as "cold boiling". It turned out that the presence of gas bubbles in the solution very much changes its ultrasonic echogenicity and provides reliable sonographic visualization of the direction and intensity of movement of these gas bubbles and solution. Cold boiling allows not only controlling this process, but also allows controlling the process of moving the solution inside the cavity (i.e. allows controlling the process of flushing the cavity). In parallel, at the same time, an invention was created that was based on controlling the movement of the drug gel inside the vaginal cavity by changing the vector of gravitational force and changing the position of the woman’s pelvic body in space (RU Patent No. 2338539). Independently, a method was invented in which it was proposed to control the tocomimetic effect of oxytocin by monitoring the process of uterine contraction/relaxation (RU Patent No. 2380110). A little later, several inventions were created, the essence of which was to register acrocyanosis in adult fingertips using a thermal imager (RU Patent No. 2422090), and in fetuses using ultrasound (RU Patent No. 2441592). These inventions are based on the assessment of the stage of hypoxic brain damage and resuscitation efficiency in adults by the dynamics of local hypothermia in fingers, palms, and forearms on the thermal imaging screen, and in fetuses by ultrasound echogenicity of the fingertip pads. In particular, a change in the echogenicity of the fetal finger pads during contractions at the beginning of vaginal labor was shown, which indicated intrauterine hypoxia of natural nature.

However, special attention should be paid to the inventions in which the presence of intrauterine hypoxia and fetal resistance to intrauterine hypoxia were proposed for the first time to be assessed by the dynamics of fetal movement using ultrasound (RU Patent No. 2432118, RU Patent No. 2441592, RU Patent No. 2511084, RU Application No. 2011109952). In general, thanks to the above inventions, by 2014 it was proposed to evaluate the fetal resistance to hypoxia by the duration of the fetal rest period recorded by ultrasound during the diagnostic Stange test. At the same time, it has been suggested that fetal resistance to hypoxia is low if ultrasonography reveals limb motor activity, respiratory rib movements, fist unclenching, and fetal finger straightening in the first 10 seconds after the onset of apnea in a pregnant woman. Moreover, during the same period, a quite exotic invention was created consisting in a breathing mask, which was designed to be put on the head of the fetus inside the uterus in order to artificially ventilate its lungs with breathing gases (RU Application No. 2010134466).

Several inventions were devoted to the diagnosis of cerebral hypoxia in the fetus during vaginal delivery and in the first seconds after birth. For example, it was proposed to assess the fetal oxygen supply in labor by the dynamics of ultrasound echogenicity of fetal finger pads and, after delivery, by the dynamics of local body surface temperature of the newborn recorded using a thermal imager in the infrared range of the tissue emission spectrum (RU Patent No. 2503414, RU Patent No. 2502485).

But for the development of the obstetric Stange test the inventions created to assess the resistance of adults and fetuses to acute diagnostic hypoxia (ischemia) (RU Patent No. 2531924, RU Patent No. 2529377) turned out to be very important. Thanks to them it was found that in normal apnea changes of local temperature and ultrasonic echogenicity of tissues in the finger pads of adults and children appear not earlier than 12 seconds from the beginning of breath-holding, and also it can be changed by training method. In this regard, repeated diagnostic periods of apnea and ultrasound (or thermal imaging) have been proposed to assess fetal (or adult) resistance to repeated hypoxia. These data were found to correlate with resting periods of fetal bodies. Therefore, it has been suggested that an increase in fetal resting period by more than 15 seconds indicates satisfactory adaptation, and a decrease in fetal resting period by less than 15 seconds indicates poor fetal adaptation to repetitive hypoxia.

During the same period of time, inventions were created in which course transamniotic injections of isotonic D-glucose solution in combination with intravenous injections of hypertonic D-glucose solution (US Patent No. 9555052) and elevation of adult lower extremities upward with application of the upper third of thigh tourniquet for clamping blood vessels up to limb ischemia (RU Patent No. 2527350) were proposed for prevention and treatment of fetal hypoxia. But these inventions did not help to improve fetal resistance during acute intrauterine hypoxia because they did not provide a direct increase in oxygen delivery to the pregnant woman and fetus and/or save their oxygen consumption.

Nevertheless, at the same time, several other inventions were developed that proposed to eliminate acute intrauterine fetal hypoxia or newborn hypoxia to increase oxygen gas delivery to the pregnant woman and/or her fetus or to reduce oxygen consumption in the infant’s brain. Specifically, hyperventilation of a pregnant woman’s lungs with oxygen breathing gases, injection of hydrogen peroxide solutions into the venous blood, brain tissue, or enteral administration of hydrogen peroxide solution (into the stomach) was suggested to increase oxygen delivery to the mother and her fetus. At the same time, it was proposed to monitor the oxygen content in the mother’s blood using a pulse oximeter sensor. In turn, to reduce oxygen consumption in the fetal brain, it was proposed to rely on the dependence of the metabolic rate on temperature (according to the Arrhenius law, when the temperature decreases by \(10^{\circ}C\), the rate of chemical reaction decreases each time by an average of 2 times). In this regard, it was proposed to take into account the diurnal rhythm of a woman’s body temperature and choose the period of the day with the lowest maternal body temperature for a scheduled delivery. On the other hand, it was suggested to cool the fetal head immediately after it leaves the birth canal to the outside. For local hypothermia, it was suggested to irrigate the head with water at \(+18^{\circ}C\) or to blow the fetal head with room temperature air using a household hair dryer. Finally, urgent intrapulmonary injection of breathing gas into the tissue of the right lung to a depth of 3 mm up to removal of amniotic fluid and mucus from the mouth and nose to the outside was suggested as resuscitation for newborn asphyxia. The essence of these inventions is presented in Table 1.

The entire list of inventions mentioned above has been sufficient to transform the commonly used Stange test into an obstetric Stange test, which provides a noninvasive assessment of fetal resistance to intrauterine hypoxia.

It should be reminded that this test is based on the general biological regularity of the dynamics of motor activity of biological objects in conditions of acute hypoxia: the more reserves of adaptation to hypoxia a biological object has, the longer it remains in a calm state. This pattern was first discovered in experiments with aquarium fish. Then it was shown that normally healthy fish and fetuses behave in acute hypoxia in almost the same way: at the beginning of hypoxia they take a motionless state, which lasts the longer the longer their adaptation reserves to hypoxia are. Then the reserves of adaptation to hypoxia are gradually used up, and when they are completely exhausted, fish and fetuses suddenly activate their motor activity. At the same time, they develop respiratory movements of the thorax [15, 29]. Therefore, recording the duration of the period of fetal immobility during diagnostic respiratory arrest in a pregnant woman is an integrative way to assess fetal resistance to hypoxia. It is essentially a Stange test modified for use in a pregnant woman to assess the resistance of her fetus to intrauterine hypoxia (i.e., it is an obstetric Stange test).

However, the obstetric Stange test has not yet been conclusively developed and is not recognized worldwide. There are only a few patents for inventions and there is a concept of medical technology for determining fetal resistance to hypoxia by the duration of fetal immobility during diagnostic maternal apnea. But there is no generally accepted technology for a Stange test modified for use in pregnant women to assess the resistance of their fetuses to intrauterine hypoxia. Consequently, there is as yet no standard for the obstetric Stange test.

It has been reported that the fetus in a pregnant woman is normally ready to tolerate intrauterine hypoxia if the pregnancy proceeds normally and the sudden onset of hypoxia remains within physiological limits. However, in cases where fetal intrauterine hypoxia exceeds physiological values and/or fetal adaptive reserves to hypoxia, hypoxia can cause fetal death during pregnancy and delivery and/or cause development of postnatal encephalopathy in the newborn, as well as cause mental failure in the child later on. Therefore, it is necessary to constantly monitor the supply of oxygen to the fetus, on the one hand, and the dynamics of fetal movement to assess fetal resistance to hypoxia, on the other hand.

Studies have shown that adult resistance to hypoxia is assessed worldwide using the Stange test, and blood oxygen saturation reliably eliminates hypoxia and preserves adult life in hypoxic conditions such as submerging underwater and climbing high in the mountains. At the same time, the standard of care for obstetric care for pregnant women does not include a test for fetal resistance to intrauterine hypoxia and technologies for urgently increasing oxygen delivery to the fetal brain, nor do technologies for urgently reducing fetal brain oxygen requirements. In this respect, our review is quite timely and justified: it provides encouraging results. It has been reported that recording the duration of fetal immobility during diagnostic maternal apnea indicates the magnitude of the fetal adaptation reserves to hypoxia similarly to that evidenced by recording the maximum duration of breath-hold in adults (Stange test).

A review of the inventions has shown that two applications of the Stange test in a pregnant woman to determine the resistance of her fetus to intrauterine hypoxia have been developed to date. Historically, the first option for performing an obstetric Stange test is as follows. A pregnant woman in the second half of her pregnancy visits an antenatal clinic at a scheduled (screening) time. At the antenatal clinic, after completing a standard fetal ultrasound scan, the pregnant woman is asked to make a video film (videorecording) of the dynamics of fetal movement against voluntary breath-holding (apnea) from the ultrasound device screen. In this case, the woman and obstetrician assume that in a normal pregnancy the fetus is motionless for more than 30 seconds, and in the case of threatened termination of pregnancy the duration of fetal immobility during the mother’s breath-hold is close to zero. In the first case, fetal resistance to hypoxia is assessed as very good, and the pregnant woman can plan to deliver a healthy and alive baby by urgent vaginal delivery. In the second case, fetal resistance to hypoxia is poor, and vaginal delivery may exacerbate hypoxic damage to fetal brain cells. Therefore, to prevent fetal hypoxic brain damage and to save the life of the newborn, urgent hyperventilation of the pregnant woman’s lungs with breathing gas with oxygen before the first symptoms of oxygen poisoning develop and urgent delivery by Cesarean section is recommended.

The second variant of the obstetric Stange test involves the individual assessment of fetal resistance to hypoxia by each pregnant woman on her own without the assistance of an obstetrician and without an ultrasound device. This option is based on tactile determination of fetal immobility inside the uterus using the woman’s fingertips. To do this, a pregnant woman chooses a period of immobility of her fetus, puts the palm of her working hand on her abdomen, feels the fetus, its head and torso inside the uterus. Then the woman fixes her hand in this position, holds her breath, and measures the time to determine the duration of fetal immobility. In this option, it is suggested that pregnancy is normal and physiological delivery is indicated if fetal immobility in an outpatient setting (without the obstetrician’s involvement) with diagnostic apnea for more than 60 seconds is detected. If, under these conditions, the duration of fetal immobility during apnea persists for less than 30 seconds, the woman decides that she needs to see an obstetrician for further investigation. Moreover, if the duration of fetal immobility during diagnostic apnea is less than 30 seconds, the woman may request a scheduled delivery by C-section. In this case, the woman has the right to request a study of her daily body temperature rhythm and to perform the C-section at the time of day when her body temperature is the lowest.

Fetal/perinatal hypoxia is one of the most frequent causes of stillbirth and neonatal encephalopathy [53]. Reliable and timely diagnosis, prevention and management of fetal hypoxia is still an unsolved problem in obstetrics and gynecology [1, 2, 3, 4, 5, 6, 7, 8]. Therefore, adherence to the current standard of obstetric care cannot predict and exclude neonatal asphyxia. Nevertheless, as early as 715 B.C., it was shown that Cesarean section reliably saved the fetus from death that could occur during physiological labor [17, 29, 39]. But only about 100 years ago the great physiologist Sir Joseph Barcroft showed that the main danger to the life and health of fetuses during pregnancy and vaginal birth was hypoxia [25, 26]. Today it is clear that the cause of acute intrauterine hypoxia, the periods of which are repeated many times during vaginal delivery, is the constriction of blood vessels in the uterus, which occurs during contractions (uterine contractions) [54]. However, fetal resistance to impending birth hypoxia has not been investigated, so values of low fetal resistance to intrauterine hypoxia are still not among the indications for Cesarean section to rule out death and asphyxia of the newborn [17, 29, 41].

Nevertheless, in 1914, Vladimir Stange, a Russian physician, was the first to suggest estimating the adaptive reserves of the adult maximum duration of voluntary apnea [42]. The method he proposed is used all over the world under the name of Stange test. However, it took almost 100 until the first inventions devoted to simple and accessible assessment of fetal resistance to hypoxia were created in Russia. These technologies are still outlined in the description of inventions and are not included in the obstetric standard. Therefore, we performed a systematic review of the essence of the inventions that formed the basis for the modernization of obstetric care aimed at preserving the life of fetal brain cells in conditions of intrauterine hypoxia. We found a total of 197 inventions, of which 27 inventions were evaluated for review.

A review of the inventions showed that sufficient inventions have now been created to modernize the commonly used Stange test in order to use it in obstetric practice to assess fetal resistance to intrauterine hypoxia. It has been shown that normally healthy fish and fetuses behave almost identically under conditions of acute hypoxia: at the beginning of the hypoxia period they take an immobile state, the duration of which is the longer, the more their adaptation reserves to hypoxia are. Then, under conditions of persisting hypoxia, the reserves of adaptation to hypoxia are gradually used up, and when these reserves are completely exhausted, fish and fetuses suddenly activate their motor activity (movement). At the same time, they show respiratory movements of the thorax [15, 29].

These data allowed to propose the recording of the duration of the fetal immobility period during diagnostic maternal apnea as an obstetric Stange test to assess the fetal resistance to intrauterine hypoxia. Initially, fetal ultrasound was proposed to record the duration of the period of fetal immobility during voluntary maternal apnea. In recent years, the tactile sensitivity of receptors located in the fingertips of the pregnant woman has been proposed for this purpose [41]. It has been suggested that the fetus can normally remain motionless for more than 30 seconds from the onset of voluntary maternal apnea. This tolerance to fetal intrauterine hypoxia will keep the fetus alive during successive periods of hypoxia during vaginal delivery. If the pregnant woman (or obstetrician) identifies a duration of fetal immobility during maternal apnea that persists less than 30 seconds, a thorough evaluation of the pregnant woman is required before the type of delivery is chosen. If, however, the duration of fetal immobility during diagnostic apnea tends to zero, there is every reason to have a C-section.

It has also been shown that regardless, a pregnant woman has the right to request a study of the diurnal rhythm of her body temperature and to perform a scheduled delivery (including C-section) at the time of day when her body temperature is lowest. In addition, hyperventilation of the mother’s lungs with respiratory gases with oxygen before symptoms of oxygen poisoning and local hypothermia of the fetal head after she leaves the birth canal to prevent hypoxic damage to the fetal brain cells are suggested.

Among the inventions devoted to monitoring fetal motor activity inside the uterus, this review showed that the fetus in pregnant women has adaptive reserves to hypoxia, which normally allow it to calmly survive at least 30 seconds of acute hypoxia. It has been found that cessation of the resting period (fetal immobility) and the appearance of respiratory movements in the chest, as well as abrupt movements of arms and legs, indicate depletion of fetal adaptation reserves to hypoxia. Therefore, it is proposed to monitor the dynamics of fetal motor activity during pregnancy and labor for timely safe diagnosis of intrauterine hypoxia and assessment of fetal resistance to it. A modified Stange test is proposed to assess fetal brain adaptive reserves for upcoming periods of hypoxia during vaginal delivery.

For this purpose, it is proposed to record fetal motor activity or the duration of fetal immobility using ultrasound and manual palpation of the fetal body and/or electronic tactile sensors during diagnostic apnea. However, if fetal immobility persists for more than 30 seconds during apnea (even better if this period exceeds 60 seconds), it is suggested that the fetal resistance to hypoxia is considered very good and a favorable outcome of the upcoming vaginal delivery is predicted. On the other hand, if the fetus remains immobile for less than 30 seconds during diagnostic apnea, it is suggested that the fetal resistance to hypoxia be assessed as low. In particular, if the fetus signals distress immediately after the onset of apnea, it should be assumed that the fetus has no reserves of adaptation to hypoxia. In these cases, delivery through the natural birth canal is contraindicated because the fetus may drown in amniotic fluid, or the newborn may develop asphyxia, encephalopathy, or pneumonia. In these cases, a planned Cesarean section may improve the prognosis for delivery.

At the same time, it has been shown that to increase the likelihood of a favorable birth outcome, it is desirable to choose the time of day when the body temperature of the pregnant woman is lowest and to lower the temperature of the fetal head during labor by artificial local hypothermia. The fact is that lowering the fetal body and/or head temperature slows aerobic metabolism, saves oxygen consumption, prolongs the period of fetal survival and postpones the onset of hypoxic damage to fetal brain cells during hypoxia. Despite this, it has been suggested that hypoxic damage to fetal brain cells should be prevented by hyperventilating the pregnant woman’s lungs with oxygenated breathing gas before she develops symptoms of oxygen poisoning. Due to the limitations of this systematic by the review of the inventions alone, additional clinical studies are needed to confirm the findings.

No special funding received for the study.

The present article no study with human participants or animals performed by any of the authors.

The authors declare no conflict of interest

NU and VS data collection and writing, AU and PS data collection & idea of research and proof reading, NU and AU analysed and interpreted data. All authors have read and approved the final manuscript.

The authors express their gratitude to the students N.Mukhutdinov, D.Suntsova and A.Stolyarenko for their contribution to drawing graphical abstract.

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