Fetal distress refers to a series of pathological conditions caused by fetal hypoxia or acidosis in the uterus, which jeopardize the health and life of the fetus and baby. How to detect and correctly diagnose fetal distress at an early stage in clinical practice is an important topic in perinatal medicine and an eternal topic in obstetrics. Timely management of fetal distress can enable the fetus to be delivered before vital organs are damaged, thus reducing or avoiding neonatal complications; but at the same time, overdiagnosis of fetal distress should be avoided, resulting in blind cesarean section, preterm labor of medical origin, and so on. Difficulties in the diagnosis of fetal distress include the lack of uniform diagnostic criteria, the indirectness of detection methods, the difficulty of detecting abnormalities of the placenta and umbilical cord during prenatal examination, and the difficulty of distinguishing between “true” fetal distress and physiological changes in the fetus during labor. Therefore, clinicians must know how to correctly use various monitoring methods to accurately assess the fetal reserve capacity, identify the true or false fetal distress, and intervene at the right time. First, the subjective signs of fetal distress, the subjective signs of fetal distress mainly refer to the performance observed by the pregnant woman herself or medical personnel through the senses, including the change of fetal movement and the change of amniotic fluid characteristics. Fetal movement: Fetal movement refers to the activity of the fetus in the uterus, which is a symbol of fetal survival, and strong fetal movement is a credible indicator of fetal health. Although the change of fetal movement is the subjective feeling of the pregnant woman, it is the earliest sign of fetal distress and should be taken seriously. Fetal movement counting is one of the indirect methods to monitor the development and functional status of the fetal central nervous system. When the fetal movement is <3 times per hour, or <10 times in 12h, or more than 30% reduction in fetal movement in 3d, the fetal movement is reduced, suggesting that there may be fetal distress. Loss of fetal movement for 12h is a fetal movement alarm signal, suggesting the possibility of fetal death. If the fetal movement suddenly increase or enhancement, called fetal movement sharp, fetal movement sharp and then stop, often suggests that the fetus due to acute hypoxia and death, common in umbilical cord prolapse, heavy placental abruption and other conditions. Fetal movement counts are greatly influenced by subjective factors, and it is important to instruct pregnant women to pay attention to fetal movement and familiarize themselves with their own fetal movement patterns. However, some clinical studies have come to different conclusions: a 2009 randomized controlled study evaluating 71,370 pregnant women in four groups concluded that there is insufficient evidence to recommend fetal movement counting for all pregnant women and high-risk women. The Society of Obstetricians and Gynaecologists of Canada (SOGC) suggests that high-risk pregnant women should monitor fetal movements daily from 26 to 32 weeks of gestation (ⅠA); healthy pregnant women should be aware of the importance of fetal movements, and perform fetal movement counts when they realize that their fetal movements are decreasing (ⅠB); hourly fetal movements <3 should be further examined; and hourly fetal movement counts <3 should be performed in all pregnant women. Fetal movement <3 times per hour should be further examined and fully evaluated and treated. 2. Change of amniotic fluid properties: The change of amniotic fluid properties is a subjective indicator observed by doctors. For a long time, amniotic fluid meconium contamination has been considered a sign of fetal hypoxia, leading to excessive clinical intervention. Simple amniotic fluid meconium contamination in late pregnancy is a sign of fetal gastrointestinal maturation and is a result of "physiologic defecation". However, when amniotic fluid-fetal feces contamination is accompanied by other abnormalities, fetal distress should be considered. Amniotic fluid volume can be used as a reference indicator, when only amniotic fluid Ⅱ ~ Ⅲ degree of pollution, if the amniotic fluid volume is normal, can continue to observe; if the amniotic fluid volume decreases, the turbid amniotic fluid is easy to induce vasoconstriction of the placenta, resulting in obstruction of the fetal airway, hypoxia and cause lung injury. If amniotic fluid is found to be contaminated to the second or third degree during labor, the pH of fetal scalp blood should be measured in order to understand the acid-base status of the fetus. The mode of delivery should be decided by considering other monitoring indexes, the progress of labor and the size of the fetus. The objective signs of fetal distress refer to the results of various auxiliary examinations, and the commonly used methods are fetal electronic monitoring, fetalbiophysicalprofilescoring (BPS), Doppler flow measurement, fetal pulse oximetry (fetalpulseoximetry, FPO) measurement and other noninvasive methods. Doppler flow measurement, fetal pulse oximetry (FPO) measurement and other non-invasive testing methods, as well as minimally invasive testing methods such as fetal scalp blood pH measurement, are described as follows. (I) Fetal electronic monitoring Fetal electronic monitoring refers to the application of a fetal heart electronic monitor to continuously monitor the fetal heart rate and/or uterine contraction pressure, and the traced cardiotocograph (CTG) is analyzed clinically, including the non-stress test (non-stresstest, NST) and the contractionstresstest (contractionstresstest). NST: NST is a non-stress test (non-stresstest, NST) and contraction stress test (contractionstresstest, CST or oxytocinchallengetest, OCT). 1, NST: Normal NST is defined as at least 3 fetal heart rate accelerations within 20 min, each with an amplitude of at least 15 beats/min and a duration of at least 15 s. The range of fetal heart rate variability is 110~160 beats/min, and the baseline range of variability is 6~25 beats/min. For gestational weeks <32 weeks, fetal heart rate acceleration is defined as an amplitude of ≥10 beats/min, and a duration of ≥10 s. NST is also defined as an acceleration of ≥10 beats/min. NST is a traditional, economical, fast and sensitive method of fetal monitoring, but due to the influence of fetal physiological sleep cycle, maternal position, medication and other factors, the artifact of non-responsive type of NST may occur, which affects the clinical judgment. In order to identify whether it is a physiological fetal sleep cycle, methods such as pushing the fetal body, sound vibration stimulation, changing the mother's position, and prolonging the monitoring time can be adopted, and further confirmation of the diagnosis can also be made by reviewing the NST within 24h or performing OCT and BPS. When the NST is repeatedly abnormal, it is often suggestive of fetal hypoxia and requires special attention. In a study by Ke Zhang et al, 204 pregnant women with 2 or more abnormal NSTs combined with sound vibration stimulation (abnormal group) were compared with 103 pregnant women with normal NSTs (control group) during the same period of time, and the proportion of pregnant women in the abnormal group who suffered from pregnancy comorbidities, umbilical cord factors, and poor perinatal prognosis was significantly higher than that of the control group, and the differences were statistically significant (all p<0.05). This study also found that when repeated abnormalities of NST are accompanied by disappearance of baseline variants, it is important to be alert to the possibility of fetal malformations, especially neural tube malformations.SOGC clinical guidelines for NST: (1) When the NST is normal, fetal movement is normal, and there is no oligohydramnios, there is no need for other investigations such as BPS (IIIB). (2) NST should be judged as soon as possible (preferably within 24h) by specially trained personnel to determine the results. (3) As soon as it is clear that the NST result is abnormal, the doctor or health care provider should be notified, the drawing should be viewed, and it should be recorded and treated (IIIB). 2.CST/OCT:CST/OCT is the main means of identifying fetal distress and fetal physiologic stress response during labor. If CST/OCT is performed for 30 minutes at the early stage of labor, it can have a good predictive effect on the progress of labor in the future. CST/OCT should be performed according to the following principles: (1) When CST/OCT is normal, intermittent monitoring can be given (unless there are other indications and prolongation of labor). (2) When CST/OCT results are suspicious, continuous monitoring should be performed. (3) When CST/OCT results are clearly abnormal, fetal reserve should be evaluated in conjunction with amniotic fluid volume, and early intervention should be performed for fetuses with low reserve. (4) CST/OCT should be monitored more closely in women with oxytocin, epidural anesthesia, and those with amniotic fluid meconium contamination. In the process of monitoring, special attention should be paid to distinguish three kinds of fetal heart deceleration: (1) late deceleration, which is characterized by fetal heart deceleration often occurs 30-40s after the peak of contraction, with a small amplitude of 10-20 beats/min, and the fetal heart returns to the original baseline 30s after the end of contraction. This type of deceleration is pathologic, suggesting that the fetal placental reserve function is poor, and should be paid special attention to. (2) Variable deceleration, which is characterized by the variable relationship between fetal heart rate deceleration and contraction, rapid deceleration and rapid recovery, suggesting the presence of umbilical cord factors. If variable deceleration occurs repeatedly, even if it is a mild V-shaped deceleration, it should be taken seriously. (3) Early deceleration, manifested as synchronization of fetal heart rate deceleration and contraction, mostly occurs after the active phase, and is generally considered to be physiological, which is a reflexive slowing of the heart rate caused by pressure on the fetal head. If the amplitude of early deceleration is too large, appears too early, and the frequency is too high, it often suggests the presence of umbilical cord factors or amniotic fluid is too small, and should also be taken seriously. Interpretation of CTG graphs: CTG graphs during labor can dynamically provide real-time information on the acid-base status of the fetus and assess the fetal status at a certain point in time, but they cannot predict the occurrence of cerebral palsy (referred to as cerebral palsy) in the distant future. The baseline variation of fetal heart rate refers to the upward and downward fluctuation of the baseline of fetal heart rate. Moderate variation with an amplitude of 6-25 beats/min suggests that the fetus is healthy; if the variation disappears or decreases with an amplitude of <5 beats/min, it suggests that the fetus may be hypoxic and needs further evaluation; if excessive variation with an amplitude of >25 beats/min suggests that umbilical cord factors may be present. In the evaluation of fetal electronic monitoring, the baseline level cannot be considered as an independent factor, but must be combined with the baseline variation, the presence of acceleration and deceleration, etc. CTG graphs are classified into three types, which can change dynamically according to the clinical situation. Type I includes the following conditions: (1) Fetal heart rate baseline 110-160 beats/min. (2) Moderate variability of the baseline fetal heart rate. (3) Fetal heart rate acceleration present or absent. (4) Late deceleration or variable deceleration absent. (5) Early deceleration present or absent. Type II includes all types except Type I and Type III, and includes the following conditions: (1) Baseline level of fetal heart rate: bradycardia (fetal heart rate <110 beats/min) without loss of baseline variability, or tachycardia (fetal heart rate >160 beats/min). (2) Fetal heart rate baseline variability: reduced baseline variability; significant baseline variability; disappearance of baseline variability (without repeated decelerations). (3) Fetal heart rate acceleration: stimulation of the fetus did not produce acceleration of the fetal heart rate. (4) Periodic or intermittent fetal heart rate decelerations: recurrent variant decelerations or late decelerations with moderate baseline variability; variant decelerations accompanied by other characteristics, such as slow return of the fetal heart to the baseline, “spiky” or “bimodal” fetal heart rate curves; prolonged decelerations ( prolonged deceleration (deceleration lasting 2-10 min). Type III includes any of the following: (1) Reduced or absent baseline fetal heart rate variability with bradycardia or recurrent late or variable decelerations. (2) Sine wave pattern. Clinical management during labor can be based on the above three types of CTG patterns:Type I is a normal pattern, predicting that the fetus is in a state of normal acid-base balance, and routine obstetric clinical operations can be followed without special treatment. Type II is an indeterminate pattern, which cannot be used to predict whether the fetal acid-base status is normal or not, but for the time being, there is not enough evidence to categorize it as type I or type III, and it needs to be reassessed after continued monitoring and taking into account other accompanying clinical conditions. Type III is an abnormal pattern, suggesting abnormal fetal acid-base status, and requires immediate evaluation and rapid intervention, including maternal oxygenation, changing maternal position, stopping the use of uterotonics, correcting maternal hypotension, etc., and terminating the pregnancy if necessary. (ii) BPSBPS is a comprehensive assessment method that applies a number of biophysical phenomena. Currently, there are mainly the Manning 5-item scale and the Vintzileous 6-item scale, as well as a variety of other improved methods based on them. Among them, Manning’s 5-item score is called “intrauterine Apgar score”, which is highly valued by perinatal medical practitioners and widely used in clinical practice. 5-item score is composed of fetal respiratory movement, fetal movement, fetal muscle tone, amniotic fluid volume and placental classification observed by NST and ultrasound, and a comprehensive score of 2 points for each item is given. A comprehensive score of 2 points for each item, out of a total of 10 points. A score of ≥8 suggests a healthy fetus; a score of 5-7 suggests suspected fetal distress, which should be retested or further evaluated within 24 h; and a score of ≤4 should lead to prompt termination of pregnancy.The SOGC clinical guidelines recommend BPS for assessing fetal health in high-risk pregnancies when available (ⅠA); and if the BPS result is abnormal, it should be highly valued, and the decision of the next step should be made based on the full clinical picture (ⅢB). B). (III) Doppler flowmetry 1. Uterine artery flowmetry: a non-invasive method of checking placental vascular resistance. In normal pregnancy, blood flow velocity increases and blood flow resistance decreases. Positive indices for uterine artery flow measurement include a mean resistance index (RI) > 0.57, a pulse index (PI) > 95th percentile, and/or a cut-off in the uterine artery flow spectrum. In some medical centers, uterine artery flowmetry has become part of routine ultrasound screening at 18 to 22 weeks of gestation.The SOGC clinical guidelines recommend that uterine artery flowmetry (IIA) may be performed at 17 to 22 weeks of gestation in high-risk pregnancies with a history of adverse perinatal outcomes, when available. Fetal middle cerebral artery (MCA) blood flow measurement: MCA is the blood vessel with the richest blood supply in the cerebral hemisphere, which can directly reflect the dynamic changes of fetal craniocerebral blood circulation, and indirectly reflect the changes of placental blood flow, thus predicting whether the fetus is hypoxic or not.The ratio of the MCA blood flow index [the maximum systolic blood flow velocity (S) and the maximum end-diastolic blood flow velocity (D)] is the ratio of the MCA blood flow index (the ratio of the maximum systolic blood flow velocity (S) to the maximum end diastolic blood flow velocity (D)). blood flow velocity (D) ratio (S/D), PI, RI] is a resistance index of craniocerebral blood circulation, which can determine fetal cerebral blood circulation. When fetal hypoxia occurs, due to the “cerebroprotective effect”, blood is redistributed throughout the body, and the MCA shows dilatation and decreases in resistance; S/D<4, PI<1.6, and RI<0.6 of the MCA are the critical values for predicting fetal hypoxia, especially when the MCA resistance index is < umbilical artery resistance index, which suggests that compensatory regulation of the MCA is unable to maintain adequate blood flow. In particular, MCA resistance index < umbilical artery resistance index, suggesting that the compensatory regulation of MCA could not maintain adequate blood circulation, thus leading to fetal hypoxia. Fetal umbilical artery blood flow measurement: It is a simple, effective, repeatable and non-invasive means to assess the fetal condition and placental function by detecting hemodynamic changes in the fetal-placental circulation through Doppler ultrasound. Numerous clinical studies have shown that the blood flow velocity waveform index of the umbilical artery provides unique information about fetal well-being and facilitates timely understanding of hemodynamic changes in the fetus, which cannot be replaced by other methods of fetal monitoring, and that its abnormal results are closely related to a variety of high-risk pregnancies and poor perinatal prognosis. Increased circulatory resistance to umbilical artery flow (S/D, PI, or RI > 95th percentile) implies a decrease in functional placental vascular units.Neilson and Alfirevic concluded that spectral biological changes in the umbilical artery during fetal hypoxia precede changes in the fetal heart rate; S/D > 3.0, PI > 1.7, and RI > 0.6 can be used as the critical values for fetal hypoxia; absence of diastolic blood flow or retrograde flow suggests very high peripheral placental vascular resistance and severe placental dysfunction, which is associated with fetal growth restriction, severe preeclampsia, and a variety of neonatal complications (respiratory distress syndrome, necrotizing small intestinal colitis, brain damage, etc.). In addition, there is a higher incidence of elevated umbilical artery resistance in malformed fetuses, whereas utero-placental blood flow resistance is often in the normal range. Therefore, when a pregnant woman has normal uterine arterial flow parameters and the umbilical artery flow spectrum shows a persistent end-diastolic flow deficit, the presence of fetal malformations such as trisomy 13-trimester, trisomy 18-trimester, and trisomy 21-trimester needs to be guarded against. Of note, neither fetal MCA flow measurements nor umbilical artery flow measurements were highly sensitive when performed alone, at 83% and 78%, respectively. In contrast, the sensitivity of the combined monitoring of the two indicators, assessed by the ratio of MCA to umbilical artery flow index, was 90%, which was more sensitive and reliable than that of the single indicator. （FPO measurement is a technique for continuous monitoring of fetal oxygen saturation during labor, which needs to be performed after rupture of membranes and is noninvasive to the fetus. Fetal oxygen saturation is approximately (50±10)% in the first stage of labor and (49±10)% in the second stage of labor, and if it is lower than 30%, it is abnormal, suggesting fetal hypoxia and acidosis.FPO measurement is suitable for suspected abnormalities of the CTG pattern, but there are different opinions on whether the combination of fetal electronic monitoring and FPO measurement can reduce the cesarean section rate: the results of a multicenter randomized controlled study in Australia showed that in the presence of suspected abnormalities, the fetal oxygen saturation was lower than that in the first stage of labor.5,6 The results of this study showed that in the presence of suspected abnormalities, the fetal oxygen saturation was higher than that in the first stage. The results of a multicenter randomized controlled study in Australia showed a significant reduction in cesarean delivery rates in the group of pregnant women who also had FPO measurement in the presence of a suspected abnormal CTG pattern, but the difference in neonatal outcome was not statistically significant. A randomized controlled study in the United States that included 360 pregnant women showed that the combined application of electronic fetal monitoring with FPO measurement did not reduce the cesarean delivery rate. More research is needed on issues such as safety and cost-effectiveness of the combined application. Currently, the SOGC and American College of Obstetricians and Gynecologists (ACOG) guidelines do not recommend the routine clinical use of FPO measurement. (e) Fetal scalp blood pH measurement Fetal scalp blood pH measurement is a valid test for determining the presence or absence of fetal acidosis, and is the gold standard for evaluating the acid-base status, gas metabolism, and substance metabolism in the fetus. It is usually considered that pH <7.20 is acidosis, pH 7.20~7.25 is suspected acidosis, and pH 7.25~7.35 is normal. Fetal scalp blood pH is determined by blood gas analysis, and partial pressure of carbon dioxide (partialpressure of carbondioxide, PCO2), partial pressure of oxygen (partialpressure of oxygen, PO2), oxygen saturation and alkali reserve can also be measured, which can distinguish respiratory or metabolic acidosis by combining with alkali reserve and PCO2. The combination of alkali reserve and PCO2 can distinguish respiratory or metabolic acidosis in clinical judgment. However, because the proportion of mixed arterial and venous blood in scalp blood is unknown, the reference value of PCO2 and PO2 is limited. The disadvantage of fetal scalp blood pH measurement is that it is not a continuous monitoring method, which can only reflect the acid-base status of the fetus at the time of blood collection, and cannot predict the changes in the future. However, fetal scalp blood pH measurement combined with CTG and other methods can improve the accuracy of the diagnosis of fetal distress, thus reducing the number of cesarean sections performed due to suspected fetal distress. It has been suggested that there is a concordance between fetal scalp blood pH changes and CTG changes, with the following relationships: (1) in early deceleration and mild variant deceleration, pH averages 7.30 ± 0.04; (2) in moderate variant deceleration, pH averages 7.26 ± 0.04; and (3) in severe variant deceleration, prolonged deceleration, or late deceleration, pH averages 7.14 ± 0.07 [14]. The ACOG guidelines recommend fetal scalp blood pH measurement to assess the true acid-base status of the fetus (IIIC) for those with gestational weeks >34 weeks, CTG abnormalities, or lack of an accelerated response to the NST; during the second stage of labor, if poor fetal status is suspected, further scalp blood pH measurement is not required and delivery should occur as soon as possible [6]. Of particular note, to reduce the probability of vertical transmission of sexually transmitted diseases from mother to child, this test should not be performed in women determined to be infected with herpes simplex virus, hepatitis B virus, hepatitis C virus, and human immunodeficiency virus. Third, other issues related to fetal distress once found signs of fetal distress, do not simply think that fetal hypoxia, rash decision to terminate the pregnancy, but also to consider the following circumstances: 1, from the maternal causes to assess the presence of fetal hypoxia: pregnant women have hypertensive disorders of pregnancy, abnormalities of glucose metabolism in pregnancy, medical diseases, immune diseases, trauma, expired pregnancy, twin or multiple pregnancy-specific complications, High uterine tension, excessive or low amniotic fluid, unexplained vaginal bleeding, premature rupture of membranes >24h, intrauterine infection. 2.From fetal causes to assess whether there is fetal hypoxia: need to consider whether there is fetal growth restriction, severe fetal hemolysis, abnormal fetal position, fetal immaturity, umbilical cord placental abnormalities and serious birth defects. 3, certain drugs may affect the results of fetal heart monitoring: (1) magnesium sulfate: can reduce the amplitude of fetal heart acceleration, reduce the baseline variation of fetal heart rate, reduce the baseline level of fetal heart rate. (2) β2-agonists: increase fetal heart rate baseline, resulting in fetal tachycardia. (3) Glucocorticoids: betamethasone reduces baseline fetal heart rate variability. (4) Anesthetic drugs: reduce fetal heart rate variability and fetal heart rate acceleration response. In conclusion, there is no uniform standard for the diagnosis of fetal distress, and multiple indicators are more helpful than a single indicator to accurately determine the intrauterine status of the fetus, and at the same time, it is also necessary to combine with the high-risk factors of the pregnant woman, gestational weeks, and the progress of the labor process and other circumstances to make a comprehensive judgment. Perinatal workers should accurately identify true and false fetal distress in a timely manner in order to reduce perinatal mortality on the one hand and avoid excessive obstetric interventions on the other. Evidence quality grading in evidence-based medicine: Level I evidence refers to large-sample double-blind randomized controlledtrial (RCT) or meta-analysis of medium-sample RCTs yielding clinically relevant results with low false-positive and low false-negative errors in randomized studies; Level II evidence refers to small-sample RCTs, RCTs that are not blinded, RCTs that employ Level II evidence refers to small-sample RCTs, RCTs not using blinding, RCTs using validated surrogate markers; Level III evidence refers to non-randomized controlled studies, observational (cohort) studies, case-control studies, or cross-sectional studies; Level IV evidence refers to the opinions of expert committees or relevant authorities; and Level V evidence refers to the opinions of experts. The strength of recommendation is divided into 4 levels from A to D: A. Conclusions of level I clinical studies with consistent results; B. Conclusions of level II and III clinical studies with consistent results or inferences from level I clinical studies; C. Conclusions of level IV clinical studies or inferences from level II and III clinical studies; D. Conclusions of level V clinical studies or contradictory or inconclusive conclusions from multiple studies at any level.