Color Doppler ultrasound is the main non-invasive method for diagnosing fetal hypoxia and predicting fetal death. At present, color Doppler ultrasound reflects the abnormalities of various organs of the fetus by measuring uterine artery blood flow, fetal hemodynamic indexes such as umbilical artery, middle cerebral artery, intravenous catheter, renal artery, and intra-abdominal umbilical vein; Use the above indicators to jointly predict fetal prognosis, such as brain-placental blood flow ratio. Based on the above indicators, the author will briefly describe the role of color Doppler ultrasound in predicting stillbirth.

 

01 Maternal hemodynamics monitoring of pregnant women

 

Uterine arteries in normal women of childbearing age showed high resistance and low diastolic blood flow during non-pregnancy and early pregnancy. Pulse Doppler showed a unimodal spectrum of systole and a bimodal spectrum during diastole. After 14 to 16 weeks of gestation, due to the need for fetal growth and development, the uterine artery "vascular recast", high resistance blood flow gradually becomes low resistance and has a very rich diastolic blood flow, systolic peak blood flow velocity / The diastolic blood flow velocity (S/D), uterine arterial resistance index (RI) and pulsatility index (PI) values ​​all decreased.

    

Uterine arteries RI and PI are sensitive indicators of uterine arterial resistance. Throughout pregnancy, increased blood flow to the uteroplacenta bed is positively associated with increased gestational age and fetal weight. By the third trimester, endometrial blood flow is increased compared with early pregnancy. 10 to 20 times to ensure the fetal oxygen and nutrient uptake and metabolic balance. In the normal third trimester, uterine arterial resistance decreased, and both RI and PI decreased. When the placenta is abnormal (such as shallow placenta, etc.), uterine “vascular recast” obstacles, resulting in increased uterine artery resistance, placental insufficiency, tissue damage caused by changes in ischemia and hypoxia, triggering hypertensive disorders during pregnancy , secondary fetal growth restriction, fetal distress, stillbirth and a series of pathological changes. Iacovella et al. screened the uterine artery RI values ​​of 9859 cases of single-pregnant early pregnancy and defined the RI resistance of the uterine artery when the RI value was higher than 90% of the normal pregnant women; the results showed that the RI value of the stillbirth and early pregnancy uterine artery. There was a correlation between the higher uterine artery RI values ​​after 34 weeks of gestation was a risk factor for delayed fetal death, but not with early-onset stillbirths. Poon et al. monitored gestational uterine arterial PI in 65 819 singletons at 20 to 24 weeks of pregnancy, and 306 (52.0%) of 306 (0.46%) stillbirths were preeclampsia (PE), placental abruption, or gestational age (SGA).

 

In cases of stillbirth, the uterine artery PI MoM values ​​were significantly higher than live births and were negatively correlated with birth weight. Among the patients who delivered less than 32 weeks of gestation, 80.6% of those who died of stillbirth combined with preeclampsia, placental abruption, and uterine arteries had a MoM value of more than 90 percentile in the SGA, 41.9% of those who delivered 33 to 36 weeks' gestation, and 37 weeks or more. There is 31.4%. The associated percentages of unmarried PE, placental abruption, and SGA in stillbirths less than 32 weeks, 33 to 36 weeks, and more than 37 weeks of delivery were 15.8%, 25%, and 12.4%, respectively. This study suggests that uterine arterial PI in the second trimester is significant for recognizing early fetal deaths associated with preeclampsia, placental abruption, and SGA, and does not include stillbirths without preeclampsia, placental abruption, or SGA. Khalil et al.'s study suggests that late pregnancy uterine artery Doppler has no obvious advantage in predicting stillbirth and perinatal mortality. In summary, there is a correlation between the RI value of uterine artery in early pregnancy and stillbirth. Reasonable interpretation is that due to the increased RI value of the uterine artery, placental blood perfusion is insufficient, and tissue ischemic and hypoxic, which leads to fetal hypoxia and fetus. Growth restriction and stillbirth occur; and uterine arterial PI in mid-pregnancy is significant for identifying early stage preeclampsia, placental abruption, and SGA-related stillbirth.

 

 

02 Fetal hemodynamic monitoring

 

The fetal arterial hemodynamic index includes S/D values, RI, and PI, which reflect the peripheral circulation resistance and blood flow perfusion of the supplied blood vessels.

 

2.1 Fetal Umbilical Cord The umbilical cord is an important channel for mother-to-child exchanging. The umbilical vein transports oxygen and nutrients through the placental tissue to the fetus and fetal metabolic waste is transported through the umbilical artery to the placenta. Therefore, the umbilical artery directly reflects the fetus-placenta. The abnormal changes in the circulation reflect the accumulation of fetal metabolic substances. The accumulation of fetal metabolites can cause intrauterine hypoxia and acidosis, which in turn leads to fetal growth restriction and even fetal death. The most commonly used in clinical practice is the S/D value. At present, there is no uniform reference standard value. Thuring et al.'s study showed that under normal pregnancy, the umbilical arterial blood flow waveforms did not change much before the 22nd week of pregnancy. The blood flow parameters of the umbilical arteries peaked at 28 to 32 weeks and then decreased. The S/D value was <3.0 for 30 weeks of pregnancy.

 

Normally, drop to 2.2 at 40 weeks of pregnancy. Studies have found that when fetal ischemia and hypoxia, fetal-placental circulation disorders, increased blood flow resistance, umbilical artery blood flow parameters increased; if ischemia and hypoxia further increase, there will be a lack of end-diastolic blood flow, Or reverse blood flow, the above conditions are more predictive of poor pregnancy outcomes. It is currently believed that for high-risk pregnancies, umbilical artery Doppler ultrasound monitoring can reduce perinatal mortality. However, maternal respiration, heart rate, body temperature, fetal respiration, heart rate, and fetal movement can affect the measurement of umbilical arterial blood flow parameters, so its assessment of fetal intrauterine hypoxia is unstable. However, for low-risk, normal-developed fetuses, perinatal mortality cannot be reduced. Therefore, routine monitoring of umbilical artery blood flow in pregnant women with normal pregnancy is not recommended.

 

 

2.2 Fetal midbrain artery (MCA) The fetal brain is most sensitive to hypoxia, and the middle cerebral artery is the blood vessel with the heaviest blood supply in the hemisphere. When the fetus is distressed, the surrounding blood vessels contract, the resistance increases, the cerebral blood vessels compensate for expansion, and the resistance decreases. Increased blood flow to ensure brain blood supply, called "brain protection effect." It has been reported that when the resistance index of mild anoxia fetal umbilical artery is still in the normal range, the impedance index of MCA is obviously reduced. Therefore, MCA has important clinical value in predicting fetal intrauterine hypoxia. However, there are also studies suggesting that a decrease in the PI value of the uterine artery has clinical value in predicting adverse pregnancy outcomes during the 28 to 32 weeks of pregnancy, but a decrease in the PI value after 34 weeks of gestation has no clinical significance. Hirshberg et al.'s study showed that if the fetal hypoxia further aggravates, the blood flow index of the middle cerebral artery decreases to the lowest point and then rises, and even the middle cerebral artery diastolic phase is lost, even a biphasic change. Therefore, I do not advocate the use of MCA indicators alone to determine fetal distress, and only based on the level of MCA blood flow indicators to assess the severity of intrauterine hypoxia is not scientific, it is recommended to jointly evaluate other blood flow indicators.

 

2.3 Fetal venous catheter and fetal intra-abdominal umbilical vein venous catheter originated in the umbilicus-portal sinus, is a small vein connecting the umbilical vein and the inferior vena cava in the abdominal cavity, terminating in the inferior vena cava into the right atrium, directly reflecting the fetal right atrium The pressure plays an important role in ensuring that the fetal brain tissue and myocardial blood supply and oxygen supply; therefore, the venous catheter blood flow parameters reflect the fetal right heart function.

 

Some scholars have suggested that the venous catheter blood flow spectrum is an important indicator reflecting the good condition of the fetus and predicting the fetal survival rate after birth. Carvalho et al. found that the increase in the rate of venous catheter shunt reflects the poor intrauterine oxygen supply status. When the fetal venous catheter does not have the blood flow signal during Doppler systole or reverse blood flow signal occurs, stillbirth occurs within 1 week. The risk is significantly increased, and the specificity of fetal death within 1 week after prediction is up to 80%, and the sensitivity can be as high as 100%. Lausman et al.'s study also pointed out that when the fetal venous catheter Doppler systolic peak decline, disappear or reflux, more prompt the fetal right heart function significantly worsened, hypoxia is in a decompensated state, if not timely intervention, then bad The incidence of pregnancy outcomes increased significantly. However, since the venous Doppler flow parameters are influenced by external factors (such as the change in posture of pregnant women, etc.), there is no clinically accepted standard value so far, and because it is not sensitive to the early state of fetal mild hypoxia, it is in the early stage. The clinical significance of predicting stillbirth is not significant.

    

The intra-abdominal umbilical vein is the main channel for the delivery of oxygen and nutrients to the fetus. Morales-Roselló et al. found that when right heart failure occurs due to intrauterine hypoxia-induced cardiac insufficiency, fetal intra-abdominal umbilical vein pulsatile signs presage that fetal hypoxia has caused irreversible damage; however, it predicts stillbirth. The disadvantages are also like intravenous catheters, which cannot predict early hypoxia and its harm.

    

In addition, some scholars have proposed a combination of multiple hemodynamic parameters to assess and predict fetal prognosis, such as the use of brain-placenta ratio (CPR) that is middle cerebral artery PI / umbilical artery PI. Bakalis et al detected the CPR of 30 780 pregnant women at 30 to 34 weeks' gestation. The results suggest that CPR is closely related to adverse pregnancy outcomes, and the incidence of adverse pregnancy outcomes is estimated to be 5% to 11%. If high-risk pregnant women are measured The incidence of up to 20% to 50%. The results of Khalil et al. further suggested that when the uterine artery PI >1.5 MoM and CPR <0.7 MoM, short-term stillbirths or poor neonatal outcomes usually occur; Khalil et al. believe that decreased CPR suggests insufficient placental perfusion and vascular endothelium Because of the damage of the organization, it can be used as an independent predictor of stillbirth. However, some scholars believe that CPR is useful for predicting fetal adverse pregnancy outcomes 34 weeks before pregnancy but cannot predict adverse outcomes of full-term pregnancy.

     

In summary, color Doppler ultrasound in the assessment of intrauterine fetal status, predicting that there is no single hemodynamic index on the stillbirth can objectively and truly reflect the fetal intrauterine status, will be pregnant women's uterine artery blood flow, fetal artery and Combined venous flow spectrum and dynamic analysis make sense. Increased uterine artery RI during early pregnancy is associated with poor placental development, which can lead to fetal distress, fetal growth restriction, and stillbirth. The increase in uterine artery PI during the second trimester is significant for recognizing early pregnancy related to preeclampsia, placental abruption, and SGA. . When fetal intrauterine hypoxia occurs, the umbilical artery index increases, and the middle cerebral artery index decreases; if the hypoxia state further aggravates, there will be a lack of end-diastolic blood flow in the arterial and venous catheters, and even reverse blood flow and umbilical veins The blood flow pulsates. At this time, the fetus is in a state of severe ischemia and hypoxia and acidosis. It indicates that the fetus is severely hypoxic and may cause irreversible brain damage. At this time, the clinic should immediately intervene to avoid the occurrence of stillbirth.