Abnormal umbilical blood flow

The pregnancy period is the most important period for women, but various problems are likely to occur at any stage of pregnancy. For example, in the final period before delivery, they often have malposition of the fetus. Or if an unusual or rare situation occurs, when a pregnant woman is approaching delivery, you should pay attention to her physical reactions and changes in the fetus. What is abnormal umbilical cord blood flow?

Umbilical blood flow refers to the ratio of the highest blood flow velocity during systole to the lowest blood flow velocity during relaxation of the umbilical artery, and indicators such as the yin force index and pulsatility index are used to reflect the blood flow situation. Checking umbilical cord blood flow is to determine the development of the fetus in the uterus.

The purpose of checking the umbilical cord blood flow is to determine the development of the fetus in the uterus, such as whether there is intrauterine growth retardation, whether there is a tendency to develop pregnancy-induced hypertension syndrome, and whether there is intrauterine fetal hypoxia. Umbilical cord blood flow may sometimes show abnormalities due to fetal chromosomal abnormalities, congenital malformations, etc. Abnormal umbilical cord blood flow may also be related to developmental defects and histological abnormalities of the placenta. The normal value is related to the number of weeks of pregnancy. There are three main items, with the S/D value as the main indicator. The normal value is usually an S/D value of less than or equal to 3.0 in the late pregnancy.

Umbilical cord blood flow is closely related to gestational age. Generally, as gestational age increases, umbilical cord blood flow shows a downward trend. The above table shows the umbilical cord blood flow parameters of full-term pregnant women awaiting delivery, which are divided into normal, loosely coiled and tightly coiled types. For the tight-wrap type, cesarean section is recommended to avoid suffocation during delivery.

The normal value of umbilical cord blood flow at 24 weeks is an average of 3.5, with an upper limit of 4.25. Values ​​exceeding this value are abnormal. The pulsatility index (PI) was 1.12±0.17, and the resistance index (RI) was 0.66±0.07. After 30 weeks of pregnancy, fetal intrauterine distress often occurs when the SV/DV of the umbilical artery is>3.0, PI>1.7, and RI>0.7. The peak systolic value (A or S) and the trough value at the end of diastole (B or D) of the arterial blood flow are used to calculate the S/D or A/B ratio. At the same time, the resistance index (RI=AB/A) and pulsatility index (PI=AB/mean A, B) can also be calculated.

In a normal pregnancy, the above values ​​decrease with the gestational age. Usually A/B>4 before 24 weeks, A/B is less than 3.0 after 30 weeks, and RI is less than 0.68. The ratio of the maximum systolic blood flow velocity (S) to the end diastolic blood flow velocity (D) of the umbilical artery blood flow (S/D, A/B) can be observed to judge the development of the placenta by observing the dynamic changes of the S/D (A/B) ratio.

Under normal circumstances, as the fetus develops, the placenta gradually increases in size, vascular resistance gradually decreases, and the S/D (A/B) ratio gradually decreases. From 25 to 41 weeks of pregnancy, the S/D ratio decreased almost linearly from 2.8 to 2.2. The fetus has intrauterine growth retardation, the placenta and fetus are poorly developed, the placenta does not increase in size, and vascular resistance does not decrease. Therefore, the S/D ratio does not decrease.

Clinically, if S/D does not decrease regularly or even increases, it indicates that the fetus is maldeveloped. In normal pregnancy, the S/a ratio of the ductus venosus decreases with increasing gestational age, from about 3 at 14 weeks of gestation to about 2 at 42 weeks of gestation. The S/a ratio of the ductus venosus is related to fetal hypoxemia.

The umbilical artery value (umbilical artery blood flow impedance) reflects the oxygen delivery in the placenta. If this value rises abnormally, the main consequence is fetal intrauterine hypoxia.