**5.5 Ultrasound features of the cervix shortening**

Bednarek et al. observed no statistically significant changes depending on the length of the cervix. Similar results were established by Klemm et al. analyzing uterine flows after radical trachelectomy. There were no changes in the uterine arterial resistance index in relation to the control group [58].

The three-dimensional evaluation of the cervical circulation accounts for a new diagnostic possibility. Samutchaikij et al. established reference values for certain measurements of the cervical vascular bed, and De Diego et al. analyzed threedimensional images of the cervix in patients at risk of preterm labor. The authors found differences in the parameters of cervical vascularization in patients with preterm labor in comparison to asymptomatic patients with a comparable length of the cervix. It is possible that three-dimensional ultrasound will become a practical tool for the actual assessment of cervical insufficiency [59, 60].

### **5.6 Pregnancy duration and vascular flows**

The study by Bednarek et al. involved the division of the study group into preterm labor below and above 32 weeks of pregnancy. Vascular flow differences were found in the umbilical artery. The pulsation, resistance and systolicdiastolic index values were higher in the younger group. Similar conclusions were presented by Chanprapaph et al. in the analysis of measurements in over three hundred healthy pregnant women. This phenomenon should be explained by the progressive increase in end-diastolic velocity with increasing gestational age, which directly translates into a decrease in the pulsation index. The authors draw attention to the fact that the value of the systolic-diastolic index above three, in a pregnancy above the thirtieth week, is more often associated with complications of low fetal body weight and birth disorders - the presence of meconium in the amniotic fluid, cesarean section and worse birth condition of the newborn [61–64].

Mari and Deter draw attention to the parabolic shape of the curve of changes in the flow rates of the central artery of the brain, the values of which are maintained in newborns until the first month of life. The curves established by the authors are applicable to the monitoring of fetuses with low body weight [65].

Degani found a clear decrease in the value of the middle cerebral artery pulsation index after the thirty-sixth week of pregnancy, which is related to the compensation mechanism that protects the fetus against a progressive decrease in oxygen tension [66].

Gadelha da Costa et al. assessed the increase in fetal middle cerebral artery resistance index up to the twenty-sixth week of pregnancy, and then a decrease to the period of full-term pregnancy [67, 68].

The lack of vascular changes in the uterine arteries in pregnant women with pre-term labor confirms the assumption that the assessment of the flow of these vessels is justified in pregnancies with placental abnormalities, such as intrauterine growth restriction, arterial hypertension or diabetes [69, 70].
