Maternal blood viscosity and uteroplacental blood flow velocity waveforms in normal and complicated pregnancies

The role of the placenta in spontaneous preterm labor and delivery with intact membranes

OBJECTIVES

To determine whether placental vascular pathology and impaired placental exchange due to maturational defects are involved in the etiology of spontaneous preterm labor and delivery in cases without histologic acute chorioamnionitis.

METHODS

This was a retrospective, observational study. Cases included pregnancies that resulted in spontaneous preterm labor and delivery (<37 weeks), whereas uncomplicated pregnancies that delivered fetuses at term (≥37-42 weeks of gestation) were selected as controls. Placental histological diagnoses were classified into three groups: lesions of maternal vascular malperfusion, lesions of fetal vascular malperfusion, and placental microvasculopathy, and the frequency of each type of lesion in cases and controls was compared. Moreover, we specifically searched for villous maturational abnormalities in cases and controls. Doppler velocimetry of the umbilical and uterine arteries were performed in a subset of patients.

RESULTS

There were 184 cases and 2471 controls, of which 95 and 1178 had Doppler studies, respectively. The frequency of lesions of maternal vascular malperfusion was greater in the placentas of patients with preterm labor than in the control group [14.1% (26/184) vs. 8.8% (217/2471) (p=0.023)]. Disorders of villous maturation were more frequent in the group with preterm labor than in the control group: 41.1% (39/95) [delayed villous maturation in 31.6% (30/95) vs. 2.5% (13/519) in controls and accelerated villous maturation in 9.5% (9/95) vs. none in controls].

CONCLUSIONS

Maturational defects of placental villi were associated with approximately 41% of cases of unexplained spontaneous preterm labor and delivery without acute inflammatory lesions of the placenta and with delivery of appropriate-for-gestational-age fetuses.

Third Trimester Umbilical Artery Doppler in Low-Risk Pregnancies and its Correlation to Estimated Fetal Weight and Birthweight

PURPOSE

 This study investigated the correlation between the umbilical artery (UA) pulsatility index (PI) and the estimated fetal weight percentile and birthweight (BW) percentile, respectively.

MATERIALS AND METHODS

 We included low-risk pregnancies, in which UA Doppler investigations after 28 weeks were performed. Cases were allocated according to BW percentiles: small for gestational age (SGA) with BW < 10^th percentile; appropriate for gestational age (AGA) and large for gestational age (LGA) with BW > 90^th percentile. We analyzed differences in the mean UA-PI and UA-PI z-score for gestational age according to the three groups. Linear regression was performed to evaluate any relationship between Doppler indices and BW percentiles. Multiple logistic regression analysis was performed to determine the independent association of UA-PI with LGA babies. In a second step, we considered data on estimated fetal weight (EFW) percentiles and performed the same analysis.

RESULTS

 We analyzed 14 554 pregnancies from 2004 to 2015. The mean UA-PI and mean UA-PI z-scores in the LGA group were lower than in the AGA and SGA groups (p < 0.001). UA-PI and UA-PI z-scores were linearly related to birthweight percentiles (p < 0.001) and to EFW percentiles (p < 0.001). Logistic regression analysis showed that low UA-PI was independently associated with neonatal LGA (p < 0.001).

CONCLUSION

 The higher the EFW and BW percentiles, the lower the UA-PI. However, reference ranges for UA Doppler are only based on gestation weeks. Further studies are needed to clarify whether customized reference ranges based on EFW percentiles are more appropriate for the evaluation of fetal wellbeing in the third trimester.

Blood transfusion in pregnancies complicated by maternal sickle cell disease. Effects on blood rheology and uteroplacental Doppler velocimetry

Doppler ultrasound gives a measure of downstream resistance to blood flow. In this study it has been used to assess the change in placental vascular resistance following blood transfusion in pregnancies complicated by sickle cell disease. Eight women with homozygous sickle cell disease were followed prospectively to assess the effect of transfusion on haemoglobin S percentage, blood and plasma viscosity and uteroplacental Doppler velocimetry. Measurements were made immediately pre and post-transfusion and after several weeks when the desired improvement in haemoglobin S had been achieved. Despite significant changes in haemoglobin S levels and consequent rheological change there was found to be no change in measurements of resistance in the uteroplacental circulation as assessed by Doppler ultrasound.

Blood viscosity and uterine artery flow velocity waveforms in pregnancy: a longitudinal study

In the course of pregnancy, whole blood viscosity (WBV) decreases, plasma viscosity (PV) increases and uteroplacental resistance to blood flow falls. According to the Hagen-Poiseuille law, viscosity influences resistance to blood flow. To investigate the contribution of these haemorheological changes to the decrease in resistance to blood flow in the uteroplacental circulation, a serial study was performed. Using blood viscosimetry and pulsed Doppler ultrasound recordings of the uterine arteries, 21 women with normal pregnancies were studied every 8 weeks from 12 weeks onwards. Statistical analysis revealed no significant relationship between WBV, PV and pulsatility index of the uterine arteries. It is concluded that the changes in blood viscosities are of minor influence on resistance to blood flow in the uteroplacental circulation throughout normal pregnancy. The vascular changes e.g. the considerable increase of the diameter of the spiral arteries have a much greater impact on resistance to blood flow.

The current status of maternal and fetal blood flow velocimetry

Significant progress has been achieved in regard to normative data for uteroplacental and fetal blood flow velocity and the factors which may affect these observations. Associations have been demonstrated between abnormal indices of maternal and fetal blood flow velocity and pregnancy-induced hypertension, fetal growth retardation, fetal blood gas measures, and intrapartum clinical fetal distress. However, in general, the predictive power is modest and no evidence has yet been provided to indicate that these measures can be used as a screening test in the general obstetric population. Continuing research is highly desirable. At the present time, the use of Doppler assessment of blood flow in pregnancy should be limited to the investigational setting.

Correlation between Doppler flow velocity waveforms and cord blood viscosity

The contribution of rheological factors to the impedance of blood flow in the umbilical artery as determined by continuous-wave Doppler ultrasound was investigated. Of the 51 pregnancies recruited, six were complicated by pre-eclampsia, 10 by intrauterine growth retardation, 15 by both pre-eclampsia and fetal growth retardation, and there were 20 controls. A significant correlation was demonstrated between both plasma viscosity and gestational age and the resistance index used to characterize the Doppler waveform. Multiple regression analysis demonstrated that changes in plasma viscosity explained 55% of the variance seen in the resistance index and that this was statistically significant. This finding is, however, unlikely to be of clinical significance as whole blood viscosity had an insignificant effect on the impedance in the umbilical artery. We postulate that vascular factors such as the number and calibre of the placental arterioles are a more important determinant of umbilical artery impedance.