Fetal and uteroplacental circulatory changes in pregnancies proceeding beyond 43 weeks

Practically all reference curves for Doppler blood flow variables in pregnancy end at 40 weeks.

To study fetal and uteroplacental circulatory changes in pregnancies proceeding beyond 43 weeks.

Longitudinal comparisons from 42 to 43 weeks and comparisons with controls at term.

Thirty-four women delivered after 43 completed weeks (301 days) and a control group of 32 women delivered at 271 days of gestation.

From 42 weeks (294 days), serial Doppler blood flow measurements were performed in the fetal descending aorta, umbilical vessels, and maternal uterine artery.

Student's unpaired t-test, Mann-Whitney U-test, and Fisher's exact test for comparison of groups; Student's paired t-test and Wilcoxon's signed-rank matched-pairs test for longitudinal data; simple linear regression analysis for comparison of variables. A two-tailed P-value of < 0.05 was considered statistically significant.

From 42 to 43 weeks, the mean flow velocity decreased significantly in the fetal aorta and increased significantly in the umbilical vein. Compared to controls, the velocity and volume flow in the aorta and the umbilical artery flow resistance were significantly lower at 43 weeks. No significant change in uterine artery flow resistance was found.

The changes in Doppler blood flow variables were all in accordance with physiological circulatory alterations enhancing continued fetal growth until the late post-term period. There were no signs of any general circulatory deterioration. Doppler velocimetry reference values at term may not be appropriate for evaluation of very advanced gestations.