Cardiovascular dynamics in relation to presentation and postural changes in normal fetuses

The role of fetal cardiovascular function in the reduced intrauterine growth rate of breech pregnancy was studied in fetuses with different presentations. Hemodynamic responses to postural changes were also investigated in fetuses in horizontal and vertical positions. Thirty-one normal pregnancies, with 15 fetuses presenting by the breech and 16 fetuses by the vertex, were included in the study. Diameter pulse waves from the fetal descending aorta were measured using equipment combining real-time ultrasound imaging with phase-locked echo-tracking. No differences in basal pulse wave parameters were demonstrated between breech and vertex fetuses nor did any differences appear when maternal posture was changed from horizontal to upright or vice versa. The present observations suggest that fetal circulatory factors are less likely to be a primary cause of the reduced intrauterine growth in breech presentation. Tilting of the fetus in late gestation seems to produce neither a redistribution of its blood volume nor secondary adaptations to an orthostatic challenge. The hydrostatic effect of amniotic fluid presumably precludes the requirement of fetal circulatory adaptation to postural changes.