Maternal bearing down efforts--another fetal risk?

A prospective study of 12 unmedicated pregnant women in the second stage of labor was undertaken to assess the effects of bearing down efforts on brachial and popliteal blood pressure. Inferior vena cava compression, usually detected by reductions in mean and/or pulse pressure in the brachial artery, did not occur at any time, whether or not the uterus was displaced to the left. Abdominal aortic compression, suggested by popliteal pressure fall, was maintained into the second stage of labor, was aggravated when the women bore down, and was alleviated only when bearing down efforts ceased and the uterus was displaced to the left. Bearing down efforts are therefore yet another mechanism by which distal aortic and possibly uteroplacental blood flow can be prejudiced. Their avoidance can be advantageous to the fetus at risk.