What is the cause of Late deceleration on an NST?

Acceleration refers to an increase in fetal hea rate above baseline and deceleration to a decrease below baseline rate. The nomenclature most commonly used in the United States is based upon the timing of the deceleration in relation to contractions--thus, early, late or variable in onset related to the corresponding uterine contraction. The waveform of these deceleration is also significant for pattern recognition. In early and Late deceleration, the slope of fetal hea rate change is gradual, resulting in a curvilinear and uniform or symmetrical waveform. With variable deceleration, the slope of fetal hea rate change is abrupt and erratic, giving the waveform a jagged appearance. It has been proposed that deceleration be defined as recurrent if they occur with 50 percent or more of contractions in any 20-minute period (NICHD Research Planning Workshop, 1997). Another system now used less often for description of deceleration is based on the pathophysiological events considered most likely to cause the pattern. In this system, early deceleration are termed head compression, Late deceleration are termed uteroplacental insufficiency, and variable deceleration become cord compression patterns. The nomenclature of type I (early), type II (late), and type III (variable) "dips" proposed by Caldeyro-Barcia and co- workers (1973) is not used in the United States.