decreased variability. These findings are more consistent with hypoxemia. Some fetuses may display a bradycardic FHR but be completely normal. It should be remembered that the range of 110-160 does not
represent all normal fetuses. The likelihood of a FHR in the range of 100-110 representing a normal variant increases as the fetus, and its nervous system, matures.Early DecelerationIn association with a uterine contraction, a visually apparent, gradual (onset to nadir 30 sec or more) decrease in FHR with return to baselineNadir of the deceleration occurs at the same time as the peak of the contraction (NICHD)An early deceleration and a late deceleration may visually appear identical. Both are smooth and curvilinear and appear to be a mirror image of the contraction. The distinction between the two is based upon the relationship of the deceleration to the uterine contraction (UC).Early decelerations correspond, temporally, to the contraction and therefore exist only as a periodic change. Early decelerations are a benign finding caused by a vasovagal response as a result of fetal head compression by the contraction. Pressure on the fetal skull alters the cerebral blood flow and this in turn stimulates the vagus nerve. The heart rate is gradually decreased as the pressure of the contraction intensifies, and the
deceleration gradually resolves as the pressure resolves. This pattern is generally limited to the active stage of labor. If the pattern is found in early labor, it may be associated with cephalopelvic disproportion (CPD).Early decelerations have not been associated with fetal hypoxemia or acidosis.Late DecelerationIn association with a uterine contraction, a visually apparent, gradual (onset to nadir 30 sec or more) decrease in FHR with return to baselineOnset, nadir, and recovery of the deceleration occur after the beginning, peak, and end of the contraction, respectively (NICHD)Though late decelerations share the same morphologic shape as early decelerations, they tend to appear late, after the onset and nadir of the contraction.Two varieties of late decelerations have been described, reflex and nonreflex. Reflex late decelerations are those which occur in the presence of normal FHR variability, whereas non-reflex late decelerations occur in association with diminished or absent FHR variability.The classically described cause of late decelerations is uteroplacental insufficiency (UPI). In UPI, there may be aproblem with a uterine perfusion or uterine activity or there may be a problem with the placenta, or both.
Uterine hyperactivity is associated with decreased time for intervillous space blood flow between uterine contractions. Similarly, maternal hypotension, even in the presence of normal uterine activity, will result in decreased volume of the intervillous space blood flow. Either of these factors may lead to a decrease in the maternal-fetal oxygen transfer.