The sinusoidal pattern is a fetal heart rate pattern that is a sign the baby is in great danger. It is a stable baseline
heart rate that has no beat-to-beat variability. So what you see is a smooth FHR tracing. It is associated with high rates of fetal death. The primary cause of a sinusoidal pattern is anemia. If the fetus had a blood loss, you might see a sinusoidal heart rate. (It is sometimes confused with a sawtooth pattern which is troubling but less ominous.)