This unusual and uncommon FHR pattern has the appearance of a sine wave, with a cyclicity of three to five cycles per minute.

This pattern must persist for at least 20 minutes to qualify as a sinusoidal pattern.

With the classical sine wave pattern, there are no additional accelerations or decelerations. Variability is usually reduced, and sometimes absent.

The precise cause is unknown, but associations have been shown between this pattern and:

• Fetal anemia
• Fetal hypoxemia
• Maternal narcotic administration

If a sinusoidal pattern cannot be resolved with conventional measures, expedited delivery is usually undertaken.