A woman at 42 weeks undergoes induction with misoprostol. Twelve hours later, cardiotocography shows a sinusoidal pattern with a frequency of 3-5 cycles per minute and amplitude of 10-15 bpm. The most appropriate next step is:

• A Continue monitoring as it is a benign fetal sleep pattern
• B Administer naloxone to the mother
• C Prepare for urgent delivery ✓
• D Perform fetal scalp stimulation

Explanation

A true sinusoidal pattern (regular, smooth, undulating sine wave with fixed periodicity of 3-5 cycles/min and amplitude 10-15 bpm, absent variability, no response to stimulation) indicates severe fetal anemia or hypoxia and mandates urgent delivery. It must be distinguished from a pseudosinusoidal pattern, which is benign and associated with fetal sleep or opiates. In a post-term fetus at 42 weeks with a sinusoidal trace, delivery should not be delayed for scalp stimulation.

Reference: Williams Obstetrics / RCOG Green-top Guidelines, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.