Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call emergency services if the baby is not breathing, turns blue or gray, becomes limp or unresponsive, or struggles severely to breathe.
Contact the baby's clinician promptly for coughing or color changes during feeding, unusual sleepiness, worsening breathing, poor feeding, fever, or events recorded by prescribed monitoring.
Follow-up may involve a pediatrician, neonatal follow-up program, eye or lung specialist, and developmental services.
The article also says
Before discharge, families should understand what the NICU considers a significant event, how long the baby has been event-free, and whether feeding-related events have been resolved.
Observation requirements vary by gestational age, event severity, caffeine use, and hospital policy.
Follow safe-sleep guidance for every sleep:
Place the baby alone on the back on a firm, flat sleep surface.
Keep pillows, loose blankets, positioners, and soft objects out of the sleep area.
Use prescribed oxygen or other equipment exactly as instructed.
Do not substitute a consumer breathing monitor for safe sleep or medical care.
Parents and regular caregivers should consider infant CPR training before discharge.
Families using prescribed oxygen, a medical monitor, or other equipment need hands-on training and clear instructions about alarms, equipment problems, and whom to call.
Every sign above is quoted from the sofidex article on Apnea of prematurity. If you are not sure, err upward — it is always reasonable to be checked.