A portable monitoring device for monitoring of the short term, mid term and long term risks of Sudden Infant Death Syndrome (SIDS) and to estimate the risk of onset of positional plagiocephaly. The monitoring device acquires one or more signals associated to the typical breath and positioning patterns of an infant through a tri-axial accelerometer, and registers the data in electronic format, where statistical analysis of the signal and pattern recognition are performed in real time by a processing unit. The device monitors the breath activity of an infant in various conditions, alerting a caregiver when a potentially dangerous situation is detected. The monitoring is attached to the clothes of the infant. Statistical analysis of the registered data is performed for quantifying the long term SIDS risk, the risk of plagiocephaly and for optimizing the functional parameters of the monitoring device.