Since Nick was our first child and I am a nervous nelly, I purchased this Angel Care Gentle Vibes Monitor. It tracks movement and goes at the bottom of the crib. If no movement is sensed, the alarm goes off. This has helped to give me some peace of mind, starting from when Nick began to sleep in his own room.
The item requires a pad to be placed under the baby's crib mattress that has a sensor in it. It can be a little sensitive. For instance, if our fan is on high enough of a speed, the sensor picks up the fan as movement. Make sure it works when the baby is not in the crib. Also, if the baby is down at one end of the crib, the alarm might go off. This happened a few times, and when you hear that alarm your heart races! Overall, we've had around 3 false alarms, but it has been a good thing to have.