Princess Bubblegum's heart monitor is an invention used to find Finn, Jake and the Ice King in the episode "Lady & Peebles." The monitor reveals that Lady is pregnant by showing five smaller heart-beat signals next to Lady's signal. This is how Princess Bubblegum finds out Lady Rainicorn is pregnant. It is also used to record diary logs and provide a map of the user's current location.
The heart monitor is contained in a brown casing and has two buttons on the left side used to activate the monitor. The monitor is similar to a GPS device, displaying a graph and points representing the heartbeats detected in the vicinity.