A lot of people wear a Fitbit or similar wearable technology to keep track of their health. The models that track heart rate are also good for pinpointing a moment of heartbreak or in this case, discovering a pregnancy.
David Trinidad was just looking for a solution to his wife's potentially malfunctioning sensors when he posted about it on Reddit.
Another user suggested that she could be under a lot of stress or she could be pregnant. Since Trinidad and his wife were trying to start a family they got it checked out and later he confirmed that was the case.