Exactly, there are a lot of requirements for this to succeed, both having the app installed, bluetooth on, location services on (does it work on the tube?) and being within a short distance of each other. I think the money spent creating this app would have been more useful going straight to the charity.

Pregnant women may well pay for the app, but I think the only reason others would pay that much, for an app that will probably never actually alert them, would be to donate money to charity.