Metropolitan Police tweeted that the girl called Epsom Hospital to say she was having a baby in a park or wooded area in Sutton, southwest London.
The teenager made the call at 7.40pm on Sunday.
More than three hours later officers had not been able to find her.
The force tweeted: “Police are trying to trace an unknown 14-yr-old girl who reported to Epsom Hospital at 7.40pm on Sunday that she has gone into labour in a park or wooded area in #Sutton.
“Officers have not been able to find the girl.
“Anyone who’s seen or heard anything is asked to call 999.”