Celeste Smith was last seen in Plymouth on Tuesday afternoon, but she was not reported missing until she failed to meet up with friends two days later.
Friends said the teenager, who lives in Saltash, Cornwall, had recently discovered she was pregnant.
Police are treating the search as a "high priority".
Miss Smith was last seen at 18:00 GMT on 31 January in the Ebrington area of Plymouth.
People from her hometown, Saltash, have organised a search of the area, handing out flyers to passers by.
Kirsty Howden, who helped set up the search party, described Miss Smith as "a young bubbly and intelligent individual" who was normally in constant contact with her friends on social media.
No-one has heard from the teenager since her disappearance.
Mrs Howden, who has known Miss Smith since she was at primary school, said she had gone missing before, but "for there to be a complete lack of communication is very unusual", adding that the teenager's mother, Julie Smith, was "beside herself".
Devon and Cornwall Police said officers were treating the teenager's disappearance as a "high priority" because family and friends were "very worried about her".
A search is under way for a 19-year-old girl who has been missing from her home for five days.