This is for when not pregnant but thought I would share anyway.
(this might sound silly but..) metho works a treat. You put a little on a cotton ear bud and dab it on the area. It dries it out and stops it from developing any further. It also kills the good skin cells though so only use if really needed.

I have had them since I was 3, but hadn't had one for I think 1 year now, but today one shows up. Stupid thing.