Already posted in Behaviour, but realised Sleep would be more appropriate...
DD (17 weeks) used to go to bed at 7pm (bedtime routine, and generally asleep by 7.30). For the last week or so, she has screamed and screamed for two or three hours until she falls asleep from exhaustion (well, obvs, but YSWIM!). It's distressing for her and us, and seems to have come out of nowhere. No changes to her bedtime routine, or anything else. She won't be cuddled, fed, held, left. We have tried to resist giving up and bringing her downstairs, but maybe this is the solution - does she just want a later bedtime?! Once asleep, she usually sleeps til 4am and then straight back asleep until 7.30 or 8, so once she's down, things are fine (for her age!). Is this just the way that sleep regression is manifesting itself?