Does anyone know how to prevent it? I'm 27 weeks with my first and my belly looks like a triangle when I try to sit up. 

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But it definitely works. Back on topic: I tried to get up to take care of another kid the other night and I felt a weird ripping sensation. I mentioned it to the midwife and she said the ripping sensation could very well be ripping, and you should not sit up from a laying down position without rolling over first. It could make that diastasis a lot harder (or impossible) to fix later.
but that was not a pleasant feeling at all and it's ripe in my mind. That triangle thing sure is weird though, I have noticed it and I usually cure it by looking in a different direction. 