Can a baby stay in a stomach for 4 months?

I'm not sure what you're asking but here are some answers to help you:

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All premature babies are small, require complex medical care, and may face serious complications both in the NICU and at home. A baby born 3 to 4 months early, though, will face far different complications from a baby born 1 to 2 months early.

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By all means, let your sleeping baby sleep. Once babies learn to roll over onto their tummies, a milestone that typically happens between 4 and 6 months but can be as early as 3 months, there's usually no turning them back (especially if they prefer snoozing belly-down)

I hope this helped :)