Are both words interchangeable? Rikai-chan defines them both as "baby, infant". So what would be the difference?
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In many contexts they are interchangeable, but strictly speaking 赤ん坊 is the actual word for baby, and 赤ちゃん is like a cute, pet name derived from it.
Also, 赤ん坊 is the spoken (colloquial) form - see sawa's comment for versions that may be more appropriate for formal writing.