Diastasis recti: 1 finger exercises?

Technically, a gap less than 2 or 2.5 fingers isn't really considered a diastasis recti. Nulliparous women (women who haven't had any kids) on average have a 1 finger width gap, which suggests that size is completely normal. Therefore, you can do any exercise that doesn't cause you to lose control of your abdomen (and pelvic floor). Losing control means your belly bulges outward rather than remains semi-braced during the specific exercise, whether it is crunches, planks, etc... Sit-ups specifically are not routinely recommended because of concerns about over flexion, and you'd need some seriously strong abs to stay braced during a sit-up. In fact, many people who technically can do one are still pooching their bellies without knowing it. If you are trying to increase abdominal strength, I'd recommend just general full body conditioning, which may or may not include crunches and planks, but definitely includes some form of resistance training. Hope that helps. If you have any more questions about diastasis recti, I write a lot about it at http://motherfigure.com (but again, at 1 finger width I wouldn't be worrying about the size of the gap).