Def don't do proper exercise until at least 6 weeks, Im surprised you don't get a check because your abs can come apart and not be back together which has happened to me so i saw a physio and wasn't allowed to do much exercise until a month ago when I got the all clear incase I got a hernia. Im a bit of an addict and worked out until the day I gave birth so was hard for me. Im on my phone so can't see how old your baby is but if you exercised while pregnant just take exercise the opposite way round so start with, so what you could do at 9 month pregnant do 6 weeks after birth and gradually work up. It takes a few weeks for your body to realise its not pregnant so your temp, heart rate and bp stay up for a while, and your ligaments are still very flexible so not as supportive. You should work on your core muscles first not the outer layer which sit ups and crunches work so things like pelvic floor and deep tummy stuff that you would do in pilates work best at knitting your tummy muscles back together then later you can add the sit ups in, 3 months is probably right if you do core stuff first that knits it back together. you have to be careful not to rush as your tummy muscles go through so much during pregnancy and you don't want to cause more damage or a hernia.
Hope that kind of helps! Walking is great to start with if that's what you have been doing x
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