When can I start exercises to strengthen my core muscles post pregnancy?

Generally, it's best to wait until about 6 weeks post delivery (assuming you had normal delivery) because although you may seemed that you're healed on the outside, your insides may not be completely healed yet. However, you can do light exercise within days of your delivery like walking, stretching and modified push ups. Please do not overdo it and it is much more important for you to fully recover. Alternative, use your baby as "weight". It's what I do at home, and it helps since I have such a clingy koala for a baby.