Brief Answer:
Yes with precautions

Detailed Answer:
Hello,
Thank you for your query.
I'm Dr. Shuba Hariprasad and will be answering your query today.

Yes, she can start to exercise.
I recommend :
-a follow up with her OBGYN to make sure her wound has healed well.
-Start with mild exercise and slowly increase in intensity over a period of 6-8weeks.
Avoid strenuous exercise as it may interfere with breast milk production.
-diet modifications is important as well. She will be Hungry frequently and needs to eat but has toto be healthy food.
- include more protein in diet (eggs, sprouts, lentils), fresh fruits and vegetables (well washed/cooked)
-avoid refined grains, processed and junk/fast foods). Include whole grains with natural fiber
-snacking also adds Weight. Instead of sweets and savories, give fruits or sprouts. Nuts are a healthy snack as well.
-regular meals(smaller but frequent) will keep away hunger pangs and avoid binging.

Yoga should be advised as for breastfeeding, post surgery woman by a certified instructor.
If she feels any pain, discomfortor unwanted chnages, she should stop immediately and consult her doctor.
Starting slow and gradually working up will avoid complications (muscle or bone injury).
She should also be on calcium and iron supplements as long as she is breast feeding. Talk to her OBGYN, if necessary.

Hope this helps.
Please let me know if there is anything else I can help you with.
Best wishes and wish you all good health.

Regards,
Dr. Shuba Hariprasad

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