Hello kalpit,
First of all, getting admission in mbbs is a huge success.
I would like to congratulate you for that.
And birth certificate is a proof of your age.
So the alternative proofs are your 10th and 12th marksheet.
So birth certificate is NOT necessary for your admission in mbbs. You can still keep it with you. But if you have misplaced it, Don't worry. It won't be a problem.
I hope this solves your concerns.
Best of luck!!