Joining New Company - Company Letters

Hello All
I am currently serving my last week of notice period in my present organization and about to join a new firm next week . I already have the joining date and offer letter. I just happened to know I am 5 weeks pregnant. Should I inform my recruiter immediately or would it be sufficient to inform them on the day I join..? Is there anyway they will refuse to accept me ? My due is likely to be in October.
Kindly assist

Kindly note my location is Bangalore and shifting my job from a Us bank to Indian company

Your new employer is not asking for Medical Test?
You have to inform the employer in advance, I opine.
Pon