1. 80 days of minimum service is required for an employee to avail the maternity leave under the law. It is only if an employee works continuously for 80 days in the last 12 months immediately preceding the date of her expected delivery that she becomes entitled to the benefit of maternity leave. During maternity leave, leave salary equal to last pay drawn is admissible.
2. Since your child's delivery took place prior to you getting the appointment letter for the job and you have not even joined yet, you do not meet the necessary legal criteria of continuous employment for 80 days to avail the maternity leave benefit. Hence, you cannot get the maternity leave.