I understand that by "our company gives appraisal in April," you mean that you provide a salary increment in April based on the performance of the employees in the preceding 12 months. If an employee is proceeding on leave, whether maternity leave or otherwise, you should have a record of her performance as evidenced by a performance appraisal, correct? Based on that, you can award an increment to her starting from April and begin paying the revised salary during her leave period.
If you choose to pay the revised salary once she rejoins work, there is nothing wrong with that approach. Similarly, if you have an incremental pay scale that guarantees an increment, it should be paid. Most private establishments do not follow incremental pay scales but instead provide increments based on performance appraisal. In such a scenario, you can withhold the increment until she resumes work. In the case of maternity leave extending to six months, it is acceptable to delay the payment. Additionally, section 5(1) of the Maternity Benefits Act stipulates that a woman employee should receive the same salary as she was receiving just before proceeding on leave. Therefore, the salary of March would meet the legal requirements.