Though an increment in a scale of pay is a right, it is not a right in an open salary structure where there is no increment offered.
Understanding Pay Scales
To explain, you should have seen scales of pay like 10000-200-11000-300-12500-500-17500. In this scale, a beginner shall get a basic salary of Rs 10,000. Every year, he will earn an increment of Rs 200; i.e., one year after joining, his salary will become 10,200, the second year it will become 10,400, and so on. In five years, it will become 11,000, and thereafter for each year, the increment will be Rs 300. This is a time scale, and the increment is guaranteed. If you want to stop an increment, there should be a sufficient reason, and it shall be done following any disciplinary action. The offer letter will contain a scale of pay as referred to above, and as such, it is a right.
Open Salary Structures
In an open scale, there will not be any increment offered at the time of employment. Only a salary and (now it is Cost to Company or CTC) and there is no guarantee that one will get an increment. As such, the management will decide what the increment should be for each person based on various parameters. Obviously, by offering an increment, the management expects more inputs from the employee. If an employee is not expected to remain with us, or if we feel that an employee may leave us (say her marriage has been fixed, and she is planning to settle in the US with her spouse after marriage), we will not pay higher increments to her. In the same way, if an employee is expected to go for maternity leave in another few months, the increment shall be kept on hold.
Legal Considerations
The law does not speak about non-payment of an increment, but the law speaks only about non-payment of salary or dismissal of women while they are pregnant. Therefore, if your wife has not been given an increment, you cannot question it. Anyway, the management has said that she will get it once she returns after leave. But if the management has communicated it by means of an order, that can be questioned, saying that the refusal to give an increment was due to her pregnancy, and that was a stigmatic order.