"What happens to employees’ contributions when they're on maternity leave?"
Pension contributions when an employee is on maternity leave, can continue to be paid into the employee’s pension.
The employer should pay the contributions based on the employee’s pay before she went on maternity leave, whilst the employee should pay contributions based on the pay that she’s actually receiving (for example Statutory Maternity Pay).
So the employee’s contribution might be nil if her earnings don’t exceed the lower earnings threshold if (for example) the qualifying earnings basis is being used.
Related: paternity leave and pension.