Whether the reduction in hours will impact your maternity pay depends on what stage you are in of your pregnancy when the change in hours happens.
Your employer should calculate statutory maternity pay based on the average of the eight weeks pay at the 15th week before your baby is due.
The 15th week before your baby is due is known as the qualifying period and is crucial in relation to your qualification for, and calculation of, statutory maternity pay.
When your maternity leave starts, your employer would need to go back to that 15th week before your baby is due and work back another 8 weeks for the purposes of calculating the average pay.
If you are paid monthly, then it would be the average of the last two pay slips. If you are paid weekly, then it is the average over the last eight pay slips.
So, if your hours are reduced in the last 3 months of your pregnancy then the change in hours and pay should not impact on the amount of statutory maternity pay you receive.