The calculation period is the eight weeks, if you are paid weekly, up to and including the 15th week before your baby is due. If you are paid monthly, the calculation period is usually based on the last two monthly payments received before the end of the 15th week before your baby is due. To work out the weekly average if you are monthly paid, add up the pay on the payslips, divide by 2 , multiply by 12 and divide that number by 52. Pay can include holiday pay, bonuses, overtime, sick pay and any previous periods of SMP but not Maternity Allowance. National pay awards, usually effective from 1st April, which fall due after the assessment period will result in the reassessment of SMP based on values derived from the increased salary. The revised SMP will be payable from the start of the maternity leave or from the effective date of the pay award (usually 1st April), whichever is later.