I'm 6 months pregnant and just discovered that the calculation for my maternity pay is made on the months of Oct - Dec 2010. As I was getting childcare vouchers during those months, the sum HMRC uses to calculate the first part (6 weeks at 90% of your gross pay) is MINUS the childcare vouchers!
I will lose about £350 because of this.
If you employer offers more weeks at 90% then you could lose a lot more...