Tax credits won't do anything until your baby is actually born but you can ask them for an application form and fill all the rest of it out so it's all ready to go. Make sure you claim fairly soon after the birth though, they will back date payments a bit but not more than 3 months I think.
It's income before tax and SMP is £128.73 but you can deduct £100 for each week you're on paid maternity leave (regardless of how much that pay was) up a maximum of 39 weeks/£3900 - as an example, all of my maternity leave falls within this tax year so I've deducted the full £3900 from my income estimate for this year.
The form asks for a Child Benefit number but you can leave this blank and still send it off. If you're not applying until the baby is born then attach a note to your claim form telling them that you've made a claim to Child Benefit but it hasn't been awarded yet and that Child Benefit have the birth certificate. Also state that you want the claim considered for backdating for the full 93 days to make sure that you're paid from the first/earliest date you're eligible.
thanks we're already claiming child tax credits for ds and dd so i just had 2 let them know of change of income. thanks for letting me know how much the pay will be (i get 6 weeks @90% b4 getting the lower rate). turns out you work out what you'd earn for the year, then they make the £100 deductions- dh also gets deduction for his paternity leave :-) so dont forget to tell them about that too