Contact HM Revenue and Customs (HMRC) within 1 month of the birth so your claim can be backdated to the date of birth. If you do not, your tax credits will only be backdated by 1 month from the date you told HMRC about the birth.
Your baby is born 1 September.
You tell HMRC on 1 January so your tax credits get backdated to 1 December.
If you’d told them by 1 October you’d be getting tax credits from 1 September.