initial pageID set in pager

Hi!, There is a way to define witch page starts displaying a Pager object? I have a pagination of invoices and want to list in ascend order but the first page displayed (first time the page is open) must be the last page. I'have tried it passing the $pager->numPages() value to this methods: $pager->getPerPageSelectBox($pager->numPages()) $pager->getLinks($pager->numPages()) but I don't know how to control this when the user change the page. Always is showed the same page, the last. Any ideas? Thanks.