Sharing a photo of herself heavily pregnant, Kate wrote: 'This tiny human busted out and joined us a tad earlier than expected & I can't wait to introduce you to her.'
'In the meantime we're taking some time to snuggle her close & slowly introduce to her to this big beautiful world.'
Andrew has two children from his previous marriages.
He shares 18-year-old son Samuel with first wife Renae Chapman, and eight-year-old son Louis with second wife Catherine Mahoney.
Andrew and Kate began dating in late 2016.
Congratulations to Kate and Andrew!