I think she is over three months - that's why they got married so quick.
the wedding photo in the sun story shows Katie's tummy to be quite rounded. I can't think she would release that particular pic if she weren't pregnant.
And for the claim she's only 5 weeks gone? Don't believe it. If she's had miscarriages before then she would wait until at least 12 weeks before making an announcement, so as not to tempt fate.