The 35-year-old singer and the 43-year-old actor will welcome their first child together later this year and Katy took to Instagram to reveal the gender of the baby.
She wrote: ''It's a girl,'' and included the words ''Girls Run the World'' in the post.
She also shared a picture of Orlando - who already has son Flynn, nine, with former wife Miranda Kerr - with his face covered in pink icing.
Katy revealed her exciting pregnancy news recently when she dropped the music video for new single 'Never Worn White', which at the end showed her cradling her baby bump.
Katy then confirmed the news in a subsequent Instagram video, admitting it is ''the longest secret'' she has ''ever had to keep''.
She said: ''I'm late ... but you already knew that. There's a lot that will be happening this summer, not only will I be giving birth - literally - but also figuratively to something you guys have been waiting for.
''So let's call it a double whammy, a two fore. I'm excited, we're excited and we're happy - and it's probably the longest secret I've ever had to keep. I knew I would tell you in the best way, which is through a piece of music because that's how I speak to you.''
And she later revealed she has always wanted to have kids, but it's only after the last couple of years that she's finally feeling ''ready'' to be a mum for the first time.
She added: ''I was that girl, or am still that girl, that had that box, the baby clothes before there was the thought, or even an Orlando Bloom.
''I'm excited about that and like I said, I have two nieces and one nephew that I'm obsessed with. I think, definitely, everything's changing.
''I wasn't ready a couple years ago, and I did the work to get ready and now I'm really ready. I see a baby and I'm like [makes gaga face]. It's just time for me and it's the right time.''
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