Brian (Peter Gunn) has been left in turmoil after learning that Julie is pregnant, fearing that he can't be the father as he had a vasectomy some time ago.
After Brian accuses Julie of sleeping with another man, the pair visit Doctor Carter (Oliver Mellor) at the medical centre later this week - hoping for an explanation.
Cavanagh told This Morning today: "[At first], Julie's got absolutely no idea why Brian's not over the moon as well. Obviously they've been trying extremely hard to make babies! It's going to completely knock her for six.
"Doctor Carter's going to put it all right. He's going to tell us that apparently, if you have a vasectomy, it's not always 100%. It's all going to come alright in the end, but Brian's really hurt her."
Asked whether Julie can forgive and forget Brian's accusation, the actress replied: "Well, Julie's got a lovely goldfish kind of memory, I think! She likes everything to be lovely.
"I love her, she's brilliant. It's lovely being nice!"
Cavanagh added that she is expecting more comic scenes for the pair as they prepare for parenthood.
She said: "Julie will love it. She'll be reading every book, she'll be looking at every possible thing - natural birth and all sorts. She'll be having Brian doing all sorts of mad stuff, I think!"
Coronation Street continues on Friday at 7.30pm and 8.30pm on ITV1.
> Coronation Street Julie is not a love cheat, says Katy Cavanagh
> Coronation Street Katy Cavanagh expects "chaotic" Julie baby scenes
> Coronation Street Kylie Platt 'goes to war with Brian'
> Read more Coronation Street spoilers and news
Watch a preview of Friday night's Coronation Street double bill in the video below: