Earlier this month, 10.2m viewers watched Walford's Ronnie Branning confess her dark secret over baby Tommy and give him back to Kat Moon before handing herself over to the police.
Writing in her latest new! magazine column, Marsh discussed the explosive plot climax as she reflected on the news that former Albert Square star Michelle Collins is joining Corrie.
Marsh commented: "It's great that Michelle Collins, who played EastEnders' Cindy Beale, is joining Corrie as Stella, the new landlady of the Rovers Return. I think I'm right in saying it's the first time that an actor has had major roles in both the top soaps.
"Talking of EastEnders, I was blown away by the outstanding episodes wrapping up the controversial baby swap storyline. Samantha Womack, who plays Ronnie Branning, and Jessie Wallace, who plays Kat Moon, were so believable, and Scott Maslen, who plays Jack Branning, did a brilliant job, too."
She added: "I watched three episodes back to back on BBC iPlayer and I cried the whole time!"
Marsh is currently on maternity leave following the arrival of her baby daughter Polly, but she is expected to return to her role as Weatherfield's Michelle Connor in September.
Read the full story in this week's new! magazine - out today.