The 32-year-old actor - who plays Ruby Allen in the BBC soap - announced in March she was expecting her first child with fiance Ben Bhanvra.
Lytton has now revealed she has become a mother by sharing a black and white picture on Instagram of her and Bhanvra holding a tiny hand.
Watch: Louisa Lytton announces pregnancy on Mother's Day
She wrote: "And then there were three… Welcome to the world, We are utterly overwhelmed with love.
Lytton - who competed in Strictly Come Dancing in 2006 - was flooded with messages of congratulations from her celebrity pals, including EastEnders co-star Jessie Wallace and Brooke Kinsella, and Strictly's Orla Jordan and Gorka Marquez.
Albert Square's Jacqueline Jossa gushed: "Congratulations darling !!"
And Olympic swimmer Rebecca Adlington, EastEnders star Adam Woodyatt and The Saturdays singer Vanessa White were also among those to send their best wishes.
Lytton announced her pregnancy on Mother's Day by filming herself opening a notecard with the words "Just a little note to say" on the front, and "there's a baby on the way" printed inside.
The Instagram video also featured herself and Bhanvra pointing at her baby bump, Bhanvra reading a Pregnancy For Men book, and their dog wearing a neckerchief with the message "My parents are getting me a human".
Lytton and Bhanvra got engaged in 2019 and in February she posted on Instagram that her wedding dress was still in the shop because she was nervous about collecting it as she wasn't sure whether weddings would be going ahead this year.
Lytton had her big break in EastEnders when she was cast as Ruby Allen in 2005 and left a year later after competing in Strictly. She starred in ITV police drama The Bill from 2007 to 2009 and competed in Channel 4 skiing show The Jump in 2016.
She returned to Albert Square in 2018.
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