The actress and husband Tom Ellis welcomed their second daughter Marnie Mae Ellis earlier this week.
Ellis tweeted: "What a couple of days...Gold medals from everywhere and...oh we had a beautiful baby girl called Marnie Mae Ellis 6lb 5oz of gorgeousness!"
The couple, who revealed that they were expecting their second child back in March, already have a 3-year-old daughter called Florence.
Outhwaite had previously confessed that she wanted more children, saying: "I don't want to leave it too late, especially as we would like to have more than one child.
"I am not worried in the least about my career taking a back seat. I want to change my priorities.
"My career and ambitions have been fulfilled, and I would be quite happy not to work again and look after the children. I don't really have that burning ambition anymore."
Outhwaite recently admitted that she never expected EastEnders to change her life as much as it did when she joined as Mel Owen in 1998.