The actress told Glamour UK that her son will take her surname rather than her husband’s name, which was changed by deed poll in 2008.
"Of course we’re not going to call it Rocknroll," she said. "People might judge all they like, but I’m a grown-up."
She also insisted that she was in no rush to put on any baby weight she may have gained. "Having just had a baby, I’m not going to be thinking about my arse," she said.
Kate gave birth to their son Bear on 7 December 2013. She has children Mia, 13, and Joe, 10, from previous relationships.
The Titanic star added that she was "baffled" by criticism leveled at her for marrying for a third time and having a third baby. Kate was previously married to film director Jim Threapleton and stage and film director Sam Mendes.
"It’s very easy to be judgmental until you know someone’s truth," she said. "People have no idea at all."
Kate and Ned began dating in 2011 after meeting at Sir Richard's idyllic Caribbean hideaway, Necker Island.
Formerly Ned Abel Smith, he changed his name by deed poll saying it would be "a fun reminder to make sure every moment of every day is spent wisely and enjoying life."
The couple married in an intimate wedding ceremony in New York, where Kate was given away by her former co-star and best friend Leonardo DiCaprio.