AceShowbiz - Actress Erika Christensen has accidentally shared the gender of her unborn child after trying to keep the news a secret.

The "Traffic" star was gushing about her second pregnancy on the red carpet at a Vanity Fair and Max Mara event in Los Angeles on Tuesday, June 12 when she let slip the surprise.

"It's fun to have secrets," she initially teased, before inadvertently referring to the baby as a "her" while discussing how her young daughter Shane felt about welcoming a sibling.

"She seems to grasp (the idea)," Erika shared. "My mum got her a T-shirt that says, 'Big sister'. She seems to grasp that there's a baby in here. She's giving the baby some kisses. Hopefully the little one will learn a lot from her sister."

Realising her gender reveal, she added, "Oh well, there you go!"

The baby will be Erika's second with husband Cole Maness, who she wed in 2015, but this time around, the actress confesses she's not been keeping as active as she did with her first pregnancy.

"I haven't challenged myself particularly," she said. "Last time, I did a lot of big adventurous hikes and things, and I think I ought to get back into that. I think I was a little fitter the last time."