The former Parenthood star broke the news on Twitter hours before she debuted her large baby bump at the Los Angeles premiere of her new TV movie Confirmation on Thursday evening (31Mar16).

"This boy is getting a little sister," she tweeted with a picture of her puppy. "A human sister... She’ll make her first appearance tomorrow at #Confirmation."

Erika, who is married to cyclist Cole Maness, told at the event that she was seven months long and her bump, which she covered with a figure-hugging and cleavage-baring dark gown, was "a really fun accessory".

She also revealed how she has adapted her exercise routine around her new body, saying, “I thought it was an automatic thing, you just lost control of your body, like, the pregnancy just took over... That didn't happen.”

“I was like, well, I'm not going to be running, that seems rude... So, (I’m) keeping (fit by doing) elliptical and walking. I just felt like continuing my lifestyle.”

The 33-year-old has been in a relationship with Cole since 2012. They tied the knot in Palm Springs, California in September, 2015. They announced their good news in an Instagram picture, which showed Erika giving a first bump and displaying her new wedding ring. She wrote, "True story. We got our rings and did the d**n thing. #weddedbliss"

Confirmations airs on cable channel HBO in the U.S. on 16 April (16). It stars Kerry Washington, who plays real-life attorney Anita Hill, who accused U.S. Supreme Court nominee Clarence Thomas of sexual harassment back in 1991.