Jessica Simpson "freaked out" when she discovered she was pregnant.

The 31-year-old singer, who is expecting her first child any day now, was not trying for a baby with fianc Eric Johnson but it was a welcome surprise.

Jessica told of her pregnancy shock in an interview with talk show host Jay Leno last night in the US.

She took a pregnancy test when she felt guilty over plans to drink cocktails with friends.

"I freaked out I was so excited, Eric didn't even know I was doing it, he was completely shocked. He cried," she said.

"We weren't trying but we were practising."

The With You singer also addressed her wedding plans with the 32-year-old former NFL player. She says their big day will have to wait until after the birth of their daughter but she can't wait to tie the knot.

"I want him to be my husband so bad. I do still want to wear the pretty gown so I have to wait," she said before adding, "Now she'll just be in the wedding."

Jessica also joked that she is now so big she looks like she is having twins. Wearing a black dress, an animal print cardigan and a pair of sky-high heels said she was "waddling at this point" but isn't due to give birth just yet.

"No, it's not twins I know it looks like it," she laughed. "It's a tough thing here and I'm wearing six inch heels, it's a lot of weight to carry on six inches."

Jessica and Eric announced their engagement in 2010 after just five months of dating.

Last November they confirmed the baby news and later revealed they are having a daughter.