Jessica Simpson is worried it'll be like a "fire hydrant" when her waters break as she has so much amniotic fluid.

The singer-and-actress is expecting her first child with her fianc Eric Johnson. Jessica is due to give birth in four weeks, but the size of her baby bump has left some fans astounded. The 31-year-old knows her stomach is surprisingly big and admits it's taken a lot of getting used to.

"I feel I have a bowling ball sitting on my hoo hah!" she laughed to talk show host Jimmy Kimmel.

"Apparently I have a lot of amniotic fluid, so whenever my water breaks it will be like a fire hydrant!"

Recent reports suggested Jessica was feeling nervous about giving birth. There were claims she was concerned because her sister Ashlee endured a lengthy labour with her son Bronx.

However, they appear to me untrue. "I'm ready to push," she insisted.

Jessica and Eric are waiting to get married until their baby daughter has arrived. She has no problem with that decision as she doesn't think a wedding will change their relationship at all.

"[He] feels like my husband already," she explained. "We are just doing it a little bit backwards. We met at my house - he just came with one of my friends to my house, we were watching March Madness or something.

"He came over and we sat on my stairwell and talked and I just haven't let him leave!"

Jessica celebrated impending motherhood with a baby shower on Sunday, where guests such as Jessica Alba and Ellen Pompeo were treated to fried Oreo cookies.