Did Jennifer Lopez's costume slip reveal a baby bump?

Her curves have made her world famous, but Jennifer Lopez's latest concert performance revealed an unexpected new contour.

Her flimsy costume swirled up to reveal a distinctly rounded midriff on the usually toned star, fuelling rumours that she is pregnant.

At one point the 38-year-old appeared to hold her stomach protectively during the stage show as the Roberto Cavalli halter top and skin-tight trousers failed to conceal her figure.

Baby Lo? Jennifer's bump was momentarily on display

An onlooker at the concert in Uncasville, Connecticut, told the New York Post: "A definitive baby bump was there. She was very quick to push down her top.

"Then she talked about how this year is full of firsts for her - and every so often her dress would fly up and you would see her belly."

Sources close to the star say she is expected to announce her first pregnancy at her New York concert in Madison Square Garden this weekend.

Bump ahead: Jennifer Lopez showed audiences what's beneth her floating Roberto Cavalli outfit during a performance on Wednesday

Audience members at her Sweet Face fashion show in New York Fashion Week last month detected a small bump as the actress and designer walked down the runway at the end of her show wearing a baggy green dress from the label.

Until now the pop star, who married Marc Anthony, 39, in June 2004, has denied rumours that she is more than three months pregnant.

She has spoken previously of her desire for a family and the couple have experimented with fertility treatments over the past two years.

Her husband has three children from previous relationships.

Material girl: during the tour Jennifer (with husband Marc Anthony) has worn only loose-fitting outfits