Jessica Simpson was ''in complete shock'' when she managed to fit into her 14-year-old True Religion jeans.

The 'Dukes of Hazzard' actress celebrated her 40th birthday in July by taking to Instagram to post a picture of herself wearing a pair of jeans she has kept in her wardrobe for more than a decade.

And now, Jessica has said she couldn't believe it when the jeans slipped on with no problem at all, because she'd tried ''so many times'' over the past 14 years to get them over her hips.

She told People magazine: ''I was in complete shock. It was a good 40th birthday present. I've tried to pull those things over my hips so many times. I just thought my hips, from having babies, would never go back, but the jeans went up.''

Meanwhile, Jessica recently admitted she ''freaked out'' over turning 40, because it took her a while to ''accept'' her age.

She explained: ''Did I turn 40? That was a really random thing, because I haven't put jeans on through the whole time, I've been in sweats. I am like embracing the quarantine life. Yeah.

''I freaked out two months before I turned 40, not knowing that that age would even mean anything to me but then its like 41, 42 ... I've never calculated my age until now ... I'm accepting it slowly [laughs].''

Jessica put on the old jeans because she enjoys ''reminiscing'' about the past.

And the 'Blonde Ambition' star joked she's glad she put her memoir, 'Open Book', out earlier this year, because it would be ''800 pages'' long if she'd waited any longer.

She added: ''I just think I am such a person that reminisces, and I hold onto so many memories, I am tragically romantic like that about my own self ... wow 40, thank God I put a memoir out at 40, because it would have been 800 pages if I waited any longer.''