30 Years Later, Lea Thompson Still Has Her Iconic Back to the Future Prom Dress

After all these years, Lea Thompson still has that dress.

As fans celebrate Back to the Future Day on Wednesday, Thompson tells PEOPLE she still has the '50s-style pastel pink prom dress she wore as part of the first film's Enchantment Under the Sea Dance.

"I can't get in it any more, though," she tells PEOPLE with a chuckle. "It was so tight! It was tight at the time, but now it’s really, really tight."

But that doesn't mean she doesn't still love it.

"It was beautifully made, and made to order for me," she says. "It's got a low-drop waist, and the way it was cut was just so beautiful. And it's still in great shape. I've still got the shoes. The shoes are even hard to get on now – my big ol' feet from having babies."

Back to the Future Day celebrates the date hero Marty McFly (played by Michael J. Fox) visited the future in the second installment of the beloved trilogy. Thompson memorably played Marty's mother in the films, and wore that pink dress in the first film when she had no clue the boy she was wooing was actually her son.

Thompson was only 24 when she filmed the first movie, and she admits the beauty of the dress she wore during the dance scene was occasionally overwhelmed by its extreme form-fitting design. "They used to make fun of it [on set] because my boobs kind of spilled out of the top of it – they used to call my boobs the MMs: the Mammoth Mammaries," she laughs.

"When we shot the big dance – the Enchantment Under the Sea Dance – it was so tight, I just took it off," she reveals. "I would wander around in my crinolines and my push-up bra. My mom was just scandalized, because of course there were like hundreds of extras, but I didn't care. I was just walking around without my dress on."

But she loved the dress enough to be allowed to keep one of them when the film wrapped and she kept it carefully stored – which ultimately turned out to be a good thing just a few years later when two back-to-back sequels to the blockbuster hit went into production. "When we did Back to the Future II, they were panicked because they couldn't find it," she reveals. "They had made three, and they had given me one, so I lent it back to them. Actually, they just shipped it out to New York to be on The Today Show."

Like most moms with a treasured dress in the back of their closet, Thompson also couldn't resist showing it off to her daughters, actresses Madelyn and Zoey Deutch, on occasion. "They wanted to wear it for Halloween, but even when they're super skinny and little, it was still tight for them!"