Rebecca Romijn has dropped nearly 60 pounds after welcoming twins Dolly and Charlie in December -- without a workout plan.

"I think within the first three weeks, I took off, like, 35 pounds without doing anything!" she tells Extra.

Chasing her four-month-old twins has kept her too busy to hit the gym, anyway.

See photos of stars with twins!

"I haven't been able to work out that much because I have twins," says the Ugly Betty star. "It's impossible to get back into a regular schedule."

Her secret?

"Breastfeeding is the very best diet I've been on. It's amazing," she says. "It's like you have to eat 5,000 extra calories a day or you can't produce enough food for them. So, that's been fun as well."

Check out celebs' bodies after baby.

But she admits she isn't completely back to her pre-baby bod.

"I'm not pretending to be back to the shape I was in beforehand anyway. I still have a ways to goWe've got Spanx underneath almost everything" she says.

Romijn -- currently filming The Love Child in Canada -- says being a working mother is tough "for a number of reasons."

See photos of stylish moms on the go.

"Our hours have been really late and we're working outside of Toronto in Hamilton, which is about an hour away [from where husband Jerry O'Connell and the twins are staying] -- and I am still nursing [the babies] around the clock," she says. "So, we have to do a lot of milk runs."

O'Connell has been "like, totally on diaper duty," Romijn continues. "He's so awesome. I think he's having a really good time also."

Romijn says her twins are "really smiley and not fussy. For four months old, it's supposed to be really difficult, but I have to say, it's just fun."