The 42-year-old actress said she and former partner Michael Sheen celebrated Lily's sweet sixteen by reenacting her birth.
"We took pictures of it!" Beckinsale told E! News.
"I wasn't naked, I was actually in a very nice cigarette pant and a high heel," the mum-of-one explained.
"Michael took one of my legs and held it up and Lily came from behind and was diving between my legs."
Well that's one way to make a 16th birthday memorable!
Beckinsale and Sheen were together for eight years and had one child together before splitting in 2003.
She said the pair remain close friends.