Berluti CEO Antoine made the happy announcement on his Facebook page.
"Hi everyone, big news today!," he wrote, alongside six touching images of the happy couple. "Giving a little brother or sister to Natalia's 3 wonderful children is something we wanted so much: AND NOW IT'S HAPPENING! :)))
CLICK ON PHOTOS FOR GALLERY
"We're going to have out first baby next year!!! #4 for her but #1 for me… I'm a little stressed! She was tired the first 3 months but is fine now…
"I hope he or she gets Mommy's look (and beautiful heart!!!) and Daddy's sense of humour!! Pleaaaase not the other way round!!! :)))"
His supermodel girlfriend shared the post on her Facebook page, adding a few words of her own.
"Please read cute post by future daddy and share our big news," she wrote.
"This will explain to many of my friends and colleagues why I have been cancelling on them so often recently.
"I am so grateful for everyone at Naked Heart and at home for patience and understanding in those tough few first months especially those who knew nothing about my present state.
"One guy (it had to be a guy) who drives me sometimes said back in September to my assistant that I am having a nervous break down because I am eating all the time in the car!
"We are due in May and all the family is very excited to welcome in a new little member S)."
Natalia and 36-year-old Antoine, the son of LVMH chairman Bernard Arnault and CEO of luxury brand Berluti, started dating in 2011.
Russian beauty Natalia, 31, has three children from her marriage to British property heir Justin Portman; Lucas, 11, Neva, seven and Viktor, six.