The 50-year-old broke the news on on Tuesday (22Mar16), revealing his 33-year-old girlfriend, Alexandra Bolotow, had given birth to sons Rex and Roman on Saturday (19Mar16).

"Watching this woman rock it through an insane delivery and push out our babies was the most intense, inspiring, exhilarating, and humbling experience of my life," he captioned a photo of the new mum cradling their newborns.

"So blessed and grateful for this sweet family. Welcome to the world Rex + Roman. March 19th, 2016 6:23pm and 6:35pm, 6lbs 1 oz and 6lbs 10oz."

Richardson, who has worked with stars such as Beyonce, Lady Gaga and Miley Cyrus, announced he was to become a first-time dad in October (15) after the news leaked in the New York Post.

As he confirmed the rumours, he admitted the couple's journey to parenthood hadn't been easy.

"Babies' first Page Six shout out!", the snapper wrote on Instagram. "We weren't planning to make this news quite so public quite so soon, because we are very cautious after going through some very difficult and heartbreaking losses, and always afraid of jinxing it, but since the cat is out of the bag... Skinny (Bolotow) and I are very excited to let you guys know that we are expecting TWINS in the spring!!!

"This has been a difficult journey and we are so thankful to be surrounded by so much love and support from our friends and family, and some incredibly smart doctors!"

Richardson first met Bolotow nearly 10 years ago when she was a student at New York University, and served as his intern. She later became his assistant and one of his models, but their relationship only turned romantic in 2014.