Clara McGregor, the Trainspotting star’s eldest child, shared the news on Instagram with a photo of her holding the new arrival.
“Welcome to the world little brother,” the 25-year-old wrote. “Congratulations to my Dad & Mary - this is the greatest gift.”
Clara's younger sister Esther, 19, revealed that McGregor and Winstead, bot actors, have named their son Laurie, as she also shared photos with him on Instagram.
“Met my little brother looking like a pirate. I recommend!,” she captioned a trio of sweet images. “Welcome to the family little Laurie.”
Winstead, 36, and McGregor, 50, had kept her pregnancy private until now.
McGregor is also father to Jamyan, 20, and Anouk, 10, from his previous marriage to production designer Eve Mavrakis, whom he divorced after 22 years.
He met Winstead on the set of the critically acclaimed TV series Fargo in 2016.
Winstead split from her husband of seven years, filmmaker Riley Stearns, in May 2017. She was seen with McGregor later that year; he filed for divorce from Mavrakis in 2018.
McGregor recently starred in Netflix’s 2021 miniseries Halston, about the iconic American fashion designer.