The 25-year-old model and her husband Elliot Grainge - who she married in April 23 - are expecting a baby girl together and she is already six months along in her pregnancy, though she has known for some time.
She told America's Vogue magazine: "I found out very, very early. I was about four weeks pregnant. I was on a trip to Milan for fashion week, and I was going to the Prada show. I was doing a really quick 24-hour turnaround, and I felt terrible but didn’t think much of it. I thought it was jet lag.”
When she got back to Los Angeles, Sofia was supposed to go to an Ed Sheeran concert with Elliot, 30, and their friends the next day but thought she should be "safe" and do a pregnancy test.
She said: “I kind of knew it was going to be a boozy night, and I was a few days late on my period, and I wanted to be generally safe, so I took a test.
“Elliot was already used to me taking tests randomly. So he didn’t really flinch when I said I was checking. I was taking one of those at-home tests, and there was the faintest of lines. I was so used to seeing negatives that I knew when something looked even kind of different.”
Her "excited" spouse then went to pick up three more pregnancy tests and they were all positive - and the couple kept them in order to break the news to their parents when Sofia was around eight weeks pregnant.
She recalled: “We went to Elliot’s parents, who live about five minutes away from us, and I think I had a box—I want to say it was a Hermès box or some sort of box—and I told everyone that I had bought them a gift from Milan Fashion Week.
“Everyone was like ‘Ooooh Hermes!’ But then they opened it and it had all three of the pregnancy tests [in it]. That’s how we told all of our parents. Watching their reaction go from like, ‘I’m about to get an expensive present’ to like ‘I’m about to get a grandchild’ was really sweet.”
Despite being aware of her pregnancy so early, Sofia felt it was important to keep the news private until now.
She said: “Pregnancy is really scary and you want to protect that space. I didn’t realise there are so many milestones you have to hit and so many tests you have to take. For me, it was really important to protect our mental health and our space as a couple.”