The model/socialite made the announcement Thursday courtesy of Vogue -- which posted a bunch of maternity shoots pics that were taken by a professional photog ... showing off Sofia's growing belly, not to mention of a bunch of fashionable outfits to boot.

The mag writes, "Congratulations are in order for @sofiarichiegrainge! The star is expecting her first child with her husband, @Elliotgrainge."

There's a quote from SR in their piece, which reads ... "I’ve learned more in the past six months than I have in my entire life." She reveals in her interview that she's having a little girl, and that the baby is due in June -- 'cause it's gonna be a Gemini kid.

In terms of how Sofia found out ... she says she caught on to the fact she was pregnant fairly early -- about 4 weeks in, she felt sick during a fashion show in Milan ... and when she got back to L.A., she started taking pregnancy tests just to be careful.

As it turns out, she was preggo -- and notes she and her husband, Elliot Grainge, had been casually trying to make a baby since they got hitched last April at their big French wedding.

Now that Sofia is in the final trimester of her pregnancy, she has this to say about the journey ... "[Y]ou just have to sit back and just be in awe, and know that everything you’re going through is completely normal and all of the ups and downs are normal. It’s made me feel better knowing that other people have gone through the emotional ups and downs. That was comforting for me."